thesis capacitor bank switching

INVESTIGATION OF CIRCUIT BREAKERSWITCHING TRANSIENTS FOR SHUNTREACTORS AND SHUNT CAPACITORS

Mohd Shamir Ramli, B.Eng

Submitted in fulfilment of the requirements for the degree of

Master of Engineering

School of Engineering Systems

Faculty of Built Environment and Engineering

Queensland University of Technology

2008

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Investigation of Circuit Breaker Switching Transients for Shunt Reactors and Shunt Capacitors

M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page i

Keywords

Antenna, arcing time, capacitor bank, capacitive coupling, circuit breaker, condition monitoring,

controlled switching, current interruption, high frequency, high voltage, non-intrusive, on-line

monitoring, overvoltages, prestrike, restrike, reignition, shunt reactor, switching, timing,

transients, voltage sensor.




M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page ii

Abstract

Switching of shunt reactors and capacitor banks is known to cause a very high rate of

rise of transient recovery voltage across the circuit breaker contacts. With improvements in

circuit breaker technology, modern SF6 puffer circuits have been designed with less interrupter

per pole than previous generations of SF6 circuit breakers. This has caused modern circuit

breakers to operate with higher voltage stress in the dielectric recovery region after current

interruption. Catastrophic failures of modern SF6 circuit breakers have been reported during

shunt reactor and capacitor bank de-energisation. In those cases, evidence of cumulative re-

strikes has been found to be the main cause of interrupter failure.

Monitoring of voltage waveforms during switching would provide information about the

magnitude and frequency of small re-ignitions and re-strikes. However, measuring waveforms at

a moderately high frequency require plant outages to connect equipment. In recent years, there

have been increasing interests in using RF measurements in condition monitoring of switchgear.

The RF measurement technique used for measuring circuit breaker inter-pole switching time

during capacitor bank closing is of particular interest.

In this thesis, research has been carried out to investigate switching transients produced

during circuit breaker switching capacitor banks and shunt reactors using a non-intrusive

measurement technique. The proposed technique measures the high frequency and low

frequency voltage waveforms during switching operations without the need of an outage. The

principles of this measurement technique are discussed and field measurements were carried out

at shunt rector and capacitor bank installation in two 275 kV air insulated substations. Results of

the measurements are presented and discussed in this thesis.

The proposed technique shows that it is relatively easy to monitor circuit breaker

switching transients and useful information on switching instances can be extracted from the

measured waveforms. Further research works are discussed to realise the full potential of the

measuring technique.




M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page iii

Table of Contents

Keywords .................................................................................................................................................i Abstract .................................................................................................................................................. ii Table of Contents .................................................................................................................................. iii List of Figures ........................................................................................................................................vi List of Tables .........................................................................................................................................ix List of Abbreviations ..............................................................................................................................x Statement of Original Authorship ..........................................................................................................xi Acknowledgments................................................................................................................................ xii CHAPTER 1: INTRODUCTION...................................................................................................... 1 1.1 Background..................................................................................................................................1 1.2 Research conducted .....................................................................................................................2 1.3 Thesis outline...............................................................................................................................3 CHAPTER 2: LITERATURE REVIEW.......................................................................................... 4 2.1 Review of current interruption in circuit breakers .......................................................................4 2.2 Reactive equipment switching .....................................................................................................6 2.3 Review on capacitor bank switching ...........................................................................................8

2.3.1 Interrupting capacitor bank...............................................................................................8 2.3.2 Energising capacitor bank ..............................................................................................13

2.4 Review of reactor bank switching .............................................................................................14 2.4.1 Interrupting shunt reactor bank.......................................................................................15 2.4.2 Current chopping ............................................................................................................16 2.4.3 Reignition .......................................................................................................................20 2.4.4 Oscillation modes ...........................................................................................................24 2.4.5 Interaction between phases.............................................................................................26 2.4.6 Energising transients.......................................................................................................28

2.5 Limitation of overvoltage transient during reactive switching ..................................................28 2.5.1 Over voltage limitation..................................................................................................28 2.5.2 Controlled switching.......................................................................................................29

2.6 Failure of circuit breaker due to restriking ................................................................................30 2.6.1 Importance of detecting restrike .....................................................................................36

2.7 Condition monitoring for circuit breakers .................................................................................39 2.7.1 Detecting restrikes or interrupter experiencing prolong restrikes ..................................40 2.7.2 Alternative monitoring methods .....................................................................................40

CHAPTER 3: NEW METHODS FOR CONDITION MONITORING OF RESTRIKING EHV CBS...................................................................................................................................................... 42 3.1 Non-invasive circuit breaker monitoring using radiometric measurement ................................42 3.2 Research methodology...............................................................................................................43 3.3 Developing measuring equipment .............................................................................................44 3.4 Active broadband Antenna ........................................................................................................45 3.5 Capacitive Coupling antenna .....................................................................................................47

3.5.1 Construction of the Passive Antenna..............................................................................47




M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page iv

3.5.2 Review on capacitive coupling.......................................................................................49 3.5.3 Single phase capacitive coupling model.........................................................................51 3.5.4 Three phase coupling inside substation ..........................................................................55

3.6 Recording instruments ...............................................................................................................56 3.6.1 Digital oscilloscopes.......................................................................................................56 3.6.2 Coaxial cable ..................................................................................................................58 3.6.3 Measurement requirement in substation .........................................................................58

CHAPTER 4: EXPLORATORY MEASUREMENT ON SINGLE-PHASE REACTOR SWITCHING AND CAPACITOR BANK SWITCHING ............................................................. 60 4.1 Field measurement at Ergon Laboratory....................................................................................60

4.1.1 Purpose of measurement.................................................................................................60 4.1.2 Restriking in Vacuum Circuit Breaker ...........................................................................60 4.1.3 Test and measurement set up..........................................................................................62 4.1.4 Reactor opening at 3kV with Passive Antenna located close to the supply transformer64 4.1.5 Conclusion......................................................................................................................67

4.2 Exploratory three phase capacitor bank switching measurement at Blackwall substation ........68 4.2.1 Purpose of site measurement ..........................................................................................68 4.2.2 Site details and arrangement...........................................................................................68 4.2.3 Measurement set up ........................................................................................................70 4.2.4 Summary of tests/measurement carried out ....................................................................72 4.2.5 Opening operation ..........................................................................................................73 4.2.6 Closing operation............................................................................................................75 4.2.7 Discussion.......................................................................................................................77 4.2.8 Improvement to be taken ................................................................................................77

CHAPTER 5: MEASUREMENT OF CAPACITOR BANK SWITCHING............................... 79 5.1 Site details and arrangement ......................................................................................................79 5.2 Test and measurement set up .....................................................................................................81 5.3 Summary of tests/measurement carried out ...............................................................................83 5.4 Background measurement..........................................................................................................84 5.5 Capacitor bank closing...............................................................................................................87 5.6 Imperfect capacitor bank closing ...............................................................................................90 5.7 Capacitor bank opening .............................................................................................................94 5.8 Summary on capacitor bank switching tests..............................................................................99

5.8.1 Closing operation............................................................................................................99 5.8.2 Opening operation ........................................................................................................100

CHAPTER 6: MEASUREMENT OF SHUNT REACTOR BANK SWITCHING .................. 101 6.1 Site arrangement ......................................................................................................................101 6.2 Test and measurement set up ...................................................................................................103 6.3 Summary of tests/measurement carried out .............................................................................105 6.4 Background measurement........................................................................................................106 6.5 Shunt reactor bank closing.......................................................................................................108 6.6 Shunt reactor bank opening .....................................................................................................114 6.7 Summary on shunt reactor bank switching tests ......................................................................119

6.7.1 Background measurement ............................................................................................119 6.7.2 Closing operation..........................................................................................................119 6.7.3 Opening operation ........................................................................................................119

CHAPTER 7: ANALYSIS OF RESULTS..................................................................................... 121




M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page v

7.1 Three phase capacitive coupling model ...................................................................................121 7.2 Observations of arcing signals in shunt reactor opening .........................................................129 7.3 Analysis in frequency-time domain .........................................................................................138

7.3.1 Analysing using Fast Fourier Transform (FFT) ...........................................................138 7.3.2 Analysing using Short Time Fast Fourier Transform (ST FFT) Analysis....................139

CHAPTER 8: CONCLUSION........................................................................................................ 144 REFERENCES................................................................................................................................. 144 APPENDICES .................................................................................................................................. 153

Appendix A: Site Measurement Procedure..............................................................................153 Appendix B : Sample of forms used for site measurement......................................................156 Appendix C : Matlab Program for Capacitive Divider Model (In Chapter 7) .........................158




M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page vi

List of Figures

Figure 2.1 Typical Circuit Interruption (from [7]) ..................................................................................5 Figure 2.2 Single Phase Capacitor Bank circuit (from [10])...................................................................8 Figure 2.3 Capacitance Switching (a) System voltage and current. (b) Capacitor voltage (c) Voltage

across CB contact. (from [9]) .........................................................................................................9 Figure 2.4 Capacitance switching showing the effect of source regulation (from [9]) .........................10 Figure 2.5 Capacitance switching with a restrike at peak voltage. (from [9]) ......................................12 Figure 2.6. Capacitance switching with multiple restrikes. (from [9]) .................................................13 Figure 2.7. Single phase equivalent circuit [11]...................................................................................16 Figure 2.8 Current chopping phenomena (from [3]).............................................................................17 Figure 2.9 Chopping Phenomena in single phase (from [11]) ..............................................................19 Figure 2.10 Reignition Windows (from[3]) ..........................................................................................21 Figure 2.11 Reignition at recovery voltage peak for a circuit with low supply side capacitance (from

[3])................................................................................................................................................23 Figure 2.12 Maximum re-ignition overvoltages (from [3]) ..................................................................24 Figure 2.13 Oscillation Mode in the reactor circuit ..............................................................................25 Figure 2.14 Load side oscillation with circuit breaker located close to shunt reactor (from[3]) .........27 Figure 2.15 Load side oscillation with circuit breaker located remote from shunt reactor (from [3]) .27 Figure 2.16 (a) Typical schematic of SF6 CB showing main contacts (1), arcing contacts (2) and

nozzle (3). (b) Voltage distribution in interrupter chamber. [21].................................................33 Figure 2.17 Analysis of voltage breakdown for main and arcing contacts along the contact gap. (from

[21])..............................................................................................................................................34 Figure 3.1 Photo of Broadband Active Antenna...................................................................................46 Figure 3.2 Gain vs Frequency for RF amplifier [62] ............................................................................46 Figure 3.3 Passive Antenna Drawing....................................................................................................47 Figure 3.4 Photo of Passive Antenna ....................................................................................................48 Figure 3.5 Electrostatic coupling between a HV conductor and secondary circuit...............................49 Figure 3.6 Capacitive Divider ...............................................................................................................51 Figure 3.7 Passive Antenna Equivalent Circuit ....................................................................................52 Figure 3.8 Bode Diagram......................................................................................................................54 Figure 3.9 Capacitive coupling between three phase conductors and three passive antennas. .............55 Figure 3.10 Recording instrument arrangement....................................................................................56 Figure 4.1 Restriking Process During CB Opening (from [58]) ...........................................................61 Figure 4.2 Measured Voltage at reactor terminal (from [58])...............................................................61 Figure 4.3 Experimental Circuit arrangement.......................................................................................62 Figure 4.4 Photograph showing Laboratory arrangement.....................................................................63 Figure 4.5 Photograph showing Laboratory arrangement.....................................................................63




M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page vii

)LJXUH��D��:DYHIRUPV�GXULQJ�RSHQLQJ�RI�YDFXXP�FLUFXLW�EUHDNHU�DW��N9��E��$UHD�$�±�5HVWULNHV�RQ�5HDFWRU�9ROWDJH��F��$UHD�$�±�5HVWULNHs detected by passive antenna.....................................65

Figure 4.7 HF restriking pulses detected on Active Antenna ...............................................................66 Figure 4.8 Magnification on one HF pulse ...........................................................................................67 Figure 4.9. Blackwall Substation Interconnection ................................................................................69 Figure 4.10 Blackwall Capacitor Bank Layout ....................................................................................70 Figure 4.11 Measuring equipment layout .............................................................................................71 Figure 4.12 Antenna waveform on CB opened at point A....................................................................73 Figure 4.13 HF pulses during opening..................................................................................................74 Figure 4.14 Three typical HF Pulses During Opening..........................................................................74 Figure 4.15 Passive Antenna waveform on closing ..............................................................................75 Figure 4.16 HF markers during closing ................................................................................................75 Figure 4.17 (a) to (f) typical HF Pulses during closing.........................................................................76 Figure 5.1 Three-phase voltage waveforms and controlled closing points for a Capacitor Bank.........80 Figure 5.2 Three-phase current waveforms and controlled opening points for a Capacitor Bank.......80 Figure 5.3 Measuring equipment layout for tests at Blackwall.............................................................81 Figure 5.4 Capacitor Bank Installation .................................................................................................82 Figure 5.5 Recording Instrumentation ..................................................................................................82 Figure 5.6 Plan view of antenna positions at Capacitor Bank installation during background

measurement.................................................................................................................................85 Figure 5.7 Waveform from Background measurement.........................................................................86 Figure 5.8 Plan view of the antenna positions for Test 5......................................................................87 Figure 5.9 Waveforms captured during CB close operation for Test 5 ................................................88 Figure 5.10 Waveforms captured on PowerliQN¶V�SRUWDEOH�UHFRUGHU�IRU�7HVW�� Figure 5.11 Plan view of the antenna positions for Test 3....................................................................91 Figure 5.12 Waveforms captured on PowerliQN¶V�SRUWDEOH�UHFRUGHU�IRU�7HVW�� Figure 5.13 Waveforms captured during CB close operation for Test 3 ..............................................93 Figure 5.14 Plan view of the antenna positions for Test 8....................................................................94 )LJXUH��:DYHIRUPV�FDSWXUHG�RQ�3RZHUOLQN¶V�portable recorder for Test 8-Open..........................96 Figure 5.16 Waveforms captured during CB open operation for Test 8...............................................98 Figure 6.1 Three-phase voltage waveforms and controlled closing points for a Shunt Reactor Bank102 Figure 6.2 Three-phase current waveforms and controlled opening points for Shunt Reactor Bank .102 Figure 6.3 Measuring equipment layout at Braemar substation..........................................................103 Figure 6.4 Shunt Reactor Installation..................................................................................................104 Figure 6.5 PASS MO Circuit Breaker.................................................................................................104 Figure 6.6 Passive and active antennas location .................................................................................105 Figure 6.7 Plan view of the antenna positions at shunt reactor installation ........................................107 Figure 6.8 Waveform on Background measurement ..........................................................................108 Figure 6.9 Waveforms captured on PowerliQN¶V�SRUWDEOH�UHFRUGHU�IRU�7HVW�� Figure 6.10 Waveforms captured by PA 1,2 and 3 during CB closing operation for Test 4 ..............110




M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page viii

Figure 6.11 Waveforms captured during CB close operation in Test 4 by each antenna ...................111 Figure 6.12 Comparison of voltage magnitude of closing pulses at each closing event.....................112 Figure 6.13 Waveforms captured on PowerliQN¶V�SRUWDEOH�UHFRUGHU�IRU�7HVW�� Figure 6.14 Waveforms captured during CB open operation in Test 7 ..............................................115 Figure 6.15 Waveforms captured during CB open operation by each antenna..................................117 Figure 7.1 Capacitances between passive antennas and three phase conductors with symmetrical

spacings ......................................................................................................................................122 Figure 7.2 Equivalent circuit for passive antenna at location 1 measuring three phase voltages. ......123 Figure 7.3 Output waveforms for Case 1 ............................................................................................125 Figure 7.4 Capacitances between passive antennas and three phase conductors with unsymmetrical

distances .....................................................................................................................................127 Figure 7.5 Output waveforms for Case 2 ............................................................................................127 Figure 7.6 Output waveforms for Case 3 ............................................................................................128 Figure 7.7 Waveforms recorded by Active antenna on capacitor bank opening ................................130 Figure 7.8 Waveforms recorded by Active antenna on shunt reactor opening during Test 7.............131 Figure. 7.9 Test 7 - AA signals without noise and load oscillation ....................................................131 Figure. 7.10 Test 7 - Cumulative energy against time ........................................................................132 Figure 7.11 Test 7 - Density of Pulses with time ................................................................................133 Figure 7.12 Test 7 - Cumulative Pulses against time ..........................................................................133 Figure 7.13 Test 5 - AA signals without noise and load oscillation .................................................134 Figure 7.14 Test5 - Cumulative energy against time ..........................................................................135 Figure 7.15 Test 5 - Density of Pulses with time ................................................................................136 Figure 7.16 Test 5 - Cummulative pulses against time .......................................................................136 Figure 7.17 RF Measurement showing arc signal UD, switch voltage Us and current Is [50]. ...........137 Figure 7.18 Reactor Opening Test 7 (a)Time domain plot of PA1 waveform for opening from 20 ms

to 50 ms (b) Frequency content of waveform in (a). .................................................................139 Figure 7.19 (a) Voltage-Time domain plot of waveforms from PA2 (b) ST FFT contour plot of PA2

waveforms for opening- from 23 ms to 28 ms ...........................................................................141 Figure 7.20 (a) Voltage-Time domain plot of AA (b) ST FFT contour plot of AA (from 0-2MHz) (c)

ST FFT contour plot of AA (from 0-10MHz) for opening from 23 ms to 28 ms ......................142




M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page ix

List of Tables

Table 2.1 Results of the overhaul of the circuit breakers (from [4]).....................................................31 Table 2.2 Results of tests made to examine effects of parasitic arcing.(from [25]) ..............................36 Table 2.3 Statistics cause of failure of circuit breaker (from [28]) ......................................................38 Table 3.1 Characteristics of Agilent Digital Oscilloscope ....................................................................56 Table 3.2 Characteristics of Yokogawa Digital Oscilloscope...............................................................57 Table 4.1 Calibration between test voltage, supply voltage and Passive antenna.................................64 Table 4.2 Summary of tests conducted at Blackwall on 21st May 2007 ...............................................72 Table 5.1 Summary of tests conducted at Blackwall on 7th August 2007.............................................83 Table 5.2 Voltage measured by each Passive antenna during background measurement.....................86 Table 5.3 Summary of CB timing and pole sequence for capacitor bank closing Test 5......................90 Table 5.4 Summary of CB timing and pole sequence for capacitor bank closing Test 3......................94 Table 5.5 Summary of CB timing and pole sequence for capacitor bank Test 8 ..................................97 Table 6.1 Summary of tests conducted at Braemar substation on 21st August 2007 ..........................106 Table 6.2 Voltage measured by each Passive antenna during background measurement...................108 Table 6.3 Summary of CB timing and pole sequence for shunt reactor bank closing Test 4 .............113 Table 6.4 Summary of CB timing and pole sequence for shunt reactor bank opening Test 7 ............118 Table 7.1 Calculated results and measured values from Braemar substation .....................................126 Table 7.2 Differences between original waveform and reconstructed waveforms with 10% error on

capacitances................................................................................................................................129




M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page x

List of Abbreviations

(Sort in alphabetical order.)

AA - Broadband active antenna

AIS - Air Insulated Switchgear

CIGRE - International Conference on High Voltage Systems, Paris

CB - Circuit Breaker

EHV - Extra High Voltage

HF - High frequency

PA - Passive antenna (capacitive coupling antenna)

PASS - Plug And Switch System

SF6 - Sulphur hexafluoride

VCB - Vacuum circuit breaker




M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page xi

Statement of Original Authorship

The work contained in this thesis has not been previously submitted to meet

requirements for an award at this or any other higher education institution. To the best of my

knowledge and belief, the thesis contains no material previously published or written by another

person except where due reference is made.

Signature: _________________________

Date: _________________________




M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page xii

Acknowledgments

First and foremost, my most sincere thanks must go to my supervisors, Associate

Professor David Birtwhistle and Dr. Tee Tang, for their advice, guidance and most of all their

patience and understanding throughout this research.

Special acknowledgement is made towards Powerlink Queensland for the financial

support given to this research project and also to Dr Jose Lopez Roldan of Powerlink for his

assistance and contribution in this research. The assistance of staff of Powerlink Queensland in

carrying out field measurements is greatly acknowledged.

I would also like to thank the Head of the School of Engineering Systems, Queensland

University of Technology, for the use of the facilities. Special thanks go to the technical staff at

Level 9, S Block, for their assistance with laboratory works, giving jokes that brighten up the

day and technical advices on equipment. I also thank my colleagues for their help throughout the

research work.

I gratefully acknowledge the management of Tenaga Nasional Berhad (TNB), Malaysia

for awarding a scholarship to me to further my studies and to embark on a very good research

that will benefit the organisation directly or indirectly.

Finally, my love and thanks go to my wife who has been giving support to me while

completing this research. Also to my children for cheering me up at times when it is really

needed.

ALHAMDULLILLAH.



Chapter 1: Introduction

M.Shamir Ramli, M.Eng Thesis, QUT, 2008 Page 1

1Chapter 1: Introduction

1.1 BACKGROUND

Modern SF6 puffer circuit breakers have been designed with far fewer interrupters per

pole than previous generations of SF6 circuit breakers. This has meant that modern circuit

breakers have to contend with far higher voltage stress in the dielectric recovery region than

previous types. The increased stress has caused dielectric re-ignition of some types of circuit

breakers on capacitive switching duties [1]. In line with this, new standards [2] have been

developed that require a large number of tests and provide a classification of circuit breakers

based on their probability of restriking for capacitive switching.

Switching of shunt reactors is recognised as a duty that causes a very high rate of rise of

transient recovery voltage across the circuit breaker contacts [3]. Restrikes of modern SF6 circuit

breakers have been observed during disconnection of shunt reactors but the high-frequency re-

ignition current is interrupted at an early current zero and often there is no external evidence of

any adverse effects on the circuit breaker interrupters.

With the ongoing development in circuit breakers, re-strike free operation of circuit

breaker is not guaranteed for the stressful capacitive and inductive switching duty. [2] states that

a very low restrike probability as the best possible performance for circuit breakers on capacitor

bank switching duty. [1] and [3] give guidance on the application of AC high voltage circuit

breakers for capacitor bank and shunt reactor bank switching respectively. Precautions are also

taken during the design stage by selecting suitable breaker duty, carrying out system studies and

evaluating methods to reduce overvoltage transients. Controlled switching to reduce switching

transients seems to be the preferred method chosen by utilities.

Despite all these measures, failures of modern circuit breaker during capacitor bank and

shunt reactor bank switching have been observed. There has been increasing evidence that

recent failures of circuit breakers have been due to restriking during reactor switching [4] and

capacitor switching. Generally, restrikes do not cause immediate failures but they gradually

degrade the nozzle over time leading to catastrophic failure. Spencer [5] has suggested that the

high-frequency re-ignition currents during interrupWLRQ�FDXVH�³SDUDVLWLF�DUFLQJ�LQ�WKH�FLUFXLW�

breaker nozzle and that this phenomena leads to gradual deterioration of the nozzle that may





eventually puncture the nozzle material and result in the failure of the interrupter. Failures due to

re-strike/re-ignition are becoming more of a concern as it is difficult to detect re-strike

occurrence. Failures can be catastrophic and they can affect the availability, reliability, safety

and cost of the system which can greatly affect the utilities.

Condition monitoring of circuit breakers is thus important in order to ensure the safe

operation and reliability of circuit breakers. To date, no specific technique has been developed to

detect re-strike. Currently, shut downs are required to physically connect monitoring equipment

to measure switching transients and re-strike. An on-line non-intrusive technique would be an

advantage in monitoring circuit breaker re-strike occurrence during switching of reactive

equipment.

Moore [6] has demonstrated the practicality of measuring time between pole-closing in

circuit breakers during capacitor switching duty from measurement of emitted radio waves. In

this thesis research is conducted to determinH�ZKHWKHU�LW�LV�SRVVLEOH�WR�H[WHQG�0RRUH¶V�

methodology to investigate switching transients produced during capacitor bank and shunt

reactor bank switching. Techniques for monitoring the magnitude and number of re-strikes

occurring during reactor switching using this or similar methods have also been explored..

1.2 RESEARCH CONDUCTED

This research investigates switching transients produced during the switching of three

phase capacitor banks and shunt reactors. The research includes the development of a monitoring

system which possesses most of the features required for the non-invasive, on-line monitoring of

EHV circuit breakers in AIS substations. The monitoring system may be used to carry out on-

line measurements at substations and the measured data are stored and analysed to give valuable

information on the switching transients.

Measurements may be correlated with the actual switching event using recorded

waveforms. Important information such as evidence of restrikes would be looked into from the

data gathered.





1.3 THESIS OUTLINE

This thesis includes a description of the development of the measuring system, results of

measurements made in EHV substations, analysis of results, comparisons with other available

techniques and suggestions for further works to be pursued.

Chapter 2 includes a literature review on topics related to the research. It reviews the

principle of current interruption, capacitor bank and shunt reactor bank switching and failures of

circuit breakers switching shunt reactor and capacitor banks. The importance of preventing

failures is highlighted with the need to develop a suitable condition monitoring method to detect

potential failures.

Chapter 3 describes the proposed new technique for monitoring CB during switching. It

starts with a review of the radiometric method used previously and describes the methodology

used in carrying out the research and development of the new monitoring system. The

measurement principles are described followed by details of the design and construction of the

high voltage transducers. . Chapters 4, 5 and 6 cover HV laboratory measurements, capacitor

bank measurements and shunt reactor bank site measurements respectively. Switching transients

are recorded and discussed. Results are analysed in time domain and important findings are

highlighted.

Chapter 7 deals with analysis of the recorded waveforms. Analysis in frequency domain

is shown to give more information and to correlate with the results from time domain analysis.

Chapter 8 reviews work done in the research highlighting important information obtained from

the measurements, the advantages of the measuring system and further work that can be done to

develop the measuring system.



Chapter 2: Literature Review


2Chapter 2: Literature Review

This chapter contains a literature review of materials pertinent to this research. A basic

review of current interruption in circuit breakers is carried out followed by a review of reactive

switching, including capacitor bank and shunt reactor bank switching. The energisation and

deenergisation phenomenon for capacitor and shunt reactor bank is described. Failures of circuit

breakers due to restriking are considered and the case for detecting restrikes is established. This

is followed by a review of the current available condition monitoring methods and their

suitability for detecting restrikes. Finally, new monitoring methods are proposed.

2.1 REVIEW OF CURRENT INTERRUPTION IN CIRCUIT BREAKERS

The primary purpose of an interrupting device such as circuit breaker is to disconnect

the circuit at the point at which it is placed. When closed, the circuit breaker must carry

continuous rated current. The insulation to ground is stressed by the power frequency voltage

and any transient overvoltages. When open, the dielectric between the contacts is stressed by the

voltage developed across the open contacts.

During the transition period from closed to open and vice versa, a range of dynamic

condition arise. For instance, during a transition from closed to open, the current must be

interrupted to achieve electrical isolation. Interruption of current normally occurs at a current

zero of the sinusoidal waveform and a voltage known as the transient recovery voltage appears

across the open contacts of the circuit breaker. The ability of a circuit breaker to interrupt the

current depends on external circuit parameters, dielectric recovery, contact separation at the time

of current zero, interrupter design and the interrupting conditions e.g. normal load, reactive

switching or fault current. The rate of rise and the peak value of the transient recovery voltage

have a significant impact on circuit breaker performance. Waveforms of a typical circuit

interruption [7] sequence are given in Figure 2.1 for a fault on the load-side terminals of a circuit

breaker.





Figure 2.1 Typical Circuit Interruption (from [7])

The circuit breaker contacts separate at Point A causing an arc to be drawn between the

contacts. This arc has a resistance that creates a small voltage drop, Va. The arc continues until

the current, I, drops to a level too small to maintain it. This occurs as the current passes through

zero, at which point the arc extinguishes and the transient recovery voltage appears across the

circuit breaker contacts. Successful interruption is achieved if the dielectric strength between the

contacts as they separate increases at a greater rate than that of the transient recovery voltage. In

addition, the breakdown strength of the gap between the contacts must exceed the peak value of

the transient recovery voltage. If not, the arc will re-establish and current interruption may occur

at a subsequent zero.

When the current ceases, the voltage between the contacts changes from virtually zero

(the arc voltage) to the instantaneously value of the power frequency voltage. This change

cannot take place instantaneously and a resultant overshoot occurs. The voltage approaches its

steady state value by a transient oscillation with a frequency that is determined by the values of

the circuit inductances and capacitances. The amplitude of the transient recovery voltage may

reach two times the steady state voltage change for the first pole to clear. However, in practice,

its value is usually less due primarily to system damping.

In addition, the instantaneous value of the recovery voltage at the instant of current

interruption is dependent on the power factor of the circuit. The amplitude of the voltage change

that occurs will depend on whether load, charging current or fault current is being interrupted.

Under fault conditions, power systems are primarily inductive. Therefore the power factor of the

circuit as seen from the circuit breaker is effectively zero lagging and the power frequency

8LMWJMKYVIMWRSXEZEMPEFPISRPMRI�4PIEWIGSRWYPXXLILEVHGST]XLIWMWEZEMPEFPIJVSQXLI5980MFVEV]





component of the transient recovery voltage has its maximum value at the instant of current

interruption as shown in Figure 2.1.

The capability of the circuit breaker [8] to successfully interrupt the current will depend

on the phenomenon of current extinction at current zero. After current interruption, the still-hot

gas between the breaker contacts is stressed by a steep rate of rise of the recovery voltage and in

the resulting electric field the present charged particles start to drift and cause a hardly

measurable so-called post arc current. The post arc-current, together with the transient recovery

voltage, results in energy input in the still-hot gas channel. When the energy input is such that

the individual gas molecules dissociate into free electrons and heavier positive ions, the plasma

state is created again and current interruption has failed. This is called a thermal breakdown.

Thermal breakdown normally occur within microsecond in a region known as thermal recovery

phase. When the current interruption is successful, the hot-gas channel cools down and the post

arc current disappears. However, if the dielectric strength of the gap between the breaker

contacts is not sufficient to withstand the transient recovery voltage, a dielectric failure can

occur. Dielectric failure normally occurs within milliseconds in a region known as dielectric

recovery phase.

2.2 REACTIVE EQUIPMENT SWITCHING

Switching of reactive equipment such as capacitor banks and shunt reactors is known to

produce overvoltage transients that may cause insulation breakdown and lead to power system

failure [8]. The reactive equipment is connected to the power system via circuit breakers, this

similar for equipment like overhead lines, transformers and generators. When circuit breakers

operate, parts of the power system are either separated from or connected to each other. This can

be either a closing or opening operation of the circuit breakers.

After a closing operation, transient currents will flow through the system. Closing of a

CB in a predominantly capacitive or inductive network may result in inrush currents. The high-

frequency inrush current can cause problems by: production of severe mechanical stresses on

equipment; production of over voltages due to the system response to the inrush current; and

induction of undesirable transients into neighbouring circuits with low power relay and control

circuits being particularly vulnerable.��After an opening operation, when a power-frequency current is interrupted, a transient recovery voltage or TRV will appear across the terminals of the

interrupting device.��The configuration of the network as seen from the terminals the circuit breaker determines amplitude, frequency, shape of the current and voltage oscillations.





When interrupting a mainly capacitive load (e.g. capacitor bank for voltage regulation)

under normal load conditions, the current and voltage are approximately 90 degrees out of phase

and the current is leading the voltage. When interrupting a mainly inductive load (e.g. large

transformer or shunt reactor) under normal load conditions, current and voltage are also

approximately 90 degrees out of phase with the current lagging the voltage.

In interrupting capacitive or inductive current, if the current is interrupted at current

zero, the interruption is normal and the transient recovery voltages are within the specified

values [8]. However when premature interruption occurs due to current chopping, interruption is

abnormal and this may cause high-frequency re-ignitions and over voltages. If the interrupter

chops high current in a reactor a high-magnitude oscillatory voltage surge may be produced.. If

this process is repeated several times due to high-frequency re-ignitions, voltage doubling may

ensue with rapid escalation of voltage. If these overvoltages exceed the specified dielectric

strength for the circuit breaker, the interrupter and other parts of the circuit breaker may be

damaged.

Re-ignition [8,9] is a phenomenon where a dielectric breakdown of the arc channel

occurs within 5ms after current interruption. It is considered not detrimental to circuit breaker

though no evidence has been found in the literature to substantiate this. Re-ignitions during

recovery voltage are expected to cause 50Hz current to be re-established with minimal

disturbance and the final interruption of current is delayed about 10ms until the next natural

current zero for some types of circuit breakers. Puffer circuit breakers in which opening times are

very critical may be more seriously affected, though there is no published research on this topic.

Re-striking [8,9] is dielectric breakdown of the arc channel after 5ms of interruption,

when the recovery voltage is close to peak. The circuit breaker gap flashes over as the recovery

voltage is greater than the dielectric strength of the gap. Re-strikes can cause high over voltages

and high magnitude HF re-ignition currents that impose sever stresses on the circuit breaker and

adjacent equipment. Numerous re-strikes and interruptions of re-ignition current may will lead to

voltage escalation.

Voltage escalation is a phenomenon where voltage across the circuit breaker is increased

by one or more interruptions of re-ignition current followed by further re-strikes. Generally

interruption at the first or third re-ignition current zero or any odd zero leads to voltage

escalation.





2.3 REVIEW ON CAPACITOR BANK SWITCHING

Shunt capacitor banks are extensively used to improve loading of the transmission lines

as well as to support system voltages. As these capacitor banks are frequently switched in and

out of duty, energisation and de-energisation transients are produced and raise important

concerns. The concerns on energisation are overvoltages and inrush current whilst for de-

energisation is restriking.

2.3.1 Interrupting capacitor bank

Capacitor switching presents circuit breakers with a difficult switching condition. While

interrupting capacitive current, the recovering circuit breaker can be severely stressed during the

time when it is prone to dielectric failure. The problem of dielectric failure arises because the

normal point of current interruption (current zero) occurs when the current leads the voltage by

around 90 degrees. At current zero, a maximum voltage occurs, resulting in a fully charged

capacitance upon disconnection from the source. The voltage due to this trapped charge creates

high stresses during the first half cycle after interruption.

To consider the phenomena associated with capacitor de-energisation, the basic single

phase circuit parameters are given in Figure 2.2.

Figure 2.2 Single Phase Capacitor Bank circuit (from [10])

A capacitor bank can be represented by a lumped capacitance, C, connected to busbar

via circuit breaker. A small capacitance, Cb represents the capacitance of the substation busbar

and other equipment. The impedance of the source is represented in the circuit by R1 and L1.






Figure 2.3 Capacitance Switching (a) System voltage and current. (b) Capacitor voltage (c) Voltage across CB contact. (from [9])

Figure 2.3 shows events occurring before and after a capacitor bank disconnection,

which in this case was performed successfully. At point A, the most favourable condition for arc

interruption is present and arc extinction occurs at the first current zero after contact separation.

Because of the relative phase current and voltage (current leads the voltage by approximately 900

), the capacitor is fully charged to maximum voltage when the switch interrupts. The magnitude

of the trapped voltage is equal to the peak value of the supply voltage, V (as shown in b).

The voltage on the supply side of the circuit breaker continues to vary at the source

power frequency (as in (a)) so that the voltage across the circuit breaker builds up sinusoidally

immediately after current interruption (as in (c)). One half cycle after current interruption, the

voltage across the circuit breaker reaches a value equal to twice the source voltage, which is

potentially dangerous. Thus, for successful interruption to be maintained, the gap between the






contacts must withstand twice the peak value of the source voltage, approximately 10ms after arc

extinction [9].

Figure 2.3 tends to oversimplify conditions to some extent in that when a capacitor is

connected to a system, the leading current that it draws, flowing through the inductance of the

system, causes the capacitor voltage to be somewhat higher than the open-circuit system voltage,

a negative regulation sometimes referred to as thH�³)HUUDQWL�5LVH��:KHQ�WKH�FDSDFLWRU�LV�

disconnected, the potential of the source side of the circuit breaker will return to this lower value,

but will do so by way of an oscillation involving the source inductance and the stray capacitance

adjacent to the breaker on the source side. A more accurate representation of the disconnecting

event is shown in Fig 2.4.

Figure 2.4 Capacitance switching showing the effect of source regulation (from [9])

Here V is the aforementioned negative regulation. It is important to recognise this

phenomenon exists as it can be important when interrupting capacitive current on relatively

weak systems [9]. A relatively weak system condition can be described where a lower voltage

system is being supplied by a higher voltage system through a step down transformer with cable

on the higher voltage side and the lower voltage breaker is called upon to interrupt the charging

current of the cable.

Some circuit breakers, when called upon to interrupt a load of fault current, do not do so

at the first current zero, but instead wait until sufficient gap has been established between their

contacts for their various arc-extinguishing effects to have a better chance of operating

successfully. The current involved in capacitance switching is frequently small, so that in most






cases the circuit breaker is capable of interrupting it at the first current zero. If this should occur

soon after the contacts have parted, a voltage of twice the system voltage will appear across the

contacts while their separation, so there is an increased likelihood of the device reigniting [9]. If

a restrike takes place precisely when the voltage reaches its peak, which in equivalent to

reclosing a perfect switch at that instant. There is in this case a series LC circuit so closing

inrush current would be expected to respond to this sudden disturbance by being a sinusoidal

oscillation with a natural frequency, fo, which is given by:

2/1)(21

2 LCfo (2.1)

where L is the inductance of the supply and C the capacitance of the bank.. A reignition or

restrike can also be viewed as an inadvertent re-energisation with a trapped charge of 1 pu on the

capacitor. The restrike current will be the instantaneous voltage across the switch at restrike

divided by the circuit surge impedance, or

tCLVpir 0

2/1

sin2 (2.2)

Neglecting damping, the voltage will swing as far above the instantaneous system

voltage as it started below [9]. This is indicated in Figure 2.5, which shows the initial 50 Hz

clearing, the trapping of charge on the capacitor, and the subsequent restrike. The transient

voltage excursion to 3Vp is an abnormal overvoltage and is the consequence of the energy stored

in the capacitor bank at the time of the restrike.

It is entirely possible that the circuit breaker will interrupt the restrike current, perhaps at

point A in Figure 2.5. If this happens, the high voltage is left trapped on the capacitor. The

source voltage, on the other hand, would continue on its way, so that after another half cycle

there would be approximately 4Vp across the interrupter. This can be shown by the sequence

drawn in Fig. 2.6 where the Rs represent sequential restrikes and the Cs subsequent clearings [9].

If a second breakdown occurs, a second oscillatory discharge would be initiated. However, since

there is now twice the voltage across the switch, the current would be twice as high, and the

voltage excursion would be from +3Vp to -5Vp (the voltage excursion, neglecting damping, is

always twice the voltage across the switch). It is technically possible for the voltage to escalate

still further by the same mechanism until an external flashover occurs or the capacitor fails [9].





Figure 2.5 Capacitance switching with a restrike at peak voltage. (from [9])

The sequence is idealized and to some extent oversimplified. Restrikes will not always

occur precisely at the voltage peak, so that the voltage, if it escalates, does so more slowly.

Again, the circuit is more complicated. Some capacitance will exist on the source side of the

breaker, which will introduce higher frequency disturbances, as was pointed out in Fig 2.5.

When the switch recovers after point A, the potential at the switch is quite high. But the

source would have it be at its potential. The source side of the switch, therefore, goes through a

high-frequency transient involving an oscillation of the aforementioned capacitance and the

inductance of the source. In fact, at this time, it is possible for a voltage of 4 pu to be developed

across the switch, a point which is often overlooked. A reignition may occur at this time rather

than half a cycle later, which will probably result in the switch conducting current for another

half cycle.






Figure 2.6. Capacitance switching with multiple restrikes. (from [9])

2.3.2 Energising capacitor bank

Energisation of capacitor banks are usually associated with transient voltages and

currents. Transients produced during energising capacitor bank are inrush currents causing a

voltage dip and overvoltages resulting from the system response to the voltage dip. It is common

nowadays to have more than one capacitor bank connected to the same bus. This has no

influence on the conditions at interruption. The inrush current during closing is affected to a high

degree [1]. Two different situations may occur:

The capacitor bank is energised from a bus that does not have other capacitor banks

energised. This situation is called isolated capacitor bank switching.

The capacitor bank is energised from a bus that has other capacitor banks

energised. This situation is called back-to-back capacitor bank switching. It is

worth noting that even energised capacitor banks in nearby substations may

contribute to the inrush current representing a back-to-back situation.






The back-to-back switching normally gives rise to an inrush current of very high

magnitude and frequency which is higher than isolated capacitor bank switching. This inrush

current needs to be limited in order not to be harmful to the circuit breaker, capacitor banks

and/or the network. The magnitude and frequency of the inrush current is a function of the

following [1]:

Applied voltage during closing i.e. point on the voltage wave at closing.

Capacitance of the circuit

Inductance in the circuit (amount and location)

Any charge on the capacitor bank at the instant of closing.

Any damping of the circuit due to closing resistors or other resistances in the

circuit.

It is assumed that the capacitor bank is discharged prior to energization. This assumption

is reasonable, as capacitor units are fitted with discharging resistors that will discharge the

capacitor bank. Typical discharge times are in the order of 5 min.

The transient inrush current to an isolated bank is less than the available short-circuit

current at the capacitor bank terminals. It rarely exceeds 20 times the rated current of the

capacitor bank at a frequency that approaches 1 kHz [1]. Because a circuit breaker must meet the

making current requirements of the system, transient inrush current is not a limiting factor in

isolated capacitor bank applications.

When capacitor banks are switched back-to-back (i.e., when one bank is switched while

another bank is connected to the same bus), transient currents of prospective high magnitude and

with a high natural frequency may flow between the banks on closing of the circuit breaker. The

effects are similar to that of a restrike on opening. This oscillatory current is limited only by the

impedance of the capacitor bank and the circuit between the energized bank or banks and the

switched bank. This transient current usually decays to zero in a fraction of a cycle of the system

frequency. In the case of back-to-back switching, the component supplied by the source is at a

lower frequency; therefore, small it may be neglected.

2.4 REVIEW OF REACTOR BANK SWITCHING

Shunt reactors are mainly used in transmission networks. Their function is to consume

the excess reactive power generated by overhead lines under low-load conditions, thus stabilize

the system voltage. They are switched in and out almost on a daily basis, following the load





situation in the system. Shunt reactors are normally connected to substation busbars, but also

quite often directly to overhead lines. They may also be connected to tertiary windings of power

transformers. The reactors may have grounded, ungrounded, or reactor grounded neutral.

2.4.1 Interrupting shunt reactor bank

Shunt reactor switching imposes a unique and severe duty on the connected system and

circuit breaker [3,11]. At high voltages, the current to be interrupted is generally less than 300A,

yet successful interruption is a complex interaction between the circuit breaker and the circuit.

Shunt reactor load currents are referred to generically as small inductive currents. The

capability of circuit breakers to interrupt small inductive currents is generally not a concern. The

circuit breaker will typically interrupt the current at the first current zero after contact parting, but

may not be immediately capable of withstanding the high magnitude recovery voltages that can

then appear across the contacts. This can result in a reignition followed by an additional loop of

rated frequency current and successful interruption.

The switching of directly grounded reactors can be analysed using the equivalent single

phase circuit shown in Figure 2.7. Basically, circuit breakers have no difficulty interrupting

shunt reactor current; in fact, the current is forced prematurely to zero, a phenomenon referred to

as current chopping. However, the chopping of the current and subsequent possible reignitions

can result in significant transient overvoltages.

The following two types of overvoltages are generated:

Chopping overvoltages with frequencies up to 5 kHz

Reignition overvoltages with frequencies up to several hundred kilohertz (kHz)

The switching process may be significantly influenced by two other circuit-breaker

characteristics:

Rise of the dielectric withstand of the contact gap after interruption which

influences the probability of re-ignitions occurring;

Capability to interrupt high-frequency currents after re-ignitions which influences

the risk of multiple re-ignitions and voltage escalation.





Ls = supply side (short-circuit) inductance

Cs = supply side capacitance

CB = circuit breaker

Lp,Cp = stray inductance and capacitance across circuit-breaker CB

Also known as first parallel circuit inductance and capacitance

Lb = inductance of re-ignition circuit

Also known as connection series inductance

CL = capacitance parallel to the reactor (load side capacitance)

L = inductance of shunt reactor

Figure 2.7. Single phase equivalent circuit (from [11])

2.4.2 Current chopping

Current chopping is caused by arc instability, which exhibits itself in the form of a

negatively damped current oscillation superimposed on the load current [3,11]. The oscillation

amplitude increases rapidly, creating a current zero at which the circuit breaker usually interrupts

as shown in Figure 2.8. The frequency of the oscillation determined by Cs, CL and Lb (Figure

2.7) and is usually several hundred kHz and therefore current chopping can reasonably be

assumed to be instantaneous for purposes of calculating load transients.

The chopping level is determined by:






WKH�FKDUDFWHULVWLF�³FKRSSLQJ�QXPEHU�RI�one interrupting unit of the switching

device;

the effective parallel capacitance;

the number of breaks in series.

a) Current through circuit breaker b) Voltage across shunt reactor

Figure 2.8 Current chopping phenomena (from [3])

For a single interrupter circuit breaker, the chopping current level is given by the

equation

tch Ci (2.3)

where

ich = current level at the instant of chopping (A)

Ct = total capacitance in parallel with the breaker (F)

chopping number for a single interrupter (AF 0.5)






The chopping number, , is a characteristic of the circuit breaker and typically given by

the manufacturer of the circuit breaker. [3] gives a typical range of chopping number for SF6

circuit breaker of 4-17 x 104.

Referring to Figure 2.7, Ct is given by the following equation

Ls

Lspt CC

CCCC (2.4)

Where

CP = circuit breaker parallel capacitance (F)

Cs = source side capacitance to ground (F)

CL = effective load side capacitance to ground (F).

CL is summation of the load side equipment capacitances to ground and the phase-to-

phase capacitance of the shunt reactor and associated connections. For many applications, the

latter may not be significant compared to former and can be ignored.

The maximum value of Ct and the worst-case condition for overvoltage generation

occurs when Cs>> CL, in which case Ct is given by

Lpt CCC (2.5)

Equation (2.3) applies as noted only to circuit breakers with a single interrupter. For

FLUFXLW�EUHDNHUV�ZLWK�³1�LQWHUUXSWLQJ�XQLWV�SHU�SROH��WKH�IROORZLQJ�HTXDWLRQ�DSSOLHV��

tch NCI (2.6)

The level of current chopping may be dependent on arcing time. This tends to be the

case for SF6 puffer type circuit breakers. Current chopping phenomena are discussed in detail in

[12,13].

Current Chopping Overvoltages

Fig 2.9 shows chopping phenomena in a single-phase circuit. When a premature current

interruption occurs at 6, the interruption is abnormal and causes an overvoltage. The energy

trapped in the load inductance and capacitance at the instant of chopping will oscillate between

this inductance and capacitance. The frequency of the oscillation is of the order of 1 kHz to 5

kHz in the HV and EHV range. It is determined by the natural frequency of the reactor load





circuit, i.e. the reactor itself and all equipment connected between the circuit-breaker and the

reactor.

Figure 2.9 Chopping Phenomena in single phase (from [11])






The first peak of the oscillation has the same polarity as the system voltage at the time of

interruption. This overvoltage is referred to as the suppression peak overvoltage. The maximum

chopping overvoltage to earth is usually the suppression peak voltage for directly earthed

reactors. Due to energy transfer between phases, the load side oscillation may in some cases

exhibit slightly higher peak values after one or two cycles of the oscillation. The highest

overvoltage to earth appears at the recovery peak for the unearthed and neutral reactor earthed

cases [11].

The magnitude of the suppression peak overvoltage, ka is given by the expression :

Lo

cha C

Lui

k *1 (2.7)

where

ich = chopped current

uo = peak system voltage to earth

L = reactor inductance

CL = load side capacitance.

For a given application (fixed uo, L, and CL), when Cs>>CL and Cp is negligible, the

overvoltage is dependent on ich only. Equation 2.4.2.5 can then be rewritten as

QNka 231

2

(2.8)

where

Q = three-phase reactor rating (V· A)

= the chopping number (AF-0.5) for a single interrupter

= 2f = angular rated power system frequency

N = number of interrupting units in series per pole

The chopping overvoltage is thus only dependent on the chopping number and the

reactive power of the reactor [3,11].

2.4.3 Reignition

The circuit-breaker, after current interruption, is stressed by the difference between the

supply side voltage and the slowly decaying load side oscillating voltage. Circuit breakers with

very high chopping levels may exhibit reignitions before or at the suppression peak. Reignitions

if they occur have mainly the effect of reducing the chopping overvoltages. Most circuit-





breakers, such as SF6 puffer type, which have low chopping levels and seldom reignite during

the suppression voltage loop.

At the recovery voltage peak the circuit-breaker is stressed by a voltage that may

approach the chopping overvoltage plus the peak of the supply side voltage. If the circuit-breaker

does not re-ignite before, or at this point, then the interruption is successful. If, however, the

instant of contact parting is such that the contact gap does not yet have sufficient dielectric

strength, then a re-ignition will occur as shown in Figure 2.10.

Figure 2.10 Reignition Windows (from[3])

Reference [3] states that all circuit-breakers will re-ignite when the interruption occurs

ZLWK�D�VPDOO�FRQWDFW�JDS��7KH�UH�LJQLWLRQ�³ZLQGRZ�PD\�EH�QDUURZ�RU�ZLGH�GHSHQGLQJ�RQ�WKH�

rate of rise of withstand capability of the increasing contact gap as illustrated in Figure 2.10. The

width depends on the design of the circuit-breaker i.e. interrupting medium, contact velocity,

electrode design, etc. Re-ignition-free interruption can practically be achieved by applying

auxiliary equipment to circuit breaker to limit overvoltages such as opening resistors, metal

oxide surge arresters and synchronous opening control devices (control switching). The latter






device opens the contacts at sufficient time before the chopping to ensure that the dielectric

strength of the gap is always greater than the chopping overvoltage.

Re-ignitions occur only for relatively short arcing times in circuit-breakers with fast

dielectric recovery, and occur therefore generally only on the first phase of attempted

interruption. A further loop of power frequency current usually follows the re-ignition as in

Figure 2.9.

Reignition Overvoltages

Figure 2.11 [3] illustrates a case where a reignition occurs, the load side voltage rapidly

tends toward the source side voltage, but overshoots producing a reignition overvoltage. The

voltage breakdown at a reignition creates a steep voltage transient that is imposed on the reactor.

The front time varies from less than one microsecond to several microseconds. Since the voltage

breakdown in the circuit breaker is practically instantaneous, the steepness is solely determined

by the frequency of the second parallel oscillation circuit, which in turn is dependent on the

system/station layout [3]. This steep transient may be unevenly distributed across the reactor

winding, stressing the entrance turns in particular with high interturn overvoltages.





Figure 2.11 Reignition at recovery voltage peak for a circuit with low supply side capacitance (from [3]).

Figure 2.12 shows the maximum attainable overvoltages without damping for a

reignition at the recovery voltage peak. It can be seen that interruption with high current

chopping produces higher overvoltages than interruption with negligible current chopping. The

high theoretical overshoot assumes that the supply side capacitance dominates over the load side

capacitance (Cs>>CL).






Figure 2.12 Maximum re-ignition overvoltages (from [3])

2.4.4 Oscillation modes

Four different oscillation modes occur during the interruption and reignition process for

a directly-ground reactor. Those oscillations are described below with reference made to Figure

2.13 which clearly shows the oscillations involved.

Load side oscillation

A successful interruption results in the slowly decaying load side oscillation with the

trapped energy oscillating between the inductance and capacitance of the load side circuit. The

frequency of the oscillation is given by

LL LCf21

(2.9)

and is in the range 1 to 5 kHz. This oscillation may be modulated due to phase

interaction as described later.






Figure 2.13 Oscillation Mode in the reactor circuit

Reignition oscillation

Reignition phenomena are described in detail in [13] but is described briefly to give

appreciation on oscillations involved during reignition. Three different oscillation circuits are

LQYROYHG�LQ�UHLJQLWLRQV��$�³ILUVW�SDUDOOHO�RVFLOODWLRQ�RFFXUV�ZKHQ�CP discharges through the

circuit breaker; the frequency of this oscillation is

PPp CLf

21

1 (2.10)

and is in the order of 1 to 10 MHz. The circuit breaker will not interrupt the current

DVVRFLDWHG�ZLWK�WKH�³ILUVW�SDUDOOHO�RVFLOODWLRQ��$�³VHFRQG�SDUDOOHO�RVFLOODWLRQ��UHLJQLWLRQ�

overvoltage oscillation) will follow, as a result of which, the voltages across Cs, and CL are

equalized, i.e., the voltage across the circuit breaker is reduced to zero for an instant. The

IUHTXHQF\�RI�WKH�³VHFRQG�SDUDOOHO oscillation is given by

sLb

sLp CCL

CCf

21

2 (2.11)

and is in the range 50 to 1000 kHz. The circuit breaker may interrupt the current

DVVRFLDWHG�ZLWK�WKH�³VHFRQG�SDUDOOHO�RVFLOODWLRQ��,I�LW�GRHV�QRW��WKHQ�D�³PDLQ�FLUFXLW�RVFLOODWLRQ�

develops. This oscillation involves the total circuit and generally leads to a new loop of current.

Main Circuit Oscillation

Second Parallel Oscillation

Load Circuit Oscillation

First Parallel Oscillation





Neglecting Lb (since it is small compared to Ls and L��WKH�IUHTXHQF\�RI�WKH�³PDLQ�FLUFXLW�

oscillation is given by

)(21

Lss

sm CCLL

LLf (2.12)

and is in the range 5 to 20 kHz. It should be noted that Equation 2.12 describes the main

circuit oscillation in its simplest possible form. In reality, the oscillation is a composite of a

number of oscillation modes dependent on the reactor installation and the local and remote

configuration of the system. Note that if Cs>>CL, no main circuit oscillation will occur.

Reignitions involve an energy exchange. Prior to the reignition, the load side energy

alternates between electromagnetic and electrostatic energy (load side oscillation). During the

³ILUVW�SDUDOOHO�RVFLOODWLRQ��WKH�HOHFWURVWDWLF�HQHUJ\�VWRUHG�LQ�Cp is dissipated with no exchange

between the source and load sides. During the ³VHFRQG�SDUDOOHO��RVFLOODWLRQ�HOHFWURVWDWLF�HQHUJ\�

is exchanged between CS and CL��'XULQJ�WKH�³PDLQ�FLUFXLW�RVFLOODWLRQ��DOO�FLUFXLW�HOHPHQWV�DUH�

involved and the energy exchange is both electromagnetic and electrostatic. This can be seen in

Figure 2.13.

2.4.5 Interaction between phases

The interaction between phases during the switching of shunt reactors is a complex

subject [3]. This is particularly the case for medium voltage transformer tertiary connected

reactors where the interaction can influence the interruption process. For the case of directly and

neutral reactor grounded reactors in the range 60 kV and above, the interaction influences only

the load side recovery voltage oscillation.

Due to mutual coupling between the individual phases, the interaction manifests itself in

the form of beating of the recovery voltage oscillation [3]. The degree of beating is dependent on

the length of the connection between the circuit breaker and the reactor and on the type of

reactor. When the circuit breaker is located close to the reactor, the interaction is minimal or

nonexistent and the recovery voltage oscillation is a clean exponentially decaying sinusoidal

function as shown in Figure 2.14. For longer connection lengths between the circuit breaker and

the reactor, beating of the recovery oscillation will occur as shown in Figure 2.15. The effect is

non-deterministic and varies even with fixed contact parting times. With regard to the reactors, if

the units are single-phase, no interaction occurs due to the reactors. If, however, the units are





three-phase (in one tank with a common core), the phase-to-phase coupling is significant and

results in beating such that the maximum recovery voltage peak can occur late in the oscillation.

[3] mentions that the interaction between phases is not a concern because the interaction

does not influence the recovery voltage in the region between current interruption and the

occurrence of the recovery voltage peak. If the circuit breaker successfully withstands the

recovery voltage peak, then no reignition will occur later even if subsequent peaks exceed the

chopping overvoltage peak value due to beating. The probability of high overvoltages occurring

due to superposition of transients from adjacent phases is considered to be remote.

Figure 2.14 Load side oscillation with circuit breaker located close to shunt reactor (from[3])

Figure 2.15 Load side oscillation with circuit breaker located remote from shunt reactor (from [3])







2.4.6 Energising transients

Energizing (switching-in) a shunt reactor is a situation similar to the occurrence of a

reignition. However, the breakdown voltage across the circuit breaker will not exceed 1 pu for

directly grounded reactors, and the peak value of the energizing transient will be 1.5 pu or less.

Reactor surge arrester operation may occur if the switching device has a slow closing

speed resulting in multiple restriking and possible voltage escalation.

2.5 LIMITATION OF OVERVOLTAGE TRANSIENT DURING REACTIVE SWITCHING

Switching of capacitor bank and shunt reactor produces overvoltage that can be harmful

to the system. The transient overvoltages may cause any of the following:-

Insulation degradation and possible failure of substation equipment

Operation of surge arrestors

Interference in the control wiring of substations

Increase in step potentials

Undesired tripping or damage to sensitive electronic equipment.

2.5.1 Over voltage limitation

Reference [1,3,14] state that several means are available to reduce the overvoltages

generated by the switching of capacitor bank. and shunt reactor.

Current-limiting reactors are normally used to reduce the current transients

associated with back-to-back switching of capacitor banks. They do not limit the

remote overvoltages.

Pre-insertion resistors limit the inrush current and remote overvoltages. It is a

basic solution widely used on transmission circuit breakers. They are usually fitted

on circuit breakers and as such add to the complexity of the equipment. Depending

on the design, the added complexity may or may not result in a reduced availability

of the equipment.

Surge Arresters are the primary means of protection against fast transients and are

usually installed very close to the protected equipment in this case capacitor bank

and shunt reactor.

Controlled switching of circuit breaker meaning the opening and/or closing of

the circuit contacts at certain points on the waveform (such as at current or voltage

zeros, which is why it is often referred to as point-on-wave control) has long been

recognised as a way of reducing stress on circuit breaker contacts and system

components during switching. A great deal of engineering is required for good





precision of this kind of control. Controlled switching seems to be the chosen

method by utilities.

2.5.2 Controlled switching

Controlled switching is a method for eliminating harmful transients via time controlled

switching operations. Closing or opening commands to the circuit breaker are delayed in such a

way that making or contact separation will occur at the optimum time instant related to the

voltage, current and phase angle. By means of controllers, both energizing and de-energizing

operations can be controlled with regard to the point-on-wave position, and no harmful transients

will be generated.

Strategies are identified for energizing all types of shunt capacitor banks and harmonic

filter banks. The strategies involve energizing the load close to voltage zero across the circuit

breaker contacts thereby avoiding energizing transients. The strategy assumes that the banks are

discharged prior to energizing. For controlled opening, the strategy is to avoid short arcing times

resulting in the highest risk for reignitions or restrikes. The need for controlled opening will

depend on circuit breaker performance, load conditions and system frequency. All types of shunt

reactors, independent of magnetic and electric circuit, can be switched in a controlled manner.

The strategy for controlled opening is to select arcing times long enough to avoid re-ignitions at

de-energizing. The strategy may vary depending on the size of the shunt reactor. The strategy for

controlled closing is to energize at instants resulting in flux symmetry (current symmetry)

thereby minimizing the risk for nuisance tripping and rotor vibrations in nearby generators due to

zero sequence current.

Reference [15] suggested that controlled switching application requires CB with stable

operating times and have high and stable dynamic electric withstand capability between contacts,

both upon making and breaking conditions. In circuit breakers with independent mechanisms for

each pole the opening and closing operations can both be controlled. However if poles are

ganged and operated by a single mechanism, there are difficulties with control switching and

generally it is only possible to control either the opening or closing operation. It is mentioned in

[15] that it is difficult to quantify the effects of the control switching on reducing the probability

of restrikes because of the wide range of variability possible in the dielectric recovery

characteristics of circuit breakers.





2.6 FAILURE OF CIRCUIT BREAKER DUE TO RESTRIKING

Modern SF6 puffer circuit breakers have been designed with far fewer interrupters per

pole. This has meant that these circuit breakers have to contend with a far greater voltage stress

in the dielectric recovery region than have previous generations of circuit breakers. The

increased stress has caused dielectric reignition of some types of circuit breakers on capacitive

switching duties [1, 16]. Because of this new standards [2] have been developed that require a

large number of tests and provide a classification of circuit breakers based on their probability of

restriking for capacitive switching.

Older CBs were designed according to the old standard which is now recognized as

being inadequate for determining restriking tendecies of circuit breaker switching

capacitor/reactor banks. The old standards IEC56-1987 on which AS2006-1986 was based

made the assumption that circuit breakers passing capacitor switching tests specified by the

VWDQGDUGV�ZHUH�³UHVWULNH�IUHH��7KH�LGHD�RI��³UHVWULNH�IUHH�FLUFXLW�EUHDNHUV�KDV�EHHQ�DEDQGRQHG�LQ�

the latest IEC62271-100. In the new standards, circuit breakers are defined as either C1 (low

probability of restrike during capacitive current breaking) or C2 (very low probability of restrike

during capacitive current breaking) types based on capacitive switching tests made on circuit

breaker.

Switching of shunt reactors is recognised as a duty that causes a very high rate rise of

transient recovery voltage across the circuit breaker contacts [3]. Restrikes have been observed to

occur during disconnection of shunt reactors but the high-frequency reignition current is

interrupted at an early current zero and often there is no external evidence of any adverse effects

on the circuit breaker interrupters. However, failures of modern circuit breakers during shunt

reactor switching have been observed [4]. There has been increasing evidence that recent

failures of circuit breaker have been due to restriking during reactor switching and capacitor

switching.

Blundell [17] has raised issues on possibility of there being some type of generic failure

mode of modern SF6 circuit breakers that leads to flashover of the circuit breaker interrupter.

This hypothesis is based on observations of failures of circuit breakers made by [17]. Explosive

failures of interrupters of circuit breakers were reported on opening of capacitor bank switching

duty and opening of circuit breakers following shunt reactor switching. It was also suggested by

[17] that similar failures of SF6 circuit breakers have been experienced by other utilities in

Australia and SE Asia.





Sawir et al [18] reported that Tenaga Nasional Berhad (Malaysia) experienced incorrect

remote feeder tripping due to inaccurate operation of point-on-wave (POW) switching during

energisation of capacitor bank. [18] stated that although point-on-wave switching of capacitor

banks is an effective method in controlling system voltages, inaccurate operation of the POW

during switching in of the capacitor banks resulted in high voltage transients and caused

nuisance trippings. [18] suggested that the point-on-wave switching operation to be monitored

periodically.

Catastrophic failures of modern SF6 circuit breakers have been reported [4] when

disconnecting 420 kV shunt reactors. Detailed investigations were carried out and field

experience with HV SF6 circuit breakers switching reactors have been documented. [4] have

made few observations on the circuit breaker interrupters used for shunt reactor during the

investigation. Table 2.2 shows observations maGH�RQ�FLUFXLW�EUHDNHUV¶�LQWHUUXSWHU�VZLWFKLQJ�

shunt reactor during the investigation.

[4] described the external arcs as existences of signs of arcing between the main contacts

on the exterior of the nozzles, the perforations of the nozzles with a diameter of around 1mm in

the proximity of the moving contact with the farthest end of the moving contact and removal of

material on the internal face of the nozzle. It is further described the commutation arcs as

existences of signs of this arc between the main contacts and dirtiness and/or burr due to grease

and metal particles on the internal face of the breaker porcelain housing.

Table 2.1 Results of the overhaul of the circuit breakers (from [4]).

Peelo et al [19] reported that British Columbia Hydro and Power Authority experienced

a number of failures when switching out 500kV 3 x 45 MVAR shunt reactor banks. Series of

8LMWXEFPIMWRSXEZEMPEFPISRPMRI�4PIEWIGSRWYPXXLILEVHGST]XLIWMWEZEMPEFPIJVSQXLI5980MFVEV]





reactor switching tests were performed over a three year period, to determine the causes of the

failures and to acquire knowledge of the switching duty in order to ensure adequate specification

of breakers for applications. Results of those field tests have provided valuable and important

information on interruption of small inductive currents. [19] concluded that high voltage shunt

reactor switching is a severe and unique duty, surge arresters play a significant role in reactor

switching application and reignitions during reactor switching can result in significant transients

in control circuits.

Khodabakchian et al [20] studied 420 kV circuit breaker failures during the opening of a

100MVAR shunt reactor in a 400kV one-and-a-half breaker transmission substation in central

part of Iran. The study shows that opposite-polarity high frequency arc-instability-dependant

oscillations caused mainly by current transformers on each side of the circuit breaker were

responsible for its thermal failures and thus the non-interruption of the low 50Hz reactor current

by the 50 kA circuit breaker. [20] mentioned the advantage in using simulation capabilities of

EMTP-RV to simulate large transients incorporating circuit parameters frequency dependency

and dynamic arc modelling which could contribute to improved reactor installation.

Lopez-Roldan et al [21] reported that Powerlink Queensland in recent years has

experienced several failures of modern SF6 circuit breakers used in Shunt Reactor switching

operations in the 275 kV network. An example of a failure was the breakdown of the CB

switching a line reactor with a neutral earthing reactor (NER) at the 275 kV substation. The CB

had been in service for over four years and had been operated almost daily. During a routine

opening operation, the dead-tank circuit breaker failed to clear on phase A and subsequently

faulted internally to ground. During the fault investigation and breaker disassembly, clear marks

of severe arcing puncture in the nozzle of the interrupter were found. The nozzle damage has

occurred prior to failure most likely due to re-striking during opening operations. Evidence of

severe nozzle puncture was also found in phase C.

The hypothesis [21] of the interrupter failure is that during the final opening, a re-strike

punctured right through the nozzle between the moving main contact and the fixed arcing

contact of the interrupter. The current within the nozzle was extinguished but ionized gases

forced though the puncture by the action of the puffer allowed power frequency current to restart

between the main contacts outside the nozzle, out of the effective area of arc interruption.

Birtwhistle [22] hypothesised that restriking of circuit breakers to be responsible for

failures of interrupters of EHV and HV circuit breakers during the switching of capacitor banks.

[22] also mentioned that risk of failure due to restrike are likely to be higher for circuit breakers





not tested to revised switching standard which include significantly upgrade capacitive

switching test [2].

Causes of restrikes

Reference [21] looked into the cause of failure of shunt reactor and circuit breaker. It

started with a review of the TRV study experienced by the CB. The EMTP study showed the

connection of the neutral reactor to the ground increases the TRV across the CB and removing

the neutral reactor reduces the TRV. It was further reported that installing surge arresters would

also reduce the TRV across the CB.

[21] also reported that the dielectric design of the interrupter chamber plays a

fundamental role in the breaker performance as most of CB failures during reactor de-

energization are of a dielectric nature. [21] described after arc interruption, a reignition occurs

because the TRV exceeds the dielectric strength of the contact gap. It is a race between the rise

of the TRV and the Rate of Rise of Dielectric Strength (RRDS) of the interrupter. [21] stated that

it is important that, in the event of a reignition, the arc remains confined between the arcing

contacts inside the nozzle area as shown in Figure 2.16 . Then the reignition will be extinguished

even though it may contribute to damage to the nozzle. However, if the arc escapes outside the

nozzle containment, either by establishing itself between the main contacts or by puncturing the

wall of the nozzle, it is likely that the interrupter will not be able to clear the arc. Therefore, it is

necessary that the breakdown voltage of the main contacts exceeds the level between the arcing

contacts for all separations of the contact gap up to the maximum arcing gap.

Figure 2.16 (a) Typical schematic of SF6 CB showing main contacts (1), arcing contacts (2) and nozzle (3). (b) Voltage distribution in interrupter chamber. [21]






Figure 2.17 shows an example of an analysis of a contact gap. In this figure, the

breakdown voltage between the arcing contacts exceeds that of the main contacts beyond a

certain point along the contact travel path. If the arcing gap exceeds this point, and the TRV

exceeds the breakdown voltage value, then a reignition outside of the nozzle will occur.

[21] also reported that the presence of surface charge on the nozzle is another important

factor to consider in the dielectric coordination analysis. Static charge on the nozzle will distort

the voltage distribution across the contacts as shown in Figure 2.16 (b) and enhance locally the

electric field. The nozzle is made of a PTFE insulating material. Due to the high volumetric and

surface resistance of the insulating material, any surface charge that occurs may remain in place

for a very long time. The charge can be as a result of corona discharges, contact or mechanical

friction [5].

Figure 2.17 Analysis of voltage breakdown for main and arcing contacts along the contact gap. (from [21])

In the interrupter diagram of Figure 2.16, the charge on the nozzle produced by the

friction with the fixed arcing contact may produce a critical local electric field enhancement

where the charge is of opposite polarity to the contact voltage. The field distortion also produces

a change in the discharge direction from the arc contacts across the nozzle. [21]

[21] highlighted the importance of the correct selection of the controlled switching

settings. Correct switching settings will ensure proper operation of the relay otherwise the

function of controlled switching could have negative effects. [21] reported that a controlled

switching function was employed for the reactor breaker which has failed. The initial settings

given by the circuit breaker manufacturer did not achieve the minimum arcing time required

during deenergisation. As a result, the relay was forcing the interrupter to clear the current at a

position with low dielectric strength for the contact path along the nozzle. Repetitive reignitions






in the same area caused compounding damage in the nozzle and the final puncturing which lead

to the catastrophic failure. After the fault investigation and discussion with the manufacturer

[21], a decision was taken to increase the arcing time by modifying the setting in the controlled

switching relay.

There have also been reports on failure due to degradation of the circuit breaker

dielectric performance which was caused by restriking with parasitic arcing during reactor

switching. In the past the main adverse effects of restriking of circuit breakers during switch off

of shunt reactors was seen as being to cause steep-fronted surges that led to deterioration of the

insulation of the reactors and of other adjacent plant such as transformers. However, work by Lui

et al [23,24] to identify causes of circuit breaker failures under reactor switching applications has

shown that modern single-interrupter SF6 circuit breakers may be affected by phenomenon

associated with arcing on reignition. The phenomenD�KDV�EHHQ�WHUPHG�>��@�³SDUDVLWLF�DUFLQJ�

which, it has been shown [5], results in arcing in the circuit breaker interrupter at frequencies of

up to several MHz both inside and outside the interrupter. The tendency of arcs to occur outside

the nozzle has been shown to be affected by the ability of the PTFE nozzle material to absorb

and store small quantities of negative charge [23]. The stored charge distorts the field around the

nozzle and causes re-ignition arcs to occur external to the nozzle. The stored charge was shown

[23] to have greater effects on parallel-sided nozzles than on nozzles with divergent geometries.

Spencer [5] suggested that charge trapped in PTFE nozzles could be stored for up to 2

\HDUV��+H�VSHFXODWHG�WKDW��³7KH�PRELOLW\�RI�FKarge in PTFE is so low that, in the presence of a

potential gradient, it may be expected to migrate only slowly. Thus with the electric field

distributions in an interrupter unit where the downstream contact is positive, negative charge

migration may occur slowly to the rim of the nozzle. This combined with charge accumulation

over successive 50Hz voltage half cycles provides a possible explanation for the late breakdowns

which have been reported in SF6 interrupter units at long times after current interruption and arc

H[WLQFWLRQ��

It was observed [5] that parasitic arcs take paths close to the surface of the nozzle and it

has been suggested that if power frequency current follows the same path as reignition currents

then damage will occur to the interrupter. A series of tests was undertaken [25] on a reduced-

scale interrupter to examine the effect of parasitic arcs on nozzle. Some results from these tests

are summarised in Table 2.3 below.





Table 2.2 Results of tests made to examine effects of parasitic arcing.(from [25])

On the basis of these tests, it was suggested [25] that molybdenum disulphide fill is

preferable as it may prevent non-uniform charge accumulation on the nozzle that is caused by

impurities in the PTFE. It was also shown that there appeared to be no correlation between

nozzle charge and the degree of parasitic arcing.

Judging from the failures described earlier it can be said that restrikes in circuit breaker

can occur due to circuit breaker tested to inadequate or old standard, inaccurate system study,

failure of voltage limiting equipment e.g. surge arrestor, resistor, incorrect operation of

controlled switching, circuit breaker wear and tear (due to daily operation) and high frequency

reignition. Generally, restrikes do not cause immediate failure but gradually degrade the nozzle

overtime increasing the probability of causing catastrophic failure.

2.6.1 Importance of detecting restrike

Bachiller et al [26] conducted a series of field tests to examine the performance of

modern SF6 circuit breakers on shunt reactor switching. From switching tests made on 110

MVAR, 420 kV shunt reactor the circuit breaker chopping number was evaluated as 15.104 to

16.104 and reignitions were observed in 25% of 68 three-phase switching operations.

Reignitions occurred for arcing times of up to almost 8 ms and the maximum switching

overvoltage with reignition was about 1.25 p.u. Tests on a 150MVAR, 420 kV shunt reactor

resulted in six reignitions for 26 switching operations. The interrupter used to switch the






110MVAR reactor was examined after the tests and found to be in good condition. Re-ignition

current magnitudes are calculated to be about 200A and were found to be considerably greater

than measured values, indicating a higher degree of damping in the system than assumed in the

model.

These tests confirmed the extent of re-ignition activity in a modern SF6 circuit breaker.

There is no indication of the type of filler used in the circuit breaker nozzles. It is not possible

therefore to draw wider conclusions from this work as to the degree of damage that might be

expected in other circuit breakers for similar duties. Failures due to restrike/reignition are

becoming more of a concern as it is difficult to detect restrike occurrence.

As mentioned earlier restrike could cause catastrophic damage to circuit breakers,

causing equipment downtime, damage to adjacent equipment and possible loss of supply. When

a circuit breaker is damaged, the equipment bay needs to be taken out of service. With the bay

being taken out, the power system is put at a higher risk as the power system could not be

operated as normal or intended. Normally when a flashover occurs it will cause catastrophic

damage to the circuit breaker, the damage is not limited to the circuit breaker only. Damage to

the adjacent equipment is possible due to the close vicinity of the circuit breaker with other

equipment. It was reported in [27] that a CB explosion shattered the CB porcelain insulator and

pieces of the porcelain insulator hit and damaged the adjacent equipment.

In a worst case scenario, the damage may cause the whole the substation to be shutdown

causing loss of supply to the customer. In addition the work and cost involved in repairing or

replacing the damaged circuit breaker can be very costly.

Restrike can cause interrupter failure which is an important failure mode compared to

the other type of CB failure. Reference [28] provides an excellent insight into the failure statistics

of the component of a high voltage circuit breaker. Table 2.3 shows statistics cause of failure of

circuit breaker.





Table 2.3 Statistics cause of failure of circuit breaker (from [28])

The table shows that most of the circuit breaker failures that have been observed in the

field can be attributed to mechanical problems and auxiliary control circuits. [28] indicates that

43-44% of the failures in circuit breakers are of a mechanical nature, 20-29% is related to

auxiliary and electrical control circuit, and 21-31% can be attributed to high-voltage components

involving the interrupters of circuit breakers. The statistics can be used as a guideline for the

selection of monitoring parameter and what components to be monitored.

The author agrees with statistic of failure shown in Table 2.4 through his experience as

HV equipment maintenance engineer. The high percentage of mechanical failure is expected as

the mechanical components are the moving parts. It is felt that mechanical failure on the circuit

breaker e.g. compressor faulty leaking energy storage could be detected during routine visual

inspection or even during scheduled maintenance. Hence it can be repaired before a catastrophic

failure occurs. This is also the case for failures caused by electrical control and auxiliary circuits.

However, in the case of interrupter failure, it is difficult to detect a problem before a

failure or explosion occurs. When this occurs, it is too late because the damage is severe and the

cost to repair the circuit breaker is high. Hence it would be a good strategy to monitor parameters

which can cause interrupter failure. Considering that switching reactive equipment consider to be

onerous duty for circuit breaker it is prudent to monitor restriking in circuit breakers switching

capacitor bank and shunt reactor bank.






2.7 CONDITION MONITORING FOR CIRCUIT BREAKERS

The importance of monitoring the condition of switchgear has been recognised for a

long time. The majority of the condition monitoring is done off line with the data obtained from

switchgear diagnostic being used to supplement maintenance reports and operational history. All

these information would then be used to understand and analyse the performance and condition

of the switchgear.

Nowadays utilities are looking more on on-line monitoring. There has been a trend away

from time-based maintenance strategies with their predefined maintenance intervals and toward

condition-based maintenance (CBM) strategies, which assess the technical condition of the

equipment. Many utilities are struggling to manage their assets and with the move toward

condition-based rather than time-based maintenance, the issue of CB residual life is actively

being pursued [29]

Prior to on-line monitoring, diagnostic of switchgear are done by measuring the timing

parameter, contact resistance between contacts, quality of insulation and visual inspections.

Throughout the years, many kinds of circuit breaker condition monitoring system have been

developed. Mechanical vibration signals have been used to detect mechanical fault [30,31,32].

Mechanical sensors such as pole position sensors with optical detectors have also been used to

detect the distance between contactor and the closing speed and opening speed [33]of the circuit

breaker contacts An electronic densimeter based on the state equation of SF6 gas has been

developed and proven as a reliable formula in its gaseous and liquid phase [34] Acoustic signals

have also been used to assess the conditrion of the circuit breakers [35,36]. The acoustic signal

generated during non-energised switching of circuit breaker was used for diagnostic [35]. The

acoustic signals recorded was analysed using digital signal processing.

Some manufacturers have produced condition monitoring equipment that monitors

several parameters which they deem to be important in evaluating the performance of

switchgear. In [37] the experience in using a condition monitoring unit (CMU) was presented.

The CMU monitors interrupter wear, integrity of the SF6 gas system, the circuit breaker

mechanical system, the electrical control system and auxiliaries. Field experiences showed that

there were some issues with the reliability of the CMU unit. Significant improvements have been

done based on the field experience resulting in better performance of the CMU [37].

Camps et al [38] reported that the design and application of on-line condition monitoring

systems for EHV SF6 circuit breakers must be considered carefully in order to minimise the





effect on the reliability of the circuit breaker. Apart from being economically viable, it is

essential that these systems measure the appropriate parameters and reliability of the condition

monitoring is of a high degree.

From literature review carried out there is no evident of currently-available monitoring

techniques in use to measure restriking produced during circuit breaker switching of capacitor

banks and shunt reactor banks.

2.7.1 Detecting restrikes or interrupter experiencing prolong restrikes

In order to measure restriking on-line, using currently-available methods monitoring

equipment must consist of high frequency voltage transformer (that need to be connected on

both sides of the circuit breaker under test) and a high-frequency current transformer or

Rogowski coil. The monitoring system should have adequate bandwidth and be capable of

resolving very fast reignition transients. Some CB installations have been installed with such

equipment i.e. for measurement of contact travel & timing, current and voltage [39]. However,

WKH�PHDVXULQJ�HTXLSPHQW�GRHVQ¶W�KDYH�HQRXJK�EDQGZLGWK�WR�PHDVXUH�+)�WUDQVLHQWV��

Furthermore, if on-line set up is desired all equipment need to be installed at commissioning and

this is very costly to have. Alternatively, for plant not equipped with on-line monitoring

shutdowns are required to connect the measuring equipment. Measuring equipment which is

relatively large needs to be brought to the substation, shutdowns need to be arranged which

reduce the reliability of the transmission network and the amount of manpower involved in

carrying out the tests is huge. Hence, it can be a costly and major exercise.

2.7.2 Alternative monitoring methods

Non-intrusive methods and on-line technique would be an advantage in monitoring

restrike occurrences during CB switching of reactive equipment. The ideal monitoring system

would measure the early high-frequency restriking phenomena and trend the magnitude of

restrike current and frequency of restriking to facilitate early identification of degradation of the

condition of circuit breakers.

Substation switching operations, faults or lightning strikes inside the substation can

cause potentially damaging levels of high frequency electromagnetic interference (EMI). This

EMI can couple into low voltage control circuits and electronic equipment and affect their

operation unless it is suitably protected.





The radio-frequency electromagnetic emissions of high-power arcs historically been

regarded as interference and have received some measure of study in regards to substation EMC

[40,41]. Recently the use of RF has begun to draw serious attention from switchgear monitoring.

An investigation into the detection of contact separation events was reported in [42] and [43]

introduces technique that can be used to identify differences between substation switching

operations using their induced radiation signatures. [43] carried out analysis of observations of

captured signals from many switching arcs carried out with a view to developing on condition-

monitoring systems.

Chapman [50] has demonstrated a suitable technique for the measurement of electrical

discharge or switching arc during current interruption in switchgear. The method makes a direct

measurement of the arcing duration based on the coupling of the VHF electric fields generated

by the arc. He noticed that there are instabilities during arc initiation during contact parting and

at arc interruption at current zero. During a period when the arc is stable, the signal strength

reduces to near zero-level. He claimed that his technique is very successful for a single-phase

interruption. However, he mentioned that the technique is not so successful for 3 phase

measurement due to interphase interference from all arcs during 3 phase switching.


circuit breakers during capacitor switching duty in AIS substation from measurement of emitted

radio waves. It is suggested that it might be possible to extend this methodology to provide

evidence about the magnitude and number of restrikes occurring during reactor switching.



Chapter 3: New Methods For Condition Monitoring Of Restriking EHV CBs


3Chapter 3: New Methods For Condition

Monitoring Of Restriking EHV CBs

3.1 NON-INVASIVE CIRCUIT BREAKER MONITORING USING RADIOMETRIC MEASUREMENT

As pointed out in the literature review in Chapter 2, switching of shunt capacitor and

shunt reactor is a great concern due to the transient produced. Re-strikes produced during

disconnection of capacitor banks and shunt reactors produces high frequency oscillations. The

high frequency oscillation cannot be measured by current transformers, voltage transformers or

monitoring systems which are known to have a rather low bandwidth and would not be capable

of resolving very fast reignition transients.

Monitoring of voltage waveforms during switching using established high voltage

measuring methods would possibly provide information about the magnitude and frequency of

small re-ignitions or re-strikes in circuit breakers that could be a precursor to interrupter failure.

However, waveform measurements at a moderately high frequency require plant outages to

connect the equipment such as voltage dividers, DQG�WKLV�LV�GLIILFXOW�DQG�FRVWO\�LQ�WRGD\¶V�

operating environment. It would be ideal to develop a non-invasive measuring system that would

detect and measure the early high-frequency restriking phenomena and trend the magnitude of

re-strike current and frequency of restriking to facilitate early identification of degradation of the

condition of circuit breakers.


circuit breakers during capacitor switching duty from measurement of emitted radio waves. It

appears possible to extend this methodology to provide evidence about the magnitude and

number of re-strikes occurring during reactive equipment switching.

Moore [6] used four broadband disckcone type antenna with a lower frequency cut-off

of 100 MHz and required a state-of-the-art measuring system to detect the impulses produced

during opening and closing. By triangulating the high frequency impulses, the source and time of





initiation could be identified. The technique is successful in determining CB interpole timing for

capacitor bank closing.

However, Moore [6] reported that his technique could not reliably determine the

opening time for capacitor bank opening due difficulty in recording the opening impulse

produced. Moore claimed that it is difficult to trigger the oscilloscope due to low level of

impulses produced during opening as compared to closing. He mentioned that setting the trigger

level very low caused repeated triggering from communication station broadcasts and rapidly

filled up the oscilloscope memory.

0RRUH¶V�>�@�WHFKQLTXH�PHDVXUHV�WKH�KLJK�frequency phenomena with frequencies above

100 MHz and therefore obtained no data about the lower-frequency transient phenomena during

switching. In our opinion, the lower-frequency transients contain a great deal of information

about the sequence of events which could help us to localise the time of occurrence of restrikes

with respect to other important phenomena such as 50 Hz current interruptions and interpole

closing/opening time.

The work done by [6] is used as basis in carrying out the investigation on transients

produced by circuit breakers switching capacitor bank and shunt reactor bank. This research

LQYHVWLJDWLRQ�ORRNV�LQWR�H[WHQGLQJ�0RRUH¶V�WHFhnique by combining the use of broadband active

antenna with a bandwidth of 1.5 MHz to 1.5 GHz with a set of three passive antennas with a

bandwidth of 30Hz to 30MHz.

3.2 RESEARCH METHODOLOGY

The complete methodology adopted in this thesis is shown below:-

Design and Development of a Measuring System

1) Design oscillatory radiating test circuits to provide e.m. disturbances of the type

expected in the field.

2) Develop measuring systems using antennas and recording instruments that can

be used in a substation environment.

3) Develop procedures for downloading and archiving test results.

4) Gain experience in practical aspects of measurement.





Laboratory Measurement at High Voltage Laboratory and Exploratory Field Measurement

1) Carry out laboratory measurement at High Voltage laboratory.

2) With industry partners identify suitable test site (close to Brisbane and with

frequent capacitor bank switching) for exploratory field measurement.

3) Identify opportunities for making coincident measurements of system voltage

and/or current from existing secondary systems.

4) Check triggering requirements and need for automated long-term measuring

system.

5) Make risk assessments and prepare necessary procedures and associated

documentation.

6) Make measurements of closing and opening operations.

7) Analyse test results.

8) Make modifications to measuring and recording systems.

Measurement on Capacitor Bank and Shunt Reactor Bank Switching Tests

1) Identify with industry partners important capacitor bank and shunt reactor

switching locations.

2) Make modifications to recording equipment.

3) Make necessary risk assessments and prepare necessary documentation.

4) Make measurements of closing and opening operations.

5) Analyse test results.

6) Review progress and make adjustments/modifications to measuring systems as

required.

Final Analysis and Documentation

1) Extend analytical procedures.

2) Review all results and assess the effectiveness of the method.

3) Prepare MEng Thesis

3.3 DEVELOPING MEASURING EQUIPMENT

There are important factors to be considered when developing equipment for measuring

transients phenomena inside a substation. The factors are as follow:





It is important to measure the required transients within the right frequency range

and with adequate sampling rate. This is to ensure measurement is done with

sufficient details recorded for further analysis.

The measuring system should be relatively easy to set up without requiring a plant

shutdown to set up the measuring equipment. If a shutdown is required, the

scheduled down time of plant should be minimum as possible.

No modification is allowed to the existing installation. Modification to the

substation primary and secondary equipment is not allowed in order to ensure that

no risk is introduced to the existing equipment.

The overall site condition needs to be considered in terms of the plant layout,

distance of equipment, suitable placement of measuring equipment, safety

requirement.

The measuring equipment should be developed with the possibilities of having

remote measurement. This will enable long term measurement to be done

automatically with measurement data transferred remotely to remote operators.

Measuring transient in substation is difficult. Electromagnetic interferences due to

corona, earth potential rises and other causes may affect high voltage equipment

and care needs to be taken to ensure that effects of the electromagnetic

interferences is kept to minimum.

Two major types of equipment were developed in this research: sensors (which are

antennas) and the measuring/recording equipment. The development of the antennas and the

theory behind the measuring philosophy will be discussed first followed by the development of

the recording equipment.

3.4 ACTIVE BROADBAND ANTENNA

The active broadband antenna (active antenna) was developed by Dr Tee Tang. The

active antenna designed for this application consists of a passive dipole antenna and a broadband

RF amplifier. The passive dipole is 150mm long, with a diameter of 1.6mm. The RF amplifier

used is of a Low Distortion RF amplifier. It is designed to have a flat frequency response from

1.5MHz to 1.5GHz and a signal gain of 12dB. The active antenna is powered by 4.5VDC

batteries and it is controlled by a switch located on the casing. The antenna is housed in a PVC

casing. The size of the casing is 83mm x 54mm x 31mm. For site measurements the antenna is

mounted on a portable tripod. The antenna is small and portable and ideal to be use for field

measurement. Figure 3.1 shows a photo of the active antenna.





Figure 3.1 Photo of Broadband Active Antenna

In operation, the RF amplifier acts as an impedance transformer, converting the high

output impedance of the electrically short dipole at the low frequency range to a low impedance

output. At the high frequency end of the amplifier response, the RF amplifier merely amplifies

the received signal. Figure 3.2 shows Gain versus Frequency for the RF amplifier. In this

research, the antenna was used to record HF pulses and also acts as high frequency marker on the

lower frequency transient recorded.

Figure 3.2 Gain vs Frequency for RF amplifier [62]






3.5 CAPACITIVE COUPLING ANTENNA

In order to obtain information about lower-frequency transients, capacitive coupling

antennas were used. For relatively low frequency transient, the wavelength is very big for

example at 50 Hz, the wavelength is 6000 km whilst at 1 kHz the wavelength is 300 km. A big

radiation antenna is required to receive a transmitted 50 Hz or 1 kHz signal. Capacitive coupling

is a phenomenon where an electrical field e.g. between two parallel conductors apart interact.

Capacitively coupled signals can be measured to give information on the magnitude and

frequency of the signals. Hence, measuring using capacitive coupling antenna (passive antenna-

PA) is needed to facilitate low frequency measurements.

3.5.1 Construction of the Passive Antenna

Figure 3.3 Passive Antenna Drawing

The capacitive coupling antenna (passive antenna) is divided into 3 major components.

The major components are the antenna head, antenna stand and antenna base. Figure 3.3 shows

the drawing of the capacitive coupling antenna. It is purposely designed as such to facilitate ease

Dia190.0mm

Aluminium Cylinder/ Passive Antenna

Weight / Sand Bag

Antenna Base Assembly

Barrel Union (Coupling) 50mm dia

PVC Tube 50mm Dia

Antenna Base/Stand Assembly

Barrel Union (Coupling) 50mm dia





of transportation for field measurement. The antenna head consists of a metallic cylinder

enclosed at both ends. The diameter of the metallic cylinder is 190mm and the length is 60mm.

The centre pin of an insulated BNC connector is connected to one end plate of the cylinder. The

overall length of the antenna head is 100mm.The antenna head is coupled with the antenna stand

using a PVC barrel union coupling.

The stand is made of PVC tube which is lightweight and non conductive. The stand is

attached to the antenna base which is made of cast iron with three legs which is heavy enough to

provide support for the whole antenna assembly. It is important that the antenna remain stable

when located on the gravel on the substation switchyard. The overall height of the antenna is less

than 1400mm which is well below the safe ground clearance inside a substation. Figure 3.4

shows photo of the complete assembly of the passive antenna.

Figure 3.4 Photo of Passive Antenna

In making measurements, the core of the input measuring cable is connected to the BNC

connector of the PA and at the other end the cable core is connected to the input of a fully-

floating oscilloscope. The cable sheath of the measuring is connected to one end which is the





earthed part of the input terminal of the oscilloscope. The terminal is connected to the substation

earth through the metal housing of the oscilloscope.

3.5.2 Review on capacitive coupling

High voltage conductors are surrounded by an electric field which can induce voltages

on nearby conductors. Such electrostatic coupling can occur between a high voltage conductor

and a secondary circuit. The principle of electrostatic coupling is illustrated in the Figure 3.5.

Figure 3.5 shows the capacitance between the conductors in the two circuits and between each of

the conductors and ground. It is assumed that the secondary circuit is floating. It is apparent that

this is a simple potential divider.

C32C12

C13

Ep Ec

Figure 3.5 Electrostatic coupling between a HV conductor and secondary circuit

The voltage will divide inversely as the capacitance, regardless of frequency, thus, if Ep

is the voltage on the power conductor,

pc ECC

CE

3213

13 (3.1)

Power frequencies signals and transient voltages can be coupled from one circuit to

another by this means. In practice, however, the secondary circuit will usually be terminated to

ground at both ends with impedances much lower than the impedance of C32. The shunting

current in these circumstances will be determined by C13 and for a voltage Ep at frequency will

be





13CEI p (3.2)

And the voltage of the control circuit conductor will be established by the product of this

current and the impedance to the ground. The example given implies that the capacitances are

known. This is usually not the case but there is standard method to calculate the capacitance

between parallel wires. A computer program can be used for complicated calculation such as

determining capacitance between three phase conductors to an object.

Estimating capacitance between a point above ground to an overhead conductor.

Determining the capacitance between a long conductor and a point above the ground

requires a complicated calculation. To roughly estimate the capacitance, the equation to calculate

capacitance between two conductors was used [53]. The capacitance equation between two

parallel conductors is given by

ad

mFC ro

2cosh

)/(1

(3.3)

Where

d = distance between the two wires in meter

a = radius of conductor in meter

0= permittivity of free space (F/m)

r = relative permittivity/dielectric constant for air (1atm) = 1

To estimate the capacitance, we use the distance, d= 5m and conductor radius,a = 0.015m, the

capacitance is calculated to be:

03.0

5cosh10*854.8*

1

12

C

mpFC

C

/79.4811.510*854.8* 12

The calculated capacitance is when the conductor is 1m in length. At a conductor length

of 0.2m which is approximately the same length as the passive antenna diameter, the capacitance

is 0.958pF/m. The value calculated is not accurate as it only takes into account the field from the

parts of the HV line which are at right angles to the secondary conductor (antenna). The field





converges on the short secondary point from all along the HV line need to be included in the

calculation of the capacitance. The actual capacitance between the antenna above a ground to the

overhead wire should be higher than the calculated value using equation 3.3. To facilitate

estimation and calculation during the research a capacitance of 1pF is estimated as the value of

C13. In the following sections, C13 will be known as C1.

3.5.3 Single phase capacitive coupling model

At the power line frequency, Vs, the passive antenna is effectively coupled to the power

lines capacitively, C1. As the coupling capacitance is small, which is estimated as 1 picofarad,

the voltage developed across the load, Vo is linearly proportional to the load impedance. This is

in effect a capacitive potential divider with a resistor connected across the low-voltage arm.

Figure 3.6 shows a diagram of single phase capacitive divider.

Figure 3.6 Capacitive Divider

The single phase capacitive coupling antenna could be represented by a simple

equivalent circuit. Figure 3.7 illustrates the equivalent circuit for the capacitive antenna for single

phase measurement.

Vo

Vs

C1

Measuring Instrument

HV conductor

Antenna Head

Details of the network to the left of the arrows are included in Figure 3.7.





Figure 3.7 Passive Antenna Equivalent Circuit

Vs = Supply phase voltage

Vo = Output Voltage measured

C1 = Capacitance from conductor to Passive antenna

Cr = Capacitance of measuring cable

R = Load resistance

From the above equivalent circuit it can be seen that at very high frequency, XCr << R.

Hence, the output voltage is determined by the capacitance. While at low frequency, R << XCr,

the output voltage is determined by the resistance. In order to analyse the frequency response of

the passive antenna, the transfer function of the passive antenna is determined.

Passive Antenna Transfer Function

The transfer function for the equivalent circuit is given by:-

1)1()1(

11

CrCRjCrCRj

CrCC

VV

S

O (3.4)

At very high frequency, the term )1( CrCRj is dominant and cancels out each other.

Giving the equation below:-

CrCC

VV

S

O

11

(3.5)

At low frequency, the term )1( CrCRj is small giving the equation below:-

Vs

C1

CrR Vo

Measuring Instrument





1)1(

11 CrCRjCrCC

VV

S

O

1RCj (3.6)

By equating s=j, the above could be expressed as;-

1)1()1(

11

CrCsRCrCsR

CrCC

VV

S

O (3.7)

The capacitance and resistance values with for field measurement are given below.

C1 = 1pF (estimated)

Cr = 20 meter * 100pF/m = 2000pF = 2nF

R = 0.5M

Replacing the above values in the Eq 3.7, the transfer function could be expressed as

1)20001(5.0)20001(5.0

200011

pFpFMspFpFMs

pFpFpF

VV

S

O

1001001.0

5 7

sse

A Bode diagram is plot from the transfer function to look at the corner frequency, c and the

relationship between the magnitude, phase angle and frequency. Figure 3.8 shows the Bode

diagram for the transfer function. From the Bode diagram:-

The corner frequency is 2000rad/s or 318.5Hz

Flat gain at -67dB.

At 50Hz or 314 rad/s, the gain is -82dB with a phase shift of about 80 degree.

At low frequency the phase shift is 90 degree and at very high frequency the phase

shift is almost zero.





Figure 3.8 Bode Diagram

The above bode diagram would be used as reference throughout the course of this research for

observations and discussion purposes. It is pointed out that the above bode diagram may not

represent the actual antenna transfer function as the above function is only an estimate for a

single phase measurement and not three phase measurement. The capacitive coupling model for

the three phase measurement would be discussed later. Furthermore the transfer function is

determined by value of C1 the capacitance between the phase conductors and the antenna. C1 is

determined by the distance between the phase conductors to the antenna and relative permittivity

of air during measurement.

Estimating the output voltage measured by the passive antenna

Considering Vs is the phase supply voltage and assuming C1 is 1pF, Vo could be estimated

using the bode diagram.in Figure 3.6 as follows:

Vs = (275kV/3)*1.414 = 222.3kV peak voltage (phase to ground)

At 50Hz or 314rad/s, gain is -82db.

Vo/Vs = -82dB. = 20log10 (Vo/Vs)

= 7.94e-5.





Therefore,

Vo = 223.3kV * 7.94e-5 = 17.7V peak voltage.

This process could be reversed to determine the supply voltage, Vs.

3.5.4 Three phase coupling inside substation

The antenna works on coupling principle. It is important to note that for a 3 phase

measurement inside a substation, an antenna placed under each three different phase conductors

will have all three phase voltage coupled to the antenna via the capacitances. This gives resultant

coupling voltage from all three phases to appear on the antenna. Figure 3.9 illustrates the

capacitive coupling between phase conductors and each passive antenna.

The capacitance is assumed to be inversely proportional to the distance. Antenna nearest

to the phase conductor will experience the strongest capacitive coupling compared to the other

two phases. This results in higher magnitude of voltage coupled from the nearest phase

conductor on the antenna. Therefore, it can be said that strongest coupled voltage on an antenna

indicates the nearest phase conductor.

Capacitive Coupling Antenna

A B C

PA

X Y Z

CAX CBX

CCX

CAY CBY CCY

CCZCBZ

CAZ

Figure 3.9 Capacitive coupling between three phase conductors and three passive antennas.





3.6 RECORDING INSTRUMENTS

Figure 3.10 shows the recording instrument arrangement. The recording instrument

consists of two digital oscilloscope, coaxial cable, portable power supply and earthing

accessories.

USB HSD

3.5 FDD RS-232

CRO 1 - 1.5GHz

CRO 2 - 100MHz

Power SupplyUPS + Surge

arrestor

12V Battery @ 250AH

Faraday Cage i.e. vehicle

Active Antenna

Passive Antenna

Figure 3.10 Recording instrument arrangement

3.6.1 Digital oscilloscopes

Two oscilloscopes were used for recording measurements. The characteristics of the two

digital oscilloscopes are shown in Table 3.1 and Table 3.2.

Table 3.1 Characteristics of Agilent Digital Oscilloscope

Make Agilent Vertical resolution 8 bits

Model 54624A Input resistance 1 M ± 1%

Available channel 4 Input capacitance 14pF

Bandwidth DC to 100 MHz Data Storage Floppy disk





Max Sample rate 200 MSa/s I/O RS-232 Serial Port

Record length 2 M/CH Sample rate @ 10ms

time div. 20 MSa/s

Table 3.2 Characteristics of Yokogawa Digital Oscilloscope

Make Yokogawa Vertical resolution 8 bits

Model DL 9240 Input resistance 1 M ± 1%

Available channel 4 Input capacitance 20pF

Bandwidth DC to 1.5 GHz Data Storage Internal HDD

Max Sample rate 10 GSa/s I/O USB Interface

Record length 2.5 M/CH Sample rate @ 10ms

time div. 25 MSa/s

It can be clearly seen that Yokogawa DL9240 is superior to Agilent 54624. Yokogawa

DL9240 has a segmented memory architecture that allows the main sampling memory of 2.5M

per channel to be segmented into separately triggered buffers according to memory length

selected. This option is known as the waveform history or History Function. History Function

allows successively triggered waveforms to be stored on the oscilloscope acquisition memory

and the waveforms could be displayed when the acquisition process stopped. This is a useful

function to record successful impulses during switching event. Similar function was used by

Moore [6] in his measurement. However, the time stamping used by Moore has a resolution of

precision 10-9s whereas Yokogawa DL9240 has only a resolution of precision 10-3s. Agilent

54624 does not have this function.

For Yokogawa DL9240, using 1CH with memory length of 2.5M, waveform saved in

.wdf format (proprietary binary format) uses 5MB of memory and this is equivalent to 25MB of

memory if saved in .csv format. With 4CH with memory length of 2.5M, a total of 20MB

memory will be used in .wdf format and time taken to save all the waveforms is around 4

minutes.

Agilent 54624 does not have the waveform save function built in but it is possible to

download the waveforms acquired on all 4 CH. The available setup allows all waveforms to be

downloaded but only one at a time. Using RS-232 connection at 57600 baud rate, with 2M

points on each waveform; downloading time for each waveform is around 12 minutes. Hence,





for 4 waveforms a total downloading time of 50 minutes is required. This is not desirable for

measurement at substations.

3.6.2 Coaxial cable

The coaxial cable used is of RG58 Cellfoil. It has low loss characteristics and is double

shielded. The cable capacitance per meter is 100 pF. To ensure similar cable capacitances from

each antenna to the oscilloscope, each cable length for the passive antenna is set to 20 meter.

3.6.3 Measurement requirement in substation

In carrying out transient measurement inside a substation, acquiring meaningful

measurement result is important. However more importantly, special attention needs to be given

to ensure safety of the operator and measuring eTXLSPHQW��,Q�WKLV�FDVH��3RZHUOLQN¶V�HOHFWULFDO�

safety procedure needs to be adhered to.

Safety Requirement

Preliminary risk assessment was done and procedures were prepared for carrying out

measurement inside live substation. On the day of measurement, at the substation, final risk

assessment was carried out by Powerlink personnel. Personal protective equipment (PPE) i.e.

safety boots, safety helmet and long sleeve shirt were worn. Powerlink personnel gave a safety

induction before starting of work and non-Powerlink staffs need to be accompanied by

Powerlink personnel at all time while in the substation.

Faraday cage

In order to prevent the influence of the electric field during measurement of switching

transients, recording devices need to be located inside a Faraday cage. To achieve this, all the

recording devices were located inside a vehicle which was earthed to the substation earthing.

Single earthing

Only a single point earthing is required to avoid multiple grounds which introduce

ground loops. Ground loops is a phenomenon in which electromagnetic fields can induce voltage

that is not only affect the original measurement signals but also may cause potential differences

between different ground terminals between equipment which can be hazardous. This was

achieved by connecting the equipment earth connection to the vehicle frame. The vehicle frame

is then connected to the substation earthing.





Portable Power Supplies

To avoid connection with the grounding of the substation supply which in turn prevent

ground loops, a portable power supply was used. The portable power supply consists of a UPS

and a large 12V DC battery.

Overvoltages Protection

To avoid potential electrical charges develop on the oscilloscope input terminals during

equipment set up or handling inside the substation, all the input CH were set to GND. As

additional precaution, all input CH were terminated with resistive load connected to ground.



Chapter 4: Exploratory Measurement on Single-Phase Reactor Switching and Capacitor Bank Switching


4Chapter 4: Exploratory Measurement on

Single-Phase Reactor Switching and

Capacitor Bank Switching

Exploratory measurements have been carried out on single-phase reactor switching at

Ergon Energy HV Laboratory, Virginia and 3-phase Capacitor Bank switching at Blackwall

Substation. The purpose, site details, test arrangements and results for both measurements are

described in this chapter.

4.1 FIELD MEASUREMENT AT ERGON LABORATORY

4.1.1 Purpose of measurement

Laboratory measurement were carried out at Ergon Laboratory, Virginia. The

objectives of the measurement were:-

To investigate the application of active antenna (AA) and passive antenna (PA) in

detecting re-ignition and restriking transients produced during single-phase

switching of reactor.

To measure transient and waveforms produced during single-phase switching of

shunt reactor.

To identify improvement required for antenna, measuring technique and recording

system.

Measurement was carried out using both the active antenna, passive antenna and

recording system developed.

4.1.2 Restriking in Vacuum Circuit Breaker

In order to investigate the application of both antennas in detecting re-strike in circuit

breaker, an experiment was set up on Single Phase Reactor Switching using vacuum circuit

breaker. Restriking is known to be produced during interruption of inductive/reactor current by

vacuum circuit breaker. Re-ignition/re-strikes in vacuum circuit breaker has been successfully

modelled and measured by Lopez-Roldan et al [58]. Expected typical result for re-striking in





vacuum circuit breaker for similar circuit arrangement has been obtained [58] and are shown in

Figure 4.1 and Figure 4.2.

Figure 4.1 and 4.2 show the measured voltage at the HV terminal of a test transformer

during the opening of the vacuum circuit breaker, approximately at the instant of maximum

current. The source voltage was 5kV rms and the maximum overvoltage measured at the

transformer terminal was 35 kV peak (pu). Numerous restrikes can be observed during a period

of 1.2ms. The frequency of the load voltage oscillation after the restrikes is a function of the L

and C at the load side of the breaker.

Figure 4.1 Restriking Process During CB Opening (from [58])

Figure 4.2 Measured Voltage at reactor terminal (from [58])







4.1.3 Test and measurement set up

Figure 4.3 outlines the circuit arrangement. The rated inductance, LB is 20H and the total

load side equipment capacitance CL is 5.0nF. The natural frequency of the LC oscillation on the

load side is given by:-

HzCL

fLB

o 50021

A step-up transformer is used to raise the test voltage to between 3kV and 5kV

simulating the network source voltage. A 12kV rated vacuum circuit breaker is chosen to switch

the reactor. The vacuum circuit breaker is manually operated without point-on-wave controlled

switching facility.

Capacitive voltage dividers were located to measure the voltage at the step-up

transformer (supply side) and at the reactor (load side). The recording equipment consisted of an

active antenna, a passive antenna and two digital oscilloscopes. The oscilloscopes were used to

record the antenna signals and voltage divider waveforms. An Agilent 54624 100MHz

oscilloscope was used to record the passive antenna signal and the voltage divider signals on a

lower-speed timescale. A Yokogawa DL9240 1.5GHz oscilloscope was used to record the active

antenna signal on a higher-speed timescale. The recorded high-speed transient was then stored in

the oscilloscope memory using the History Function. Both oscilloscopes were powered by an

isolation transformer located inside the laboratory control room.

Figure 4.3 Experimental circuit arrangement

Figure 4.4 and figure 4.5 show photographs of the laboratory arrangement. Figure 4.4

shows the vacuum circuit breaker, capacitive voltage divider measuring the load side voltage,

step-up transformer and the 1nF capacitor.





Figure 4.5 shows a single core XLPE cable connected from the vacuum circuit breaker

(which is not visible in the photograph) to the 20H reactor. The passive antenna can be seen to

be located next to the reactor. The active antenna and oscilloscopes are located inside the

laboratory control room. A series of tests were carried out at varying test voltages and varying

antennas location for both passive antenna and active antenna. Selected results are displayed and

discussed

Figure 4.4 Photograph showing Laboratory arrangement

Figure 4.5 Photograph showing Laboratory arrangement

Vacuum

Circuit Breaker

Capacitor

1nF

Step Up

Transformer

Reactor

Passive Antenna

1C XLPE Cable

Potential

Divider





4.1.4 Reactor opening at 3kV with Passive Antenna located close to the supply transformer

In this test, the passive antenna was located close to the supply transformer and the

active antenna was located in the control room. Prior to the opening test being carried out,

measurement was carried out to calibrate the passive antenna using the supply voltage. The

calibration was carried out with the vacuum circuit breaker in closed position. Table 4.1 shows

the calibration result between the test voltage, supply voltage and passive antenna waveform.

Table 4.1 Calibration between test voltage, supply voltage and Passive antenna

TEST VOLTAGE,

V (RMS)

PASSIVE ANTENNA

VCH1

V (P-P)

SUPPLY VOLTAGE

VCH2

V (P-P)

500 0.84 0.77

1000 1.66 1.47

1500 2.47 2.17

2000 3.22 2.88

2500 3.97 3.59

3000 4.75 4.34

Figure 4.6 (a) shows the signal measured by passive antenna, supply voltage, reactor

voltage. It was observed that the signal picked up by the passive antenna leads the supply voltage

and reactor voltage due to the capacitive coupling nature of the passive antenna by almost 900.

On opening of the vacuum circuit breaker, the supply voltage indicated HF restriking

followed by voltage distortion and settled to 50Hz. The reactor voltage showed HF restriking

followed by decaying transient oscillation at 500Hz. The 500Hz oscillation is a function of the L

and C on the load side of the circuit breaker and corresponds with the calculated values earlier.

The passive antenna picked up the high frequency restriking and the 500Hz transient LC

oscillation superimposed on the 50Hz component. Restriking on the VCB occurred at the early





stage of opening/interrupting as shown in circled area A. Area A is magnified to further look at

the restriking event.

)LJXUH��D��:DYHIRUPV�GXULQJ�RSHQLQJ�RI�YDFXXP�FLUFXLW�EUHDNHU�DW��N9��E��$UHD�$�±�5HVWULNHV�RQ�5HDFWRU�9ROWDJH��F��$UHD�$�±�5HVWULkes detected by passive antenna.

Figure 4.6 (b) and (c) show restrikes measured on the reactor voltage and passive

antenna waveform. It can be seen that numerous re-strikes occurred for a period of 1ms. Re-

strikes detected by passive antenna and measured by the voltage divider at the reactor side are

similar in frequency and pattern but vary in terms of magnitude.

A

Passive antenna

Supply voltage

Reactor Voltage

(a)

(b) Reactor Voltage

Passive antenna

(c)5V

200µs





In Figure 4.6(b), the pattern and the magnitude of the restrike voltages measured at the

reactor voltage terminal are different from the expected result of Figure 4.2. The huge difference

in the expected result may be due to the high frequency earth loop current passing through the

isolation transformer (which was plugged into the same supply as the main supply transformer)

supplying the oscilloscope.

In Figure 4.6 (c), the passive antenna detected the HF restriking current and small

magnitude of restriking voltage. The time difference between restriking pulses ranges from 20s

initially to 200 s.

Figure 4.7 HF restriking pulses detected on Active Antenna

Figure 4.7 shows the HF restriking pulses detected by the active antenna recorded on the

high speed oscilloscope. The active antenna blocks out the 50Hz component and the 500Hz

transient load oscillation. The recorded HF pulses occurred for 1 ms and consistent with the

restriking pulses recorded by the passive antenna. The constant amplitude on the HF pulses was

due to active antenna amplifier going into saturation during restriking.

1V/div





Figure 4.8 Magnification on one HF pulse

Further observation found that every pulse has similar waveform pattern and 1 HF

restriking pulse was further analysed. In Figure 4.8, the 2nd part of the figure show that the

analysed HF pulse contains 2 distinct frequency oscillations. Applying Fast Fourier Transform

(FFT) on the HF pulse indicated that each HF pulse contained frequency component of 10MHz

and 2.6MHz. This HF pulse may be related to the ³ILUVW�SDUDOOHO�RVFLOODWLRQV�WKDW�RFFXUV�GXULQJ�

re-ignitions.

4.1.5 Conclusion

The circuit arrangement for the experiment successfully produced restrike during single

phase reactor switching. From series of tests carried out, the passive antenna managed to detect

50Hz signal, re-strikes during switching of shunt reactor and transient oscillation after

interrupting the shunt reactor. Location of the passive antenna determines the coupling voltage.

The active antenna also detected the HF re-strikes during shunt reactor switching. The only

drawback was the earth loop effect which affected the result slightly.

The experiment has showed that the use of both low frequency capacitive coupling

antenna and high-frequency active broadband antenna are an interesting, non-invasive technique

for measuring transients and detecting re-strikes during single phase shunt reactor switching. It is

thought that this technique could be extended to measurement of three phase shunt reactor or

capacitor bank switching.

1V/div





4.2 EXPLORATORY THREE PHASE CAPACITOR BANK SWITCHING MEASUREMENT AT BLACKWALL SUBSTATION

4.2.1 Purpose of site measurement

Site measurements were carried out at Blackwall substation on 21st May 2007. The

objectives of the measurement were:-

To investigate the application of active antenna and capacitive coupling antenna in

detecting re-ignition and restriking transient produced during three phase capacitor

bank switching.

To carry out measurement on transient and waveforms produced during three phase

capacitor bank switching.

To identify improvements required in terms of antenna requirement, measuring

technique and recording system.

To determine the background noise inside a substation.

4.2.2 Site details and arrangement

Blackwall substation is a 275 kV substation interconnected with other important

VXEVWDWLRQV�LQ�3RZHUOLQN�4XHHQVODQG¶V��N9�QHWZork. The substation is interconnected with 6

other 275kV substations via 275kV overhead lines. Blackwall substation also has Static VAR

Compensator (SVC) installation and two Capacitor Bank bays with 60MVAR rating each. The

two capacitor bank bays are identified as Capacitor Bank No. 5 and Capacitor Bank No. 6

respectively. Figure 4.9 shows Blackwall substation interconnection. Measurements were carried

out on Capacitor Bank No.5. Figure 4.10 shows the capacitor banks layout.

The monitored circuit breaker type is an ABB 275kV Hybrid PASS MO. It is a dead

tank circuit breaker, using SF6 as the interrupting medium, single pole operation with hydraulic

drive mechanism. Controlled switching is employed for both opening and closing operation.

Computerised sequence of event (SOE) information is normally available from this type of

circuit breaker but secondary voltage and current waveforms were not available for this

exploratory measurement.





Figure 4.9. Blackwall Substation Interconnection

The capacitor bank rated capacitance, FCB 94.4 and the series inrush inductance,

mHLB 86.2 .These values were supplied by Powerlink.

The inrush current frequency for capacitor bank could be calculated as follows [1]:

Isolated Capacitor Bank

121

CLLf

BSi

where

if is the inrush frequency (Hz)

sL is the estimated source inductance,

at fault current of 40kA, sL is kAkV 40/3275 =12.63mH

LB is equal to 2.86 mH

1C is equal to FCB 94.4

Hence, Hzfi 3.575

Back-to-back Capacitor Bank

For a back-to-back capacitor bank with similar rating

SOUTHPINE

ROCKLEA

SWANBANK

BELMONT

CAPACITOR BANK

MT ENGLAND

TARONG

BLACKWALL

SVC





eqeqi CLf2

1

where

if is the inrush frequency (Hz)

eqL is series inductance of bank 1 and bank 2, 5.72mH.

eqC is the series capacitance of bank 1 and bank 2 , F47.2

Hence, kHzfi 34.1

These two frequencies are important as they would indicate the inrush frequency during

transient measurement for the capacitor bank closing.

Figure 4.10 Blackwall Capacitor Bank Layout

4.2.3 Measurement set up

Figure 4.11 shows the measuring equipment layout. The passive antenna is connected to

CH1 on Agilent CRO whilst the active antenna is connected to CH1 on Yokogawa CRO. The

active antenna was also connected to CH2 of Agilent 54624 to act as a High frequency marker.

Agilent 54624 oscilloscope was used to record low-speed transients detected by the passive

antenna using 20ms/div with a sampling rate of 20MSa/s. Yokogawa DL9240 was used to

Control Building

Capacitor Bank 6 Capacitor Bank 5

Capacitor Bank

Series Reactor

Switchgear ABC

*Not to scale





record high speed transients detected by active antenna by using 1s/div with a sampling rate of

10GSa/s. The high speed transient is then stored in the oscilloscope memory using the History

Function which allows successively triggered waveforms to be stored on the oscilloscope

acquisition memory and can be displayed when waveform acquisition stopped.

A few triggering methods were applied. The best technique was when the active antenna

triggered the Yokogawa oscilloscope using conventional amplitude triggering and the

Yokogawa oscilloscope in turn triggered the Agilent oscilloscope which was set for single shot

operation.

During the measurement the passive antenna and active antenna were located on the

busbar side of the circuit breaker.

Figure 4.11 Measuring equipment layout

USB HSD

3.5 FDD

Yokogawa DL9240

Agilent 54624

Active Antenna

PassiveAntenna


arrestorFaraday Cage e.g. vehicle

B/B

CB SERIES REACTOR

CAPACITOR BANK

CT

12V Battery @ 250AH

Trigger

275 kV busbar





4.2.4 Summary of tests/measurement carried out

Table 4.2 shows a summarised record of the tests carried out at Blackwall.

Table 4.2 Summary of tests conducted at Blackwall on 21st May 2007

EVENT AGILENT 54624 YOKOGAWA DL9240

Background Measurement

Three phase waveform measured. Triggering by Ext trig from Yokogawa.

Triggering by Active antenna on CH1 with level set at 50mV.

CB6 Closing Not properly triggered Not properly triggered

Test 1

CB5 1st Opening

Not properly triggered Inaccurate waveform recorded

Test 2

CB5 1st Closing

Trig: CH1 Level : 7.5V

Closing transient recorded by Passive Antenna and HF markers recorded by active antenna. Both signals were recorded in 40ms window.

Trig: CH1 Level : 200mV

6 individual HF waveforms recorded within 60ms.

Test 3

CB5 2nd Opening

Trig: CH1 Level : 7.5V

Not triggered

Trig: CH1 Level : 200mV

3 similar HF waveforms triggered in approximately 10ms.

Test 4

CB5 2nd Closing

Trig: CH4 (Ext-Yokogawa)

Closing transient recorded by Passive Antenna and HF markers recorded by active antenna. Both signals were recorded in 40ms.

Trig: CH1 Level: 200mV

6 individual HF waveforms recorded within 60ms. Similar as 1st closing.

Test 5

CB5 3rd Opening

Trig: CH4 (Ext-Yokogawa)

Opening transient recorded by passive antenna and HF markers recorded by active antenna. Both signals were recorded in 20ms.

Trig: CH1 Level :200mV

3 individual HF waveforms were recorded within 20ms. Similar as 2nd closing.

Samples of opening and closing waveforms recorded on low speed oscillograph and

high speed oscillograph are presented and discussed in the following sections.





4.2.5 Opening operation

Low Speed Oscillograph

Figure 4.12 Antenna waveform on CB opened at point A

Figure 4.12 shows the waveform received by the passive antenna when it was positioned

under the conductor on the busbar side (supply side) of the circuit breaker. The passive antenna

picked up the summation of the three phase 50Hz voltages. It can be observed that at about 17

ms (point A) there was sudden reduction of voltage after CB opening i.e. disconnection of

capacitor bank. There are also appear to be indications of other events at about 23 ms and 27-28

ms. The event at 17 ms may be due to an oscillation similar to that shown in Figure 2.4. The

lumpy staircase-like voltages observed in Figure 4.12 were due to the limited resolution of the

oscilloscope.

Figure 4.13 shows that the active antenna picked up a series of HF pulses during CB

opening. It can also be observed that the voltage distortion on the passive antenna 50 Hz

waveform corresponds to the HF markers measured by the active antenna. The HF markers

occurred for a duration of approximately 10ms.

CH1-PA CH2-AA

V before V after

A





Figure 4.13 HF pulses during opening

High Speed Oscillograph

(a)

(b)

(c)

Figure 4.14 Three typical HF Pulses During Opening

Figure 4.14 shows three typical high frequency pulses recorded by the active antenna on

the high speed oscillograph during Test 5. It is interesting to note that the active antenna

triggered 3 HF events. This may correspond to the three pole separation events. However, HF

Timescale:

1µs/div

Volt scale:

200mV/div

10 ms /div

2 ms /div Zoomed view of time window above

1V /div





markers on Figure 4.13 recorded four HF pulses during opening. Comparing this with the HF

pulses recorded by the high speed oscilloscope, the HF pulses probably do not indicate just pole

separation. Unfortunately, it is not possible to determine the time difference between each of the

HF pulses as the time stamping facility on the Yokogawa oscilloscope has a resolution of 10ms.

4.2.6 Closing operation

Low Speed Oscillograph

Figure 4.15 shows the passive antenna picked up the summation of the three phase 50Hz

component and transient oscillations during CB closing. It can be observed that there was an

increase of 50 Hz voltage after the CB closing as would be expected from a connection of

capacitor bank. The transient oscillations picked up by the passive antenna at point A shows the

effect of inrush current during CB closing. The transient oscillation frequency recorded is

approximately 400Hz. This value is less than 573.5 Hz that calculated earlier in section 4.2.2.

This is probably due to the actual circuit condition with overhead line connection and SVC

which were not taken into consideration in the earlier calculation.

Figure 4.15 Passive Antenna waveform on closing

Figure 4.16 HF markers during closing

CH1-PA CH2-AA

V before V after A

CH2-AA





Figure 4.16 shows the active antenna picked up series of HF pulses during CB closing.

The HF pulses also correspond to the voltage distortion/transient recorded on the passive antenna

as shown in Figure 4.15. The duration of the HF pulses and transient is approximately 40ms.

High Speed Oscillograph

(a)

(b)

(c)

(d)

(e)

(f)

Figure 4.17 (a) to (f) typical HF Pulses during closing

Figure 4.17 shows HF pulses picked up by the active antenna and recorded on

Yokogawa using the history function. The pulses are arranged according to their sequence

occurrence. Pulse (a) was the 1st pulse and pulse (f) was the last pulse. Figure 4.17 (a) and (b)

show that the 1s/div scale used is not adequate to show the HF pulse waveform. By analysing

other HF waveforms, it was found that some of the HF pulses lasted for almost 20s. It can also

be seen that the picked up signal by the active antenna overloaded the oscilloscope. Figure 4.17

(e) and (f) show that positive only HF transient were recorded on Yokogawa scope. However,

Time scale: 1µs/div Volt scale: 200mV/div





from figure 4.16, it can be seen that positive only and negative only transients were recorded on

slower scale during closing operation.

Similar HF transient were recorded during other closing operation on the Yokogawa

oscilloscope at 1s/div scale using the history function. However, the sequence of the HF pulses

between closing operations is different.

4.2.7 Discussion

The exploratory measurement at Blackwall gave a good insight into the technique of

measuring transients in substation using the broadband active antenna and the capacitive

coupling passive antenna. Opening and closing of capacitor bank transients were detected by

both antennas and recorded by the recording instrument. This would be used as reference for

future measurement.

At this stage it was difficult to completely explain the waveforms recorded by both

antennas and to determine the timing, pole sequence and whether re-strikes occurred. However,

it was thought to be possible to improve the measuring system to give more information that

would facilitate understanding of the waveforms. The exploratory measurement gave positive

results in measuring transient using antennas and illustrated the importance of earthing which

could be improved by using single point earthing methods. The measuring system managed to

measure the transients produced during capacitor bank switching.

4.2.8 Improvement to be taken

After the exploratory measurement carried out, improvements were identified on the

measuring system, recording technique and other information required from Powerlink.

In order to relate measurements to the travel of the circuit breaker and to assist in

analyse of the acquired waveforms, additional information will be needed from

Powerlink. These include secondary current and voltage waveforms and sequence

of event (SOE).

As for the measuring system, it was proposed to use three Passive antennas, one for

each phase to give better information about transient waveforms recorded. Further

to this, varying the location of the passive antenna need to be investigated to give





greater understanding of the sensitivity of this type of antenna to distance from HV

sources.

As for the recording technique, improvements could be made in terms of triggering

settings, oscilloscope memory length, volts/div, suitable time scale for opening and

closing and data storage medium.

It is also worth noting that the field measurement involves actual equipment connected

to the transmission network and requires authorised staff to operate the system. Hence, the

number of tests are limited and time is always a constraint. Measurements need to be planned

and executed efficiently.



Chapter 5: Measurement of Capacitor Bank Switching


5Chapter 5: Measurement of Capacitor

Bank Switching

Field measurements were carried out on three phase Capacitor Bank switching at

3RZHUOLQN�4XHHQVODQG¶V�%ODFNZDOO�6XEVWDWLRQ�RQ��th August 2007. The purpose of the test, site

details, test arrangements and results from the field measurement are described in this chapter.

Purpose of the site measurement

. The objectives of the field measurement carried out at Blackwall substation are:-

To investigate the application of active antenna and three passive antenna in

detecting re-ignition and restriking transient that may be produced during 3 phase

capacitor bank. switching

To measure transient waveforms produced during 3 phase capacitor bank

switching.

To identify effectiveness on improvements made in antennas, measuring techniques

and recording systems.

5.1 SITE DETAILS AND ARRANGEMENT

Measurements were carried out at the 275kV, Blackwall Substation. As mentioned in

Chapter 4 the substation is interconnected to five other 275kV substations via 275kV overhead

lines. Blackwall substation also has Static VAR Compensator (SVC) installation and 2 Capacitor

Bank bays with 60MVar rating each. The 2 capacitor bank bays are again identified as Capacitor

Bank No. 5 and Capacitor Bank No. 6 respectively.

The monitored CB is a ABB 275kV Hybrid PASS MO circuit breaker used for reactive

switching of Capacitor Bank No.5. The PASS MO is a dead tank circuit breaker, using SF6 as

the interrupting medium, with single-pole operation with an hydraulic drive mechanism. As

expected for a reactive switching the circuit breaker, the circuit breaker operates everyday.





The capacitor bank is rated at 60MVar. The rated capacitance, FCB 94.4 and the

series inrush inductance LB= 2.86 mH. The inrush current frequency as was calculated and

described in Chapter 4 (Section 4.2.2).

To reduce harmful transient overvoltages produced during capacitor bank switching

operations, controlled switching is employed on the PASS MO circuit breaker for both opening

and closing operations. In energising the capacitor bank, circuit breaker closing is synchronised

to occur at a zero voltage crossing. While in de-energising, circuit breaker opening is done with

adequate arcing time before the first current zero to avoid reignitions or restrikes. As described in

Chapter 2 (Section 2.3.1) short arcing time gives higher risk of reignition or restrike.

Figure 5.1 Three-phase voltage waveforms and controlled closing points for a Capacitor Bank

Figure 5.2 Three-phase current waveforms and controlled opening points for a Capacitor Bank

Current Interruption at zero with 3.3ms

between phases

I

t

Zero Voltage crossing with 6.6ms

Between phases

V

t





Figure 5.1 shows control closing time used by Powerlink for a grounded-neutral

capacitor bank at Blackwall substation. The circuit breaker is closed at zero voltage crossing

with 6.6ms interval between poles. Figure 5.2 shows control opening time. The current

interruption occurs at first current zeros with 3.3ms interval between poles. A portable recorder

owned by Powerlink was used to provide secondary voltage and secondary current waveforms

available from the monitoring system built into the PASS MO circuit breaker during switching .

5.2 TEST AND MEASUREMENT SET UP

Figure 5.3 Measuring equipment layout for tests at Blackwall

Figure 5.3 shows the measuring equipment layout. The measuring equipment consists of

3 passive antenna, 1 active antenna, 2 digital oscilloscopes and a portable power supply. The 3

USB HSD

3.5 FDD RS-232

YOKOGAWA DL9240

AGILENT 54624

Active Antenna

3X Passive Antenna


arrestor

12V Battery @ 250AH


B/B

SERIES REACTOR

CAPACITOR BANK

CTPASS CB

275 kV busbar





passive antennas were connected to Agilent 54624 oscilloscope. The Agilent 54624 was used to

record the lower speed transients using 20ms/div with sampling rate of 10MSa/s. The active

antenna was connected to the 4th channel of Agilent 54624 for triggering and to act as a High

frequency marker. Conventional amplitude triggering of the scope was used.

The active antenna was also connected to Yokogawa DL9240 oscilloscope. The

Yokogawa DL9240 was used to record high speed transients using 1s/div with sampling rate of

10GSa/s. The high speed transient is then stored in the oscilloscope memory using the History

Function.

Figure 5.4 Capacitor Bank Installation

Figure 5.5 Recording Instrumentation

Figure 5.4 shows photograph of the circuit breaker and capacitor bank installation. To

the right of the photo is the capacitor bank. Next to the capacitor bank is the reactor used to limit

Passive Antenna

Agilent 54624

Yokogawa DL9240

Single point earthing connection to substation earth

Circuit Breaker





the inrush current effect. The MO Pass Circuit breakers are after the reactors. It is a dead tank

circuit breaker where the current interruption is done inside an earthed interrupting chamber. To

the left of the circuit breaker is the conductor leading to the 275kV busbar. The passive antennas

were located closed to the circuit breaker as shown in the circle. At the far end is the Capacitor

bank no.6. The 3 passive antennas are circled. They are located on the ground on the substation

gravel below the overhead conductors connecting the 3 circuit breaker terminals to the 275kV

substation busbar.

Figure 5.5 shows photograph of the recording instrumentation located inside the

measuring vehicle. The oscilloscopes were positioned next to each other. The portable power

supply is located behind the 2 oscilloscopes. All the equipments were connected to a single point

earth on the vehicle. The single point earth is then connected from the measuring vehicle to the

substation earth grid.

5.3 SUMMARY OF TESTS/MEASUREMENT CARRIED OUT

Series of circuit breaker closing and opening tests were made on capacitor bank 5.

Positions of the 3 passive antennas and active antenna were varied during the e tests. Table 5.1

shows a summary of tests carried out. Only selected results are displayed and discussed. After

evaluating the results, it was concluded that the HF pulse measured by the active antenna and

recorded on the high-speed oscilloscope was very difficult to correlate with the low speed

transient recorded. This is due to the sampling rate and the random nature of the arc produce

during opening which will be mentioned later. However, the low speed transient measurements

seemed to contain considerable information.

Table 5.1 Summary of tests conducted at Blackwall on 7th August 2007

EVENT DESCRIPTION


Background measurement by the 3 Passive antenna and Active antenna.

7HVW��±�&ORVLQJ� $OO�SDVVLYH�DQWHQQDV�ORFDWHG�RQ�WKH�VRXUFH�EXVEDU�VLGH�RI�WKH�FLUFXLW�breaker. Active antenna was located just outside the measuring vehicle.

3 HF events were recorded on the lower speed scope.

7HVW��±�2SHQLQJ� $OO�SDVVLYH�DQWHQQDV�ORFDWHG on the source/busbar side of the circuit breaker. Active antenna was located just outside the measuring





vehicle.

No significant transient or voltage changes recorded.

7HVW��±�&ORVLQJ� ��3DVVLYH�DQWHQQD�ORFDWHG�RQ�ORDG�FDSDFLWRU�EDQN�VLGH�XQGHU�SKDVH�A and B and 1 Passive antenna at the source/busbar side under phase A.


7HVW��±�2SHQLQJ� $OO�SDVVLYH�DQWHQQDV�ORFDWed on the load/capacitor bank side of the circuit breaker. Active antenna was located just outside the measuring vehicle.

Significant voltage changes recorded.

7HVW��±�&ORVLQJ� $OO�SDVVLYH�DQWHQQDV�ORFDWed on the load/capacitor bank side of the circuit breaker.

Active antenna was located under phase conductor B at 3.5m from B phase CB.

3HF events were recorded on the lower speed scope.

7HVW��±�2SHQLQJ� $OO�DQWHQQDV�ZLWK�VDPH�ORFDWLRQ�DV�LQ�7HVW��


7HVW��±�&ORVLQJ� $OO�SDVVLYH�DQWHQQDV�ORFDWed on the load/capacitor bank side of the circuit breaker.

Active antenna was located at B phase CB inspection window


7HVW��±�2SHQLQJ� $OO�DQWHQQDV�ZLWK�VDPH�ORFDWLRQ�DV�LQ�7HVW��


Only selected results are discussed in this thesis. Background measurements are

discussed first followed by results of closing tests, i.e. Test 5 and Test 3, and opening test of

capacitor bank circuit breaker i.e Test 8.

5.4 BACKGROUND MEASUREMENT

Figure 5.6 shows position of the 3 passive antennas and active antenna when

background measurements were carried out.

Figure 5.7 shows the 50 Hz waveforms measured during background measurement

before switching on of the capacitor bank . Passive antennas 1, 2 and 3 were connected to CH1,





CH2 and CH3 respectively of the oscilloscope. It can be seen that the 3 waveforms have same

50Hz frequency, but they have different magnitudes and the phase angle difference between

them is not 1200. The magnitude of each waveform is determined by the position of the antenna

hence the capacitive coupling effect.

Figure 5.6 Plan view of antenna positions at Capacitor Bank installation during background measurement

PASS Switchgear

Control Building

123

Capacitor Bank 6 Capacitor Bank 5

Capacitor Bank

Series Reactor

ABC

CapacitiveCoupling Antenna

Active Antenna

13m

4.0m

2.5m





Figure 5.7 Waveform from Background measurement

CH1 has the biggest magnitude as antenna 1 is experiencing strong capacitive coupling

effect from phase A hence indicating phase A. CH2 for antenna 2 being in the centre position,

phase B, has the lowest magnitude as antenna 2 is experiencing coupling effect from all 3

phases. Hence, indicating phase B. CH3 has higher magnitude than CH2 but lower than CH1 as

antenna 3 is experiencing strong coupling effect from phase C but also coupling from difference

phases from adjacent bay i.e. Cap Bank 6. Hence, indicating phase C.

Table 5.2 Voltage measured by each Passive antenna during background measurement

Antenna Measured Vpeak

(Load, R = 500 k)

PA1 10V

PA2 7V

PA3 7.5V

AB

C





5.5 CAPACITOR BANK CLOSING

Figure 5.8 Plan view of the antenna positions for Test 5

Result for closing capacitor bank for Test 5 is discussed. Figure 5.8 shows the position

of the 3 passive antennas and active antenna for THVW��±�&ORVLQJ��$OO�WKH�SDVVLYH�DQWHQQDV�ZHUH�

located at the capacitor side. Passive antenna 1 (PA1) was located under phase A, PA2 under

phase C and PA3 under phase B. The active antenna was located on the busbar side under phase

B.

Figure 5.9 shows the waveform recorded during closing of capacitor bank 5 circuit

breaker. From the three passive antennas, the inrush current effect during closing can be clearly

seen on the voltage waveform. The voltage waveform increased indicating CB closing and point

of current flowing with transient oscillation followed by steady state waveform.

ABC

12 3

Capacitor Bank 5

Capacitor

Series Reactor

PASS CBCapacitive

Coupling Antenna

Active Antenna





Figure 5.9 Waveforms captured during CB close operation for Test 5

The instant of individual phase closing could be determined by the comparing the

magnitude of voltage detected by the passive antennas. Passive antenna that detected the largest

voltage magnitude represents the closing phase. In this test, during the first pole closing, antenna

1 under phase A detected the highest voltage compared to antenna 3 and 2 located under phase B

and C respectively. For the second pole closing, antenna 3 under phase B has the highest

magnitude compared to the other 2 antennas thus indicating phase B closing. This was followed

by phase C closing, where antenna 3 under phase C has the highest voltage magnitude.

Therefore, by comparing the passive antennas signal the sequence of CB pole closing could be

determined. The closing sequence for this test is A-B-C.

1V

10 ms

X2X1

PA1 under Phase A

PA3 under Phase B

PA2 under Phase C

Pulses from

Active antenna

@ 50mV/div





The active antenna picked up the HF pulses or HF markers. The HF markers are in line

with the starting of voltage transient on the passive antennas. The time difference between the 3

HF markers is indicated by X1 and X2. Both X1 and X2 were measured to be approximately

6.6ms. X1 and X2 indicate the timing between pole closing.

Test 5 -Close

6.6ms6.6ms

1

APhase

APhase

BPhase

BPhase

CPhase

CPhase

Time

)LJXUH��:DYHIRUPV�FDSWXUHG�RQ�3RZHUOLQN¶V�SRUWDEOH�UHFRUGHU�IRU�7HVW��

Figure 5.10 shows the voltage and current ZDYHIRUP�IURP�WKH�3RZHUOLQN¶V�SRUWDEOH�

recorder. The voltage waveforms are indicative of the busbar voltage whilst the current

waveforms represent the capacitor bank current. It can be seen that CB was closed at zero

voltage crossing causing minimal inrush current to flow into the capacitor bank. The timing

between each pole closing is 6.6ms and the pole sequence is A-B-C.

The waveforms from all the antennas gave same timing difference and pole sequence as

ZLWK�ZDYHIRUP�IURP�3RZHUOLQN¶V�SRUWDEOH�UHFRrder. They are summarised in Table 5.3.





Table 5.3 Summary of CB timing and pole sequence for capacitor bank closing Test 5

X1

ms

X2

ms

Sequence

Measured from antennas

waveform

6.6 6.6 A-B-C

Powerlink waveform 6.6 6.6 A-B-C

From Figures 5.9 and 5.10 it can be seen that the voltages recorded on the PA are a

composite of the three-phase voltages with the magnitude of the component of each phase

depending on the distance of the PA from the phase busbar. For example in Figure 5.9 in the

region X1 the phase A voltage is very similar to the start of the phase A voltage in Figure 5.10.

The phase B voltage in the region X1 is lower than that in phase A and the phase C voltage is

less than that in phase B: this is to be expected because the distances from phase A to the A

phase PA, the B phase PA and the C phase PA are each greater than the preceding value. It can

be seen that the shape of the voltage waveforms in Figure 5.9 are very similar to the current

waveforms in Figure 5.10. This was due to the shunt resistance connected across the output of

the passive antenna introducing a phase shift of almost 90 degrees in the output voltage signal.

The pulses of the AA are superimposed on each phase voltage in Figure 5.9 and provide

accurate identification of each pole closure. Timing of the closing operation may be obtained

directly from Figure 5.9 and values X1 and X2 are comparable with similar measurements from

Figure 5.10.

5.6 IMPERFECT CAPACITOR BANK CLOSING

Result for closing onto the capacitor bank for Test 3 is discussed in this Section. Figure

5.11 shows the position of the 3 passive antennDV�DQG�DFWLYH�DQWHQQD�IRU�7HVW��±�&ORVLQJ��

Passive antenna 1 and 3 were located at the capacitor side while passive antenna 2 was located at

the busbar side. Passive antenna 1 (PA1) was located under phase A, PA2 under phase A and

PA3 under phase B. The active antenna was located on the busbar side close to measuring

vehicle.






Figure 5.12 shows the voltage and current ZDYHIRUP�IURP�WKH�3RZHUOLQN¶V�SRUWDEOH�

recorder. From the voltage waveform for phase B , it can be seen that CB was closed prior to

zero voltage crossing. This caused big inrush current to flow into the capacitor bank as indicated

by the current waveform on phase B. The inrush current also caused a voltage dip on the B phase

busbar voltage. Similar occurrence can be seen on phase C and phase A. The timing between

each pole closing is 6.6ms and the pole sequence is B-C-A.


Active Antenna

ABC

1

2

3

Capacitor Bank 5

Capacitor

Series Reactor

PASS CB






Figure 5.13 shows the waveform recorded by the passive antennas and active antenna

during closing of capacitor bank. Only waveforms for passive antenna 1, passive antenna 3 and

active antenna were shown. Waveform for passive antenna 2 was not shown because passive

antenna 2 was located on the busbar side as in figure 5.12 hence the waveform detected by

passive antenna 2 could not be compared reliably with waveforms from passive antenna 1 and

passive antenna 3.

On waveforms recorded on passive antennas 1and 3, the inrush current effect during

closing can be clearly seen on the voltage waveform. The sudden voltage increase or voltage

spike on antenna indicated the CB pole closing and point of current flowing with transient

oscillation followed by steady state waveform. The transient oscillation in this test is more severe

when compared to transient oscillation detected in test 5. This is due to the incorrect zero voltage

crossing closing which was shown in Figure 5.12. The magnitude of the 50 Hz components in

Figure 5.13 looks smaller than in Figure 5.9 because waveforms in Figure 5.13 were measured

using 5V/div while waveforms in Figure 5.9 were measured using 1V/div.

C Phase

C Phase

B Phase

A Phase

B Phase

A Phase

Test 3 - Close Current

Voltage

6.6 ms 6.6 ms





Figure 5.13 Waveforms captured during CB close operation for Test 3

Top trace 50mV/div: Second trace 5V/div: Third trace 50 mV/div: Bottom trace 5V/div,

In this test, by comparing the voltage magnitude at point of closing the pole closing

sequence could be determined. Antenna 3 under phase B was the 1st to close as it has higher

transient voltage compared to Antenna 1. Antenna 1 under phase A was the last to close as it has

the higher voltage magnitude than antenna 3. The closing sequence for this test is B-C-A.

The active antenna picked up the HF pulses or HF markers. The HF markers are in line

with the starting of voltage transients from the passive antennas. The time difference between the

3 HF markers is indicated by X1 and X2. Both X1 and X2 were measured to be approximately

6.6ms. X1 and X2 indicate the timing between pole closing.

The waveforms from all the antennas gave the same timing difference and pole sequence

DV�ZLWK�ZDYHIRUP�IURP�3RZHUOLQN¶V�SRUWDEOH�UHFRUGer. They are summarised in Table 5.4. From

results in Table 5.3 and 5.4, in both tests the circuit breaker was controlled close at 6.6ms

interval with similar pole sequence. The 6.6ms interval may suggest correct zero voltage

PA1 under Phase A

PA3 under Phase B X1

X2

Pulses from

Active

antenna @

50mV/div

5V/div

10ms/div





crossing closing between phases. However for test 3, by looking at the transient waveforms

detected by the passive antenna it can be said that the closing was not done exactly at zero

voltage crossing.

Table 5.4 Summary of CB timing and pole sequence for capacitor bank closing Test 3

X1

ms

X2

ms

Sequence


waveform

6.6 6.6 B-C-A

Powerlink waveform 6.6 6.6 B-C-A

5.7 CAPACITOR BANK OPENING

Result for capacitor bank opening for Test 8 is discussed in this section. Figure 5.14

shows the position of the 3 passive antennas and DFWLYH�DQWHQQD�IRU�7HVW��±�2SHQLQJ��3DVVLYH�

antenna 1 and 3 were located at the capacitor side while passive antenna 2 was located at the

busbar side. Passive antenna 1 (PA1) was located under phase A, PA2 under phase A and PA3

under phase B. The active antenna was located on the busbar side close to measuring vehicle.


ABC

12 3

Capacitor Bank 5

Capacitor

Series Reactor

PASS CB Capacitive Coupling Antenna

Active Antenna





Figure 5.15 shows the voltage and current�ZDYHIRUP�IURP�3RZHUOLQN¶V�SRUWDEOH�

recorder. From the current waveform for phase A , it can be seen that phase A was the first to

open and current was interrupted at first current zero. At current zero, the busbar voltage for

phase B is at maximum. Similar occurrence can be seen on phase C and phase B. The timing

between each current interruption is 3.3ms and the pole sequence is A-C-B.

Figure 5.16 shows the waveform recorded during opening of capacitor bank. In this the

passive antennas showed in each phase the recorded voltage had initially an approximately 50

Hz waveform but after a time there appears to be a sudden reduction in this 50 Hz value due to

current interruption. We term the apparent instant of the change in the waveform magnitude as

the inflection points. The instant of pole opening can be approximately determined by

determining the inflection points on the waveforms. This is rather difficult but the approximate

inflection point for each voltage waveform is indicated by the arrow in Figure 5.16. It is clear

that more accurate determinations of the inflection points could be made using advanced signal

processing techniques on digitised waveforms. Unfortunately only the analogue signals were

recorded due to limitations of equipment available at the time that tests were done. It was not

possible to save these signals.

In this test, during the first pole opening, antenna 1 under phase A detected the largest

voltage drop compared to antenna 3 and 2 located under phase B and C respectively. For the

second pole opening, antenna 2 under phase C has the largest voltage drop compared to the other

2 antennas thus indicating phase C opening. This was followed by phase B opening, where

antenna 3 under phase B has the largest voltage drop. Therefore, by comparing the passive

antennas signal the sequence of CB pole closing could be determined. The closing sequence for

this test is A-C-B.





)LJXUH��:DYHIRUPV�FDSWXUHG�RQ�3RZHUOLQN¶V�SRUWDEOH�UHFRUGHU�IRU�7HVW��2SHQ��

As mentioned before, the instant of current interruption between poles could be

determined by looking at the inflection point on the waveforms. The inflection point for each

voltage waveform is indicated by the arrows in Figure 5.16. The time difference between the

inflection points are indicated by X1 and X2 which is also the time between zero current

interruption. X1 was measured to be approximately at 3.4ms and X2 was measured to be

approximately 3.6ms. The inflection points identified are thought to have errors between ±0.5ms

from the actual current interruption point. The error is to represent inaccuracies when identifying

the inflection point using only the diagram. A more precise technique would be to digitise the

record and carry out analysis using digital signal processing.

Ic

Ib

Ia

Vc

Vb

Va

Test 8 - Opening

Current

Supply Voltage





The waveforms from all the antennas gave similar timing difference and pole opening

sequence as those determined from waveform�PHDVXUHG�E\�3RZHUOLQN¶V�SRUWDEOH�UHFRUGHU��

They are compared in Table 5.5.

Table 5.5 Summary of CB timing and pole sequence for capacitor bank Test 8

X1

ms

X2

ms

Sequence


waveform

3.4 3.6 A-C-B

Powerlink waveform 3.3 3.3 A-C-B

The active antenna picked up the HF pulses as shown in Figure 5.16. The HF pulses on

opening seemed to be occurring in more random manner as compared to HF pulses detected

during closing. It is also noticed that the HF pulses are concentrated in three groups . These

groups occur just before phase A current interruption (A Open), just before phase C current

interruption (C Open) and just before phase B current interruption (B Open). It is interesting to

note that HF signals were also detected by all passive antennas before individual phase current

interruption. This shows that the HF signals are also coupled to the passive antennas. In the area

before phase A current interruption, the highest magnitude of HF pulses detected by the active

antenna correspond to the highest magnitude HF signals detected by passive antenna 1 located at

phase A. Similar observation can be made for area phase C and phase B. . At this stage it is

difficult to correlate the HF pulses detected by the active antenna and the low frequency signals

detected by the passive antennas. The HF signals detected during pole opening may be related to

the arcing aftercontact parting, arc generation in the interrupter chamber and final current

interruption at current zero. In a 3 phase switching, these HF events may overlap with one

another making it difficult to differentiate the HF events occurring in each pole opening.





Figure 5.16 Waveforms captured during CB open operation for Test 8

Inflection points (current interruption) shown by arrows.

X1X2

2 ms

1V

Phase A

Phase B

Phase C

A Open B OpenC Open

200mV





From all the four Capacitor Bank opening tests carried out, none produced any re-

strikes. It is thought, on the basis of measurements made at the Ergon laboratory in Chapter 4 of

this thesis that when a restrike does occur, the magnitude of current and transient voltage would

be sufficiently large to give very distinct indication of the existence of the restrike on the low

frequency transient passive antenna signals and corresponding HF pulses from the active

antenna.

In general, opening of capacitor bank circuit breakers generally does not lead to any

significant switching transients. The major reason is that circuit breakers are designed to have

very low risk of restrikes upon interruption of capacitive current. Application of controlled

switching should further reduce the small statistical risk that a re-strike may still occur. This is

achieved by the circuit breaker being controlled in such a manner that short arcing times are

avoided.

5.8 SUMMARY ON CAPACITOR BANK SWITCHING TESTS


Closing operations were successfully detected using the active antenna and passive

antennas. Pulses from the active antenna act as HF markers and give clear closing times. The

passive antennas give clear indication of the instant of pole closing, timing differences and the

pole sequence. The timing information by the antennas could be used to further determine the

circuit breaker pre-arcing time. This can be achieved by synchronising the coil energisation time

with the recording instrument. The closing time could be determined from the circuit breaker

timing test result, alternatively more accurate timing could be obtained via precision CB

auxiliary contacts. The make time is the time from coil energising until the current making which

is indicated clearly by the antenna signal. The pre-arcing time is simply the difference between

the closing time and the make time.

Results from capacitor bank closing on Test 3 and Test 5 indicate that this technique

could also be used to check the control switching relay effectiveness during zero voltage closing.

In Test 5, the transient oscillation detected upon CB closing is smooth as compared to transient

oscillation in Test 3. The transient oscillation detected by the passive antennas in test 3 indicated

that closing was not properly done at zero voltage crossing.






Opening operations were not as successfully measured as in closing operations. The HF

pulses detected by the active antenna do not provide clear current interruption instants. The HF

markers show consistent indication of arc disturbance before current interruption in each phase.

The HF signals detected may be related to the arcing contact parting, arc generation in the

interrupter chamber and final current interruption at current zero.

The passive antennas give indications of the instant of current interruption, the timing

difference between poles and the pole sequence. The current interruption could be determined

from the inflection point on the waveform recorded by the passive antenna. More precision on

the interruption timing could be achieved by applying digital signal processing (DSP) e.g.

Wavelet Transforms. At this stage it is difficult to correlate the HF pulses s detected by the active

antenna and the low-frequency transients detected by the passive antennas.

The opening operations carried out did not produce any re-strikes. It is thought that

when a restrike occurs, the magnitude of current and voltage transients involved would be

sufficiently large to give similar results to those recorded in this thesis during closing operations.



Chapter 6: Measurement of Shunt Reactor Bank Switching


6Chapter 6: Measurement of Shunt Reactor

Bank Switching

Field measurements were carried out on three phase Shunt Reactor Bank switching at

3RZHUOLQN�4XHHQVODQG¶V�%UDHPDU�6XEVWDWLRQ�RQ��st August 2007. The purpose of the test, site

details, test arrangements and results for the field measurement are described in this chapter.

Purpose of the site measurement

The objectives of the field measurement carried out at Braemar substation are:-

To investigate the application of broadband active antenna and three capacitive

coupling antenna (Passive antenna-PA) in detecting re-ignition and restriking

transient that may be produced during 3 phase shunt reactor bank. switching

To measure transient and waveforms produced during 3 phase shunt reactor bank

switching.

To identify effectiveness on improvements made in antennas, measuring technique

and recording systems.

6.1 SITE ARRANGEMENT

Measurement was carried out at 275kV, Braemar Substation. The substation is

interconnected with other 275kV substations via 275kV overhead lines. Braemar substation has

Static VAR Compensator (SVC) installation. There are two 275 kV Shunt Reactor with a rating

of 35MVAR each. The shunt reactors are connected to the 275kV overhead lines leading to

Tarong substation. Only one shunt reactor bank is currently in service and this shunt reactor was

used for the switching test.

The monitored CB is similar to the type used for capacitor bank switching in Chapter 5.

The circuit breaker is used for reactive switching of directly grounded shunt reactor bank and the

circuit breaker is operated daily.

The shunt reactor bank is rated at 35MVar. The rated capacitance was given to be,

nFCB 863.2 and the rated reactance, HLB 85.8 . It is a three phase unit in a single tank..





The load oscillation frequency is given by equation 2.4.4.1 in Chapter 2 and can be

calculated as follows:

BBL CLf21

Hence, HzfL 1000

Controlled switching is employed on the circuit breaker for both opening and closing

operation of the shunt rector to reduce switching transients. In energising the shunt reactor bank,

in order to give the smallest inrush current the circuit breaker closing is done at maximum power

frequency voltage. While in de-energising, circuit breaker opening is done with an adequate

arcing time before the first current zero to avoid reignitions or restrikes. As described in Chapter

2 (Section 2.4.1) short arcing time gives high risk of reignition or restrike.

Figure 6.1 Three-phase voltage waveforms and controlled closing points for a Shunt Reactor Bank

Figure 6.2 Three-phase current waveforms and controlled opening points for Shunt Reactor Bank

Maximum Voltage with 3.3ms Between phases

V

t

Current Interruption at zero with 3.3ms between phases

V

t





Figure 6.1 shows the controlled closing time strategy used by Powerlink for the directly

grounded shunt reactor bank at Braemar substation. The circuit breaker is closed at maximum

voltage with 3.3ms interval between poles. Figure 6.2 shows the controlled opening time. The

current interruption occurs at first current zero with 3.3ms interval between poles. A portable

recorder owned by Powerlink was used to record secondary voltage and secondary current

waveforms during switching.

6.2 TEST AND MEASUREMENT SET UP

Figure 6.3 Measuring equipment layout at Braemar substation

Figure 6.3 shows the measuring equipment layout. The measuring equipment consists

of 3 passive antenna, 1 active antenna, 1 digital oscilloscopes and a portable power supply. The

three passive antennas and active antenna were connected to the Yokogawa DL9240

oscilloscope which was used to record the low speed transients using 10ms/div with sampling

rate of 25MSa/s. The active antenna was used to trigger the Yokogawa oscilloscope.

Conventional amplitude triggering of the scope was used.

USB HSDYOKOGAWA DL9240


arrestor

12V Battery @ 250AH


O/H

3X Passive Antenna

Active Antenna

SHUNT REACTOR

SACB





The oscilloscope stored 2.5Mpts per triggered event. The file is saved in .wdf binary

format for fast analysis in Yokogawa oscilloscope. It can also be exported to PC in .csv format

for further analysis. The oscilloscope was powered by a portable power supply. All the

measuring equipment were located inside a vehicle and all the equipment were earthed through a

common point from the vehicle to the substation earth grid.

Figure 6.4 Shunt Reactor Installation

Figure 6.5 PASS MO Circuit Breaker

Shunt reactor

Ø AØ B

Ø C

Surge arresters

Circuit breakerVehicle for measuring

equipment Post Insulators





Figure 6.6 Passive and active antennas location

Figure 6.4 shows photograph of the circuit breaker and shunt reactor bank installation.

To the left of the picture is the 30MVar Shunt Reactor. It is a 3 phase oil immersed reactor. To

the right of the reactor are the surge arrestors to protect the reactor from overvoltage. On the

right side of the photograph is the Pass MO Circuit breaker. The conductor phases are shown

next to the CB bushing. Figure 6.5 shows side view of the PASS MO circuit breaker. It is a dead

tank circuit breaker where the current interruption is done inside an earthed interrupting

chamber. The hydraulic mechanism housing is located on the left side of the circuit breaker. The

right side of the circuit breaker bushing is connected to the 275kV overhead/busbar while the left

side bushing is connected to the post insulator, surge arresters and shunt reactor. Connections are

made using aluminium conductors. Figure 6.6 shows the location of the 3 passive antennas.

They were located under the circuit breaker conductor as shown in the circle. The nearest passive

antenna in the photo is located at phase C while the furthest passive antenna is located at phase

A. The active antenna is located close to phase C.

6.3 SUMMARY OF TESTS/MEASUREMENT CARRIED OUT

A series of closing and opening of the circuit breakers for shunt reactor bank were

carried out. The positions of the three passive antennas and active antenna were fixed throughout

the tests. Table 6.1 shows a summary of tests carried out. Background measurement would be

discussed first followed by results on closing tests i.e. Test 4 and opening test of shunt reactor

bank circuit breaker i.e. Test 7.

Passive antenna

Active antenna





Table 6.1 Summary of tests conducted at Braemar substation on 21st August 2007

EVENT DESCRIPTION


Background measurement by the 3 Passive antenna and Active antenna. Circuit breaker in closed position and reactor energised.

7HVW��±�2SHQLQJ� /&�WUDQVLHQW�RVFLOODWLRQV detected however the magnitude exceeded the oscilloscope scale. Each CH1, CH2 and CH3 was paralleled with 1M Ohm resistor.

7HVW��±�&ORVLQJ� ��GLVWLQFW�+)�HYHQW�WUDQVLHQW�UHFRUGHG��

7HVW��±�2SHQLQJ� $GGLWLRQDO�YROWDJH�UHGXFtion was added and it was observed that the magnitude of the transient oscillation was still big.

CH1 and CH3 further paralleled with 100K ohm resistor and CH2 with 10K resistor.

7HVW��±�&ORVLQJ� ��GLVWLQFW�+)�HYHQW�ZLWK�WUDQVLHQW�UHFRUGHG��

7HVW��±�2SHQLQJ� $GGLWLRQDO�YROWDJH�UHGXFtion was further added and it was observed that the magnitude of the transient oscillation recorded was suitable for analysis.

CH1 and CH2 paralleled with 10K, CH3 with 100K.

7HVW��±�&ORVLQJ� 1RW�WULJJHUHG�DV�WKH�SRUWDEOH�SRZHU�VXSSO\�ZDV�UXQQLQJ�ORZ�RQ�power.

7HVW��±�2SHQLQJ� /&�WUDQVLHQW�RVFLOODWLRQV�UHFRUGHG��

On circuit breaker opening, tt was found that the transient load oscillation produced

considerably large magnitude. In order to be able to measure the signal produced, the signal was

scaled down. This was achieved by connecting resistor in parallel with each channel. For Test 7,

CH1 and CH2 were paralleled with a 10K resistor. CH3 was paralleled with a 100K resistor.

The signals were recorded digitally for further analysis.

6.4 BACKGROUND MEASUREMENT

Figure 6.7 shows the positions of the 3 passive antennas and the active antenna This

arrangement was used for the all closing and opening tests.





Figure 6.7 Plan view of the antenna positions at shunt reactor installation

Passive antenna 1 was located under phase A, passive antenna 2 under phase B, passive

antenna 3 under phase C and active antenna located close to phase C. Passive antennas 1,2 and 3

were connected to CH1, CH2, and CH3 of the oscilloscope respectively. The active antenna was

connected to the CH4.

Figure 6.8 shows the waveforms measured with the 3 passive antennas during

background measurement. It can be seen that the three waveforms have same 50Hz frequency,

different magnitudes and the phase differences between them are not exactly 1200. The

magnitude of each waveform and the phase angle is determined by the position of the antenna

which determines the capacitive coupling effect to the conductors.

PA3 has the biggest magnitude it is experiencing strong capacitive coupling effect from

phase C. PA2 being in the centre position has the lowest magnitude as it is experiencing net

coupling effect from all 3 phases. PA1 has higher magnitude than PA2 and almost the same as

PA3 as PA1 is experiencing strong coupling effect from phase A.

CBA

31 2

Reactor Bank

Shunt Reactor

PASS CB


Active Antenna





Figure 6.8 Waveform on Background measurement

Table 6.2 Voltage measured by each Passive antenna during background measurement

Antenna Measured Vpeak

(Load, R = 500 k)

PA1 19.04V

PA2 8.32V

PA3 20.85V

6.5 SHUNT REACTOR BANK CLOSING

Results for Test 4 (closing shunt reactor bank) is discussed. Figure 6.9 shows the voltage

DQG�FXUUHQW�ZDYHIRUPV�IURP�WKH�3RZHUOLQN¶V�portable recorder. The voltage waveforms

represent the busbar voltages whilst the current waveforms represent the shunt reactor bank

currents. It can be seen that CB was closed at maximum voltage causing minimal inrush current

to flow into the capacitor bank. The timing between each pole closing is 3.3ms and the pole

closing sequence is B-A-C.

1 3

2





Figure 6.10 shows the waveform recorded during closing of the shunt reactor bank. The

first pole closing occurred at 30ms. This was followed by closing of another 2 poles. The closing

operation caused some transient oscillation followed by the steady state 50 Hz oscillation which

is similar to the background measurement.


Va

Ib

Vc

Ia

Vb

Ic

A Close

B Close

C Close

Test 4 - Close





Figure 6.10 Waveforms captured by PA 1,2 and 3 during CB closing operation for Test 4

Figure 6.11 shows the closing signals detected by each antenna. All passive antennas

showed similar starting point and time difference between pulses. The active antenna picked up

the HF pulses (HF markers). The HF markers are in line with the starting of voltage transient on

the passive antennas.

The time difference between the 3 HF markers is indicated by X1 and X2. Both X1 and

X2 were measured to be approximately 3.3ms. X1 and X2 indicate the timing between pole

closing. At this stage, it is not possible to determine the sequence of closing. In order to

determine the sequence of closing, it is best to analyse the closing pulse at each closing.

Steady state at 50 Hz





Figure 6.11 Waveforms captured during CB close operation in Test 4 by each antenna

X1 X2

PA1 under phase A

PA2 under phase B

PA3 under phase C

AA under phase C





Figure 6.12 Comparison of voltage magnitude of closing pulses at each closing event

Figure 6.12 shows the zoom-in-view of the closing pulses detected by each passive

antenna at each closing instant. At first closing, by comparing the magnitude of waveforms on

each passive antenna phase B has the highest magnitude. At second closing, phase A has the

highest magnitude and at the third closing, phase C has the highest magnitude. By comparing the

magnitude of waveforms on each antenna at the individual closing pulse, the pole sequence

could be determined. The highest magnitude at each closing instant indicate the closing phase.

Hence, the closing sequence was B-A-C.

1st closing

2nd closing

B phase

(PA2)

A phase

(PA1)

3rd closing

C phase

(PA3)





The waveforms from all the antennas gave slight difference in closing but similar pole

VHTXHQFH�DV�ZLWK�ZDYHIRUP�IURP�3RZHUOLQN¶V�SRUWDble recorder. They are summarised in table

6.3.

Table 6.3 Summary of CB timing and pole sequence for shunt reactor bank closing Test 4

X1

ms

X2

ms

Sequence


waveform

3.7 3.0 B-A-C

Powerlink waveform 3.3 3.3 B-A-C





6.6 SHUNT REACTOR BANK OPENING

Results for Test 7 (reactor bank opening) is discussed here. Location of the antennas is

as per Figure 6.7. Figure 6.13 shows the voltage�DQG�FXUUHQW�ZDYHIRUPV�IURP�WKH�3RZHUOLQN¶V�

portable recorder. From the current waveform for phase B, it can be seen that CB phase B was

the first to open and current was interrupted at first current zero. At current zero, the busbar

voltage for phase B is at maximum. Similar occurrence can be seen on phase A and phase C. The

timing between each current interruption is 3.3ms and the pole sequence is B-A-C.


IaVa

VbIb

VcIc

Current Interruption

at A


at B


at C

Test 7 - Open





Figure 6.14 Waveforms captured during CB open operation in Test 7

Figure 6.14 shows the waveforms recorded by the passive antennas during opening of

shunt reactor bank. CH1, CH2 and CH3 represent passive antennas 1, 2 and 3 respectively.

Passive antennas 1,2 and 3 were located under phase A, B and C, respectively. Referring to the

opening waveforms recorded on CH2, the opening process could be clearly divided into 3 stages.

The 1st stage is before pole opening or pre-current interruption which is before 20ms, where no

Oscillation at 1kHz

Beating on voltage oscillation


PA1 under phase A

PA2 under phase B

PA3 under phase C

Pre Current

Interruption





transient was observed at this stage. The 2nd stage is the current interruption which occurred from

20ms to 33ms. In this stage, the 3 phase CB poles started to open, causing arc to be drawn

followed by current interruption at or near current zero. This process occurred on all 3 phases

and explained the disturbances observed on the waveforms. The 3rd stage is the transient voltage

oscillation of the reactor. The oscillation detected by the passive antennas started from 33ms and

last until more than 100ms. The frequency of the oscillation was 1.03 kHz. This oscillation is

also known as load side oscillation as described in Chapter 2 (Section 2.4.4). The oscillation

waveform in Figure 6.14 does not look sinusoidal due to resolution of the graph produced.

On the waveform recorded on CH3, beating on the voltage oscillation could be clearly

seen. Beating is due to coupling between individual phases. For a three phase unit reactor (in one

tank), beating is considerable as the phase to phase coupling is significant. The oscillation

frequency of each phase may differ slightly from each other.

Figure 6.15 shows a close up view of the opening waveforms from 18ms to 38ms. It

shows more detailed information on the current interruption region. The transient recovery

oscillation generated after the 1st current interruption could be clearly seen at 26ms on all three

graphs. This was followed by another oscillation superimposed on the earlier transient oscillation

at around 29.3ms, followed by another oscillation superimposed at around 32.7ms. Each starting

instant of the oscillations can represent current interruption at zero or current chopping of phase.

The current interruption instant of each CB pole could be determined by comparing the

magnitude of voltage on each CH at the starting of oscillation. The CH with the largest

magnitude of voltage represents the current interruption phase. In this test, during the first

current interruption, CH2 detected the highest voltage compared to CH1 and CH3, indicating it

was phase B. For the 2nd current interruption, CH 1 detected the highest voltage compared to

CH2 and CH3 hence indicating phase A. This was followed by interruption on phase C, where

CH3 has the highest voltage. The opening sequence for this test is thus B-A-C.





Figure 6.15 Waveforms captured during CB open operation by each antenna

As mentioned before, the instant of current interruption could be determined by

comparing the voltage magnitude at the start of each oscillation. The time difference between the

current interruption points can be easily determined and they are indicated by X1 and X2 (e.g. on

CH3 waveform, Figure 6.15). X1 was measured to be approximately at 3.3ms and X2 was

measured to be approximately 3.4ms.

1st open 2nd open 3rd open

X1 X2

PA1 under phase A

PA2 under phase B

PA3 under phase C

AA under phase C





The waveforms from the passive antennas gave similar timing differences and pole

VHTXHQFH�DV�ZLWK�WKH�ZDYHIRUP�IURP�3RZHUOLQN¶s portable recorder. They are summarised in

Table 6.4.

Table 6.4 Summary of CB timing and pole sequence for shunt reactor bank opening Test 7

X1

ms

X2

ms

Sequence


waveform

3.3 3.4 B-C-A

Powerlink waveform 3.3 3.3 B-C-A

The active antenna (CH4) picked up the HF pulses (HF markers) as shown in Figure

6.15. The HF markers on opening seemed to be occurring in more random manner as compared

to the HF markers detected during closing. It is also noticed that the HF markers are concentrated

within 3 areas. The areas are (1) area before phase B current interruption (B Open), (2) area

before phase A current interruption (A Open) and (3) area before phase C current interruption (C

Open). It is interesting to note that the HF signals were also detected by all passive antennas

before individual phase current interruption. This shows that the HF signals are also coupled to

the passive antennas. At this stage it is difficult to correlate the HF markers detected by the active

antenna and the low frequency signals detected by the passive antennas. The HF signals detected

during pole opening may be related to the arcing contact parting, arc generation in the interrupter

chamber and final current interruption at current zero. During a 3 phase switching, these HF

events may overlap with one another making it difficult to differentiate the HF markers

associated with each pole opening. It is also interesting to note that active antenna picked up

more HF pulses prior to reactor current interruption compared to capacitor bank current

interruption.

From Test 5 and Test 7 of the shunt reactor opening tests, none of them produced any

re-strike. Re-strikes with parasitic arcing if occurred would produce high frequency, low-

magnitude restriking current. This would be shown by discontinuities in the oscillatory

transients. These discontinuities should occur early in the oscillatory transients. It is thought that

when a restrike occurs, the magnitude of current and transient voltage would be considerably big

to give distinct changes on the low-frequency transient on the passive antennas signals as well as

the HF markers on the active antenna.





6.7 SUMMARY ON SHUNT REACTOR BANK SWITCHING TESTS

6.7.1 Background measurement

During the background measurement, the 3 Passive antennas capacitively coupled with

the three phase 50Hz voltages. The magnitude of each received waveform is different. The phase

angle difference between them is not 1200. The magnitude of each waveform and the phase

angle between them is determined by the position of the antenna because of the capacitive

coupling effect.


Closing operations were successfully detected using the active antenna and passive

antennas. During closing, the passive antennas and active antenna easily detected the inrush

current effect on each pole closing. The timing of the inrush current could be easily measured to

give the time difference between closing of the 3 phases. The sequence of the pole closing was

determined by identifying the highest voltage detected by the passive antennas at each closing

instant.


Opening operations were successfully measured by combining the waveforms from the

passive and active antennas. Opening sequence could be easily determined by comparing the

magnitude of oscillations produced after each current interruption. The passive antennas gave

indication on the instant of current interruption, the time difference between current interruption

and the pole opening sequence. The HF markers detected by the active antenna did not provide

clear current interruption instant but showed consistent indication of arc disturbance before

current interruption in each phase. The HF signals detected may be related to the arcing contact

parting, arc generation in the interrupter chamber and final current interruption at current zero.

The opening operations carried out did not produce any re-strike. It is thought that if a

restrike occurs, the magnitude of current and transient voltage involved would be considerably

big to give similar results as in closing operation.

During opening, the passive antennas detected large transient oscillations. The large

magnitude is due to the magnifying effect (high-pass characteristics) of the measurement system

A sample characteristics for the measuring system is shown in Figure 3.8 in Chapter 3 (Section





3.5.3) The high voltage amplitude of transient oscillation detected by the passive antennas

requires the signal to be reduced for measuring and recording purposes. In this test, the reduction

was achieved by putting a resistor in parallel with the oscilloscope input.



Chapter 7: Analysis of Results


7Chapter 7: Analysis of Results

This chapter presents the three-phase capacitive coupling model and analyses the results

of field measurements. For the three-phase capacitive coupling model, the effect of distance and

coupling capacitance is discussed. For the field measurement results, analysis is carried out to

determine the arcing characteristics during shunt reactor opening. A time-frequency domain

analysis of the shunt reactor opening is also presented.

7.1 THREE-PHASE CAPACITIVE COUPLING MODEL

In Chapters 5 and 6, the background measurements for the capacitor bank at Blackwall

substation and shunt reactor bank at Braemar substation were discussed. It was observed in both

cases, the location and distance of the passive antennas from the phase conductors had great

effect in the waveforms received. The background waveforms measured indicated the magnitude

and phase angle of the waveforms were affected by the distance of the passive antenna from the

high voltage conductors: hence the capacitances from the HV conductors to the passive antennas

are an important parameter.

The distances between the passive antennas at Blackwall and Braemar substations were

not symmetrical as each substation has a different physical layout. Hence, the coupling

capacitances would have different values. This explains the difference in magnitude and phase

angle of the waveforms recorded at Blackwall and Braemar substations. In Chapter 3, it was

VKRZQ�WKDW�WKH�SDVVLYH�DQWHQQD¶V�WUDQVIHU�Iunction for a single phase capacitive coupling at low

frequency is determined by C1, (the capacitance between the phase conductors and passive

antenna) and R, (the resistance in parallel with the input of the measurement oscilloscopes) .

Resistance,R could be pre-determined prior to measurements whereas capacitance C1 needs to

be measured or derived to enable a full model to be constructed for the measurement system..

A three-phase capacitive coupling model is used to show the importance of coupling

capacitance when measuring three-phase voltages at low frequency. Figure 7.1 shows all

relevant capacitive coupling between phase conductors and passive antennas.





A simple model assumes that the distances between the antennas and phase conductors

are symmetrically disposed about the centre conductor, as shown in Figure 7.1. Consider the

passive antenna, PA1. C1A is the capacitance between it and phase A directly above the antenna.

C1B is the capacitance between it and phase B and C1C is for phase C furthest phase from PA1.

C1A has the strongest capacitance followed by C1B and C1C. Here, the capacitances are assumed

to be inversely proportional to distances. Capacitances for the remaining antennas are as shown

in Figure 7.1.


A B C

PA2

L L

C1A C1B

C1C

C2B

C3C

C2A C2C

C3B

C3A

V1 V2 V3

PA1 PA3

H

Figure 7.1 Capacitances between passive antennas and three phase conductors with symmetrical spacings

In this simplified model, C1A has the same value as C2B and C3C The capacitances C1B ,C2A,C2C and C3B are the same. Finally, C1C=C3A.

The capacitive coupling of three-phase to any antenna can be represented by an

equivalent circuit as shown in Figure 7.2. At low frequency e.g. 50Hz, the transfer function for

single phase capacitive coupling is given by Eq. 3.6. Capacitance Cr has no effect on the output.





VA

VB

VC

C1A

C1B

C1C V1RCR

Figure 7.2 Equivalent circuit for passive antenna at location 1 measuring three phase voltages.

Provided that V1 is extremely small compared to the phase voltages in Figure 7.2 the

currents flowing into R are a function of only the phase voltages and the coupling capacitances.

The voltage V1 can however vary as the total current through R changes. Under these conditions

it can be shown that, for PA1, the voltage contribution from each phase is coupled through

capacitance C1A, C1B and C1C giving a resultant voltage

)( 1111 CCBBAA VCVCVCRjV (7.1)

Similar equations are applied to the remaining two passive antennas with their respective

capacitances, giving

)( 2222 CCBBAA VCVCVCRjV (7.2)

)3333 CCBBAA VCVCVCRjV (7.3)

The above equations give the output voltage of each passive antenna.

Using the measured voltage from each passive antenna and by reversing the technique in

determining the output voltage, the supply voltage could be determined. The supply voltage

could be reconstructed using the equation below:-

RjCV

CV

CV

VAAA

A3

3

2

2

1

1 (7.4)

Similar equation is used to reconstruct the supply voltage of the remaining two phases.





RjCV

CV

CV

VBBB

B3

3

2

2

1

1 (7.5)

RjCV

CV

CVV

CCCC

3

3

2

2

1

1 (7.6)

A Matlab program was written to simulate the capacitive coupling model incorporating

Equation 7.1 to 7.6. The program assumes (a) the highest capacitance, CP is associated with the

antenna located directly beneath the phase conductor, and (b) the capacitance is inversely

proportional to the distance from another phase conductor to the passive antenna. For simplicity,

CP is chosen to be 1 pF. Temperature and atmospheric pressure effects are neglected. The model

calculates the voltages picked up by the passive antennas and reconstructs the phase voltages by

reversing the calculation.

The parameters used for the simulation are:

Highest capacitance, Cp = 1 pF

Measurement resistance, R = 0.5 M

System frequency, f = 50 Hz

Phase to ground voltage (peak) = 275kV/3*1.4142 Vpeak = 224.5 kV

Horizontal distance, L = 4 m

Vertical distance, H = 3 m





Case 1 – Simulation with symmetrical distances between passive antennas and reconstruction of phase voltages with no error introduced to the capacitance values

Figure 7.3 Output waveforms for Case 1

Figure 7.3 shows output waveforms from simulation for case 1. The first plot (a) shows

the original waveforms of phase A, B and C representing the three-phase voltage supply. The

peak voltage is 224.5 kV and the phase difference between all adjacent waveforms is 6.6 ms

(1200).

The second plot (b) shows waveforms calculated at the passive antennas 1,2 and 3. PA1

and PA3 have a peak voltage of 20 volts whilst PA2 has the lowest peak voltage of 14 volts. The

PAs waveforms are shifted; the phase difference between PA1 and PA2 is approximately 5.5 ms

(990), PA2 to PA3 is approximately 5.4 ms (97.20) and PA3 to PA1 is approximately 9.1 ms

(1630). The waveforms generated are similar to the result of the background measurement at

Braemar substation (in Chapter 6). Table 7.1 shows the calculated results and measured results

from Braemar substation and the differences in percentage.

AB C

PA3 PA2

PA1

(a)

(b)

(c)





Table 7.1 Calculated results and measured values from Braemar substation

Calculated

waveform

Measured values from

Braemar

Difference

s

PA1 20.0 19.04 4.8%

PA2 14.1 8.32 40.1%

PA3 20.0 20.85 4.25%

Phase Difference PA1-

PA2

5.5ms 5.8ms 5.45%


PA3

5.4ms 5.2ms 3.8%


PA1

9.1ms 9.0ms 1%

The third plot (c) shows the reconstruction of the phase voltage. Using the passive

antennas voltages and assuming similar capacitances values as before, the phase voltage is

reconstructed. The reconstructed voltage is exactly the same as the original waveforms of plot

(a).

Case 2 – Simulation with unsymmetrical distances between passive antennas

CapacitiveCoupling Antenna

A B C

PA2

L L

C1A

C1B

C1C

C2B

C3C

C2A C2C

C3B

C3A

V1 V2 V3

S

PA1 PA3

H





Figure 7.4 Capacitances between passive antennas and three phase conductors with unsymmetrical distances

This case looks into the effect of varying the passive antenna location on the waveforms

recorded by the passive antennas. Referring to Figure 7.4, PA1 was shifted 2 meter away from

its previous position. Figure 7.5 shows the simulation output waveforms.

Comparing with plot (b) of Figure 7.3, it can be seen that the peak voltage picked up by

PA1 has decreased from 20 to 17.1 volts. PA2 and PA3, as expected do not show any change to

the peak voltage picked up. The reduction in voltage picked up by PA1 indicates that the

increased distance reduces the coupling capacitances, hence reducing the coupling voltages. It is

also worth noting that the phase differences are also affected. The phase difference between PA1

and PA2 is altered to approximately 5.8 ms (104.40) and PA3 to PA1 to approximately 9.1 ms

(1630). No changes on the phase difference between PA2 to PA3 (which is still approximately

5.4 ms (97.20)).


PA1 PA2

PA3

(a)

(b)





Case 3 – Simulation on reconstructing phase voltages with 10% error introduced to the capacitance values

This case looks into the accuracy in reconstructing the phase voltages using the passive

antenna voltages. A -10% error was introduced on capacitance C1A, C1B, C1C. Figure 7.6 shows

output waveforms from simulation for case 3. Table 7.2 shows summarised differences between

the original waveforms and the reconstructed waveforms. From the results, phase A has the

largest reconstructed voltage error at 9.2% and phase difference error of 2-2.5%. This is expected

as C1A has the strongest capacitive coupling to phase A compared with C1B to phase B and C1C to

phase C.


A simple model for three phase capacitive divider using passive antenna has been

developed. It gives reasonably good indication of voltages coupled to the passive antennas.

However, the model relies on the value of capacitances between phase conductors and passive

ACB

(a)

(b)





antennas. The capacitance values used in this model are only estimations with the assumption

that capacitance is inversely proportional to distance without considering any other factors.

Table 7.2 Differences between original waveform and reconstructed waveforms with 10% error on

capacitances

Original waveform Reconstructed waveform Error

Phase A 224.5 kV 245.1 kV 9.2%

Phase B 224.5 kV 226.7 kV 1%

Phase C 224.5 kV 224.3 kV 0.09%

Phase Difference A-B 1200 122.40 2%

Phase Difference B-C 1200 120.60 0.5%

Phase Difference C-A 1200 1170 2.5%

In order to achieve better accuracy, capacitances between the passive antennas and the

phase conductors need to be determined as accurate as possible. This would require access to

3D em field calculation software which was not available to the author during the period of the

research project.

7.2 OBSERVATIONS OF ARCING SIGNALS IN SHUNT REACTOR OPENING

Arcing signals can be indicated by the HF pulses detected by the active antenna (AA).

During capacitor bank opening, the amount of arcing signals detected by the AA is low

compared to arcing signals detected during shunt reactor opening. Figure 7.7 shows the pulses

detected by the AA during capacitor bank opening as described in Section 4.8. The arrows

indicate the current interruptions which are in line with the inflection points mentioned in

Chapter 5 (Section 5.7)





Figure 7.7 Waveforms recorded by Active antenna on capacitor bank opening

Shunt reactor opening produces more arcing signals from the AA than capacitor bank

opening. Figure 7.8 shows the arcing signals detected by the AA during controlled shunt reactor

opening as described in Chapter 6. It is hypothesised that the increase of arcing signals detected

indicates the presence of instability oscillations in switching arcs prior to current interruption

during shunt reactor opening. The regions of instability are indicated by the three dotted ellipses

in Figure 7.8. The arcing signals from the AA consist of three clearly-discernible groups of

pulses with different magnitudes in each group. The density of pulses in the groups appears to

increase towards the point of final current interruption in each phase. The points of interruption

determined from PA waveforms are indicated by arrows in Figure 7.8.

2 ms/div

200mV/div

B phase current interruption

C phase current interruption

A phase current interruption








Figure 7.8 Waveforms recorded by Active antenna on shunt reactor opening during Test 7

The waveforms above were recorded in digital form and stored as time and voltage

arrays. This data was then analysed in MATLAB. The low level signals (noise) and the low-

frequency load oscillations (to the right in Figure 7.8) were first removed from the waveform. By

graphical inspection, the noise level was estimated to be between -0.03 V and 0.03 V. The final

current interruption occurs at 32.62 ms. Figure 7.9 shows the AA waveform with the noise and

load oscillations removed.

Figure. 7.9 Test 7 - AA signals without noise and load oscillation

The cumulative energy of the pulses within a sliding window is plotted against time. The

cumulative energy of the signal is defined by2V . A sliding window with a width of 0.5 ms,

stepped at 0.1 ms interval, was used. Figure 7.10 shows a plot of cumulative energy against time

for the waveforms in Figure 7.9. It is, however, difficult to interpret the plot. Hence another

method was used to analyse the HF pulses.





Figure. 7.10 Test 7 - Cumulative energy against time

The second approach was to look at the number of pulses with respect to time. This

approach gives the pulse density. It is done by converting the signals into pulses with equal

magnitude. The pulses are then plotted against time as shown in Figure 7.11. From Figure 7.11,

it can be seen qualitatively that the pulse density is high at each current interruption (Refer

Figure 7.8).

Cumulative pulse counts within a sliding window are then plotted with respect to time.

The cumulative pulses is given by pulses. A sliding window with a time width of 0.5 ms

stepped at 0.1 ms interval was used to show the cumulative pulses with time. Figure 7.12 shows

plot of cumulative pulses against time. The plot indicates the high pulses concentration area has a

similar shape to the results shown in Figure 7.9. These areas appear to correspond to the three

current interruption events.





Figure 7.11 Test 7 - Density of Pulses with respect to time

Figure 7.12 Test 7 - Cumulative pulses against time

B phase arcing area




A phase arcing area

C phase arcing area








Figure 7.12 shows the current interruption of pole C has the highest pulse density.

Pulses before current interruptions could be due to arc instability oscillations prior to chopping

and the smaller blocks of pulses appearing after each opening could be due to transient

oscillations in the network produced by the current interruptions.

Shunt Reactor Opening – Test 5

In order to verify the technique used earlier, similar analysis was done on results from

7HVW��±�VKXQW�UHDFWRU�RSHQLQJ��7KH�RSHQLQJ�VHTXHQce is B-C-A and the instants at which current

interruption occur in Figure 7.13 are 25 ms, 28.2 ms and 31.7 ms respectively.

Figure 7.13 Test 5 - AA signals without noise and load oscillation

Figure 7.13 shows the HF pulses recorded by the AA. The waveform is similar to that in

Figure 7.8. The regions of instability are indicated by the three areas. The arcing signals consist

of random pulses with different magnitudes and the density of pulses is high in the three areas.

Noise and load oscillations were then filtered out from the waveform as described in

Test 7 earlier. The cumulative pulses with respect to time is then plotted in Figure 7.14. The plot

indicates the areas of energy concentration are in line with those in Figure 7.13.








Figure 7.14 Test5 - Cumulative energy against time

The signals were then converted into pulses with equal magnitude. The pulses were

plotted against time as shown in Figure 7.15. From Figure 7.15, it can be seen that the density of

pulses is high at each current interruption. Windowed cumulative pulse counts was plotted next

using the technique described earlier. Figure 7.16 shows plot of cumulative pulses against time.

The plot indicates the pulses concentration which visually agrees with Figure 7.13. Figure 7.16

also shows three areas with high pulse density and these areas correspond with the three current

interruption events.

Comparing Figure 7.12 and Figure 7.16, it can be seen that both figures have similar

features e.g. three arcing areas. The pattern in each corresponding arcing area is similar although

the magnitudes are different. The steep negative slope after each peak indicates the end of arcing

and probably the point of current interruption as the slope ends at a time which is very close to

the given opening time. The width of the arcing time for each phase is comparable. In Figure

7.16, the widths of B, A, C and overall arcing duration are approximately 3.2 ms, 3.2 ms, 3.5 ms

and 11.9 ms respectively. In Figure 7.12, the widths of B, A, C and overall arcing duration are

approximately 3.8 ms, 3.5 ms, 3.4 ms and 13.1 ms respectively.





Figure 7.15 Test 5 - Density of Pulses with time

Figure 7.16 Test 5 - Cummulative pulses against time




B phase arcing area

A phase arcing area

C phase arcing area





Figure 7.17 RF Measurement showing arc signal UD, switch voltage Us and current Is (from[50])

Chapman [50] reported measurement of arcing time using RF frequency. Figure 7.17

shows the result on arcing measurement carried out by Chapman. The rising edge of the sensor

voltage indicates a sudden increase of RF energy due to ignition at contact separation. The signal

level increase again before current zero probably due to the cooling of the arc by the auto

puffer[50]. The moment of current zero can be identified by an increase of signal level followed

by abrupt disappearance of the RF signal [50].

7KH�SDWWHUQ�RI�&KDSPDQ¶V�ZDYHIRUP�VKRZQ�LQ Figure 7.17 is similar to the pattern of

our measured windowed cumulative pulse counts in Figure 7.12 and Figure 7.16. The arcing

area in Figure 7.12 and Figure 7.16 may indicate contact separation, auto puffer cooling action of

the arc and current zero interruption.

Chapman in his paper said that his method does not easily identify two simultaneous

switching-arc durations if the sensor is coupled to the signal of both arcs. He also said that his

method is not suited to distinguish between the simultaneous arcing signals of a three pole circuit

breaker without shielding of the radio-frequency electric field.

From the measured active antenna waveform, it is relatively easy to identify three areas

of arcing corresponding to the three poles. However, it is difficult to identify the point at which

the circuit breaker contact separates. This is due to the fact that the active antenna picked up all

HF pulses generated by all 3 phases during the contacts parting, arcing and current interruption.

Pulses from contact separation of the subsequent two poles are masked by the arcing pulses.






7.3 ANALYSIS IN TIME-FREQUENCY DOMAIN

In Chapters 4,5 and 6, observations and discussions were made on waveforms measured

in voltage-time domain. Information about inter-pole closing times, opening times and

closing/opening pole sequence can be obtained by analysing the voltage-time waveforms.

To obtain further information from the recorded results, the recorded waveforms were

analysed in the time-frequency domain. Data recorded during Test 7 for shunt reactor opening at

Braemar substation was used as an example for analysis in time-frequency domain.

The analysis was carried out to examine into the frequency content at opening event. In

Chapter 2 it was mentioned that a reignition during opening of a shunt reactor bank will cause an

oscillation with higher frequencies than the load oscillation. This frequency changes could be

more easily seen in the time-frequency domain. During shunt reactor opening, there were a lot of

HF spikes prior to current interruption as shown in Figure 7.8. These HF spikes may indicate

contact parting, HF arcing and/or reignition.

7.3.1 Analysing using Fast Fourier Transform (FFT)

FFT is commonly used to find the frequency components of a signal buried in a noisy

time domain signal. Waveform recorded during shunt reactor opening for PA1 was analysed by

applying FFT to give the frequency spectrum. Figure 7.18 shows (a) the opening waveform from

20ms to 50ms and (b) the frequency spectrum. The frequency spectrum in Figure 7.18 (b)

indicates that the opening waveform recorded is swamped by the 1 kHz signal which is the load

oscillation after the opening of the circuit breaker.





(a)

(b)

Figure 7.18 Reactor Opening Test 7 (a)Time domain plot of PA1 waveform for opening from 20 ms to 50 ms (b) Frequency content of waveform in (a).

7.3.2 Analysing using Short Time Fast Fourier Transform (ST FFT) Analysis

It was observed that there were a lot of HF activities at each current interruption

recorded. In order to analyse these HF activities, a ST FFT with sliding window was used. This

technique takes successive windowed of fast Fourier transforms to represent the time-changing

frequency characteristics of the waveforms. A Matlab program was developed to analyse the

opening waveforms using ST FFT. It gives a basic time-frequency domain representation.

Below are the parameters used in implementing ST FFT :

Duration of event to be analysed : 5 ms

Data sampling rate : 40 ns

Sliding window width : 0.1 ms

Interval time between windows : 0.05ms

No. of time bin : 100

1.03 kHz





The analysis was carried out at each individual opening. Each opening waveform

recorded by the passive antennas and active antenna was analysed. The duration of each opening

event analysed was 5ms. The first 3 ms looks into activities prior to interruption while the

remaining 2 ms looks into activities after interruption. The time-frequency relationship is then

shown on a contour plot. The contour plot for the PA used 200 frequency points while the

contour plot for the AA used 200 and 1000 frequency points. As the sliding window width was

0.1 ms, the frequency observed for this ST FFT analysis was 10 kHz and above. Limits were

introduced inside the Matlab programme to suppress the very low signals from being shown on

the ST FFT contour plot.

The analysis was focused on the passive antenna waveforms and active antenna

waveform during the phase opening. For example for the 1st opening, phase B was the opening

phase and occurred at 26 ms after the waveform was recorded (see Figure 6.15 in Chapter 6).

Hence, the ST FFT analysis focused on waveforms of PA2 and active antenna for times

between 23 ms and 28 ms. The analysis for ST FFT was carried out for the 1st opening.

Figure 7.19 (a) shows the opening waveform and (b) the ST FFT contour plot for

waveforms from PA2 during the 1st opening. At current interruption there was a significant

change on the frequency content. The oscillation after the current interruption is represented by a

high concentration of lower-frequency components.

Figure 7.20 (a) shows opening an waveform and (b) and (c) the corresponding ST FFT

contour plot for AA for 1st opening of Test 7. The HF spikes occurring in (a) can be observed in

(b) and (c). Both (b) and (c) indicate that the HF spikes in Figure 7.20 (a) have significant high

frequency components. In (c), some of the HF spikes contain strong 2 MHz and 8 MHz

component. This is probably due to some local parallel capacitance at the circuit breaker

discharging during the arcing process. At the instant of opening, no significant changes in the

frequency content was observed. In (b), the active antenna picked up a faint continuous 850 kHz

signal which probably comes from a local AM radio station.





Test 7 - 1st Opening – PA2

(a)

(b)

Figure 7.19 (a) Voltage-Time domain plot of waveforms from PA2 (b) ST FFT contour plot of PA2 waveforms for opening- from 23 ms to 28 ms

An analysis of 2nd opening and 3rd opening also gave similar findings as in the 1st

opening. The instant of opening could be easily determined by the sudden changes in the low

frequency content of the time-frequency plot for the passive antenna. This is similar to

determining the instant of opening from the sudden changes in the voltage magnitude of the time

domain plot.

In all opening events, it was observed from the ST FFT contour plots that the AA

detected HF spikes with frequency contents ranging from 2 MHz to 8 MHz. It also detected the

850 kHz signal in all the opening events. The PAs detected the low frequency component (1

kHz) strongly compared to the HF spikes.

current interruption

High

concentration of

lower frequency






Test 7 - 1st opening AA

(a)

(b)

(c)

Figure 7.20 (a) Voltage-Time domain plot of AA (b) ST FFT contour plot of AA (from 0-2MHz) (c) ST FFT contour plot of AA (from 0-10MHz) for opening from 23 ms to 28 ms

HF spikes

containing 8MHz

and 2MHz.


Clipped spikes

The AA picked up

800kHz






The technique used in carrying out the time-frequency domain analysis using ST FFT is

a simple technique. A fixed width sliding window was used to perform the ST FFT which

restricts flexibility when analysing high frequency and low frequency interchangeably. For

example, a 0.1 ms sliding window will look at frequencies higher than 10 kHz while a 1 ms

sliding window will give inaccuracy in giving the time-changing characteristics of the

waveform. The analysis of experimental results did not detect any reignition event as there were

no indications of corresponding frequency changes through out the opening sequence.

Using a more advanced technique, it may be able to differentiate the phase producing

the HF spikes. This could be achieved by comparing the magnitude of HF spike on each PA. The

highest magnitude of HF spike present on the PA should indicate the phase. Then, the HF signal

containing the HF spike for each phase could be reconstructed. However, problems would arise

when two spikes from different phases occur at the same time causing the PAs to pick up the

resultant signals in which case may be misleading.

For future works, more sophisticated digital signal processing techniques could be

applied to extract further information from the acquired waveforms and to provide automatic

recognition and quantification of features indicative of impeding circuit breaker failures. Wavelet

Transform has been used widely in the power community in recent years. Fernandez et al [59]

mentioned that wavelet transform is better suited for the analysis of certain types of transient

waveforms in power systems applications. It is also better than windowed ST FFT because it

does not have fixed width window function which is a limitation of ST FFT. Wavelet Transform

has been reported to have been used in assessing the condition of power transformer OLTC [60]

and in the diagnostics of circuit breaker condition [61]. In both reports Wavelet Transform was

used to analyse the vibration signals which have characteristics similar to CB switching

transients.



Chapter 8: Conclusion


8Chapter 8: Conclusion

Non-contact measurement techniques have been developed using capacitive coupling

antennas and active broadband antenna to monitor the electrical transients produced during

capacitor bank and shunt reactor bank switching. A capacitive coupling model was developed

for both for single and three phase measurements. The model indicated that the passive antenna

characteristics rely on the capacitances between phase conductors and the passive antenna, and

the resistance of the measuring devices. The capacitances between the passive antennas and earth

are also an important factor for high frequency signals.

Field measurements were carried out for capacitor bank switching at Blackwall

substation and shunt reactor bank switching at Braemar substation. Both substations are air

insulated substations. Switching transients were measured and recorded for analysis. High

frequency signals from the active antenna act as timing markers for the slow frequency transient

oscillations recorded by the passive antennas. The combined signals detected by both types of

antenna give a more complete picture of the switching events.

Results of field measurements showed that it is relatively easy to measure oscillatory

switching transients. The monitoring system can provide:

inter-pole closing times and the pole sequence for circuit breakers switching

capacitor banks and shunt reactor banks,

inter-pole current interruption times during shunt reactor switchings,

information on the accuracy of point-on-wave control switching equipment, and

information on pre-strike during CB closing.

The technique developed in this project is tKRXJKW�WR�EH�VXSHULRU�WR�0RRUH¶V�PHWKRG�>�@��

0RRUH¶V�PHWKRG�UHOLHV�RQ�GHWHFWLQJ�+)�LPSXOVHV�ZKLFK�FDQ�EH�DIIHFWHG�E\�WKH�SUHVHQFH�RI�

multiple HF pulses during opening of three-phase CB and potential interference from other

sources. By contrast, the technique used in this research uses the HF impulses as HF markers and

any information on a switching event is obtained from the low-frequecny transient recorded by

the passive antennas.

The significant advantages of the proposed measurement technique are that

measurements can be taken while the CB is energised. No physical connection to the high-





voltage system is required during site installation. The equipment is portable and can be easily

moved between different AIS circuit breakers. It is easy to set up and is not restricted to any

particular manufacturer of AIS circuit breakers.

Analysis on the arcing signals detected by the active antenna during shunt reactor

RSHQLQJ�ZDV�FDUULHG�RXW��7KH�SURSRVHG�³VOLGLQJ�ZLQGRZ�SXOVH�GHQVLW\�PHWKRG�GHPRQVWUDWHG�

that the cumulative pulse count with time could be used to indicate the arcing area during current

interruption. The duration of each arcing area may indicate the arcing duration of each phase and

the negative slope at the end of each arcing area indicates the point of current interruption. The

pattern of the arcing area is comparable to works done by other researchers.

The results from field measurements have demonstrated the usefulness of the technique

where CB switching transients can be measured and valuable information can be extracted from

the measurement.

Further Work

Further work is planned to improve the measuring equipment. Improvements for the

measuring system are given below:

Depending on measurement objective, the passive antenna transfer function should

be adjusted to give a suitable voltage gain and frequency response when measuring

the desired event. This is to ensure that signals level is measured within limits. This

is achieved by adjusting the resistance value of the measuring equipment and/or the

value of the capacitance parallel to the resistance.

Currently only one active antenna (AA) is used for the measuring system. Results

from measurement carried out suggest that having an AA located for each phase

would give more information on HF signals produced by circuit breakers during

switching.

The recording equipment could be improved by using a 4 channel digitiser or PC-

based data acquisition system. These equipments are smaller in size compared to

the oscilloscopes used. Provision should also be made for remote monitoring

function.

In this research, the effect of capacitive coupling between the PAs was not

observed in great detail. Capacitive coupling between the PAs also affects the





accuracy of the measurement. Better accuracy could be achieved by reducing

capacitive coupling between PA to conductors of other phases and between PA to

PA. An improved shape of capacitive coupling antenna possibly with shielding

could be developed to reduce capacitive coupling between other phases and PA.

Further investigations are also planned to increase the utility of this measurement

method and explore the potential of its technique. The following investigations will be looked

into:-

1) Measurement of circuit breaker inter-pole current interruption time during

capacitor bank opening: Currently, the interruption instant is determined

graphically by the voltage inflection on the passive antenna waveform. An

improved identification technique using DSP e.g. wavelet analysis on digitised

waveforms would be able to identify the inflection point accurately indicating the

point of current interruption.

2) Improvement in modelling the three phase capacitive divider model: complex

calculations are required to accurately model the three phase capacitive divider

model. The effects of capacitances on voltage magnitude, phase shift and phase

difference between phases need to be established. An accurate model could be used

to estimate transient voltage produced during switching or even replacing the

conventional voltage transformers. Further studies are planned in this area.

3) Monitoring of circuit breaker switching over period of time: the field measurement

carried out in this research was conducted as a series of tests with limited time

duration. This does not reflect the actual situation where the circuit breaker is

operated once or twice daily. Automatically monitoring circuit breakers switching

over a period of time would give better information on the circuit breaker

operations and could be readily realised using existing computer and

communications technology.

4) Measurement of overall CB operating time: the overall CB operating time can be

measured by taking reference signals from the circuit breaker. This can be done by

either measuring the circuit breaker coil current or using precision CB auxiliary

contacts. This method will require connection to the CB wiring.





5) Measurement of restrike or reignition: The field measurement did not detect any

restrike. A restrike or reignition if occur would produce sufficiently large transient

voltage change and reignition oscillation which can be detected by the passive

antenna. The reignition oscillation frequency could be easily extracted from the

passive antenna waveforms using DSP. The active antenna would detect the high

frequency pulses of the restriking current similar to during circuit breaker closing

indicating the instant of restriking. Further measurement is anticipated to be carried

out by Powerlink on circuit breakers identified to have problems with restriking.

6) Monitoring the accuracy of the point-on-wave control switching relay: it is known

that controlled switching application require accurate control of switching times

and need CB with stable operating times. The CB must also have high and stable

dynamic withstand capability between contacts. One inaccurate zero voltage

closing was observed during capacitor bank closing measurement. Further

measurements are required to establish the effectiveness of this technique in

monitoring the accuracy of the point-on-wave control switching. Measurements

could also be extended to circuit breakers with single control operation on closing

or opening i.e. circuit breakers with three pole operation.

7) Measurement of arc duration time: analysis of the arcing signature and arcing time

during shunt reactor opening was carried out in Chapter 7. It indicates similarities

with international research. The analysis could be improved by having three active

antennas to monitor each phase of circuit breaker. Further work is needed to

confirm the suggested improvement.




References

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[10] K. J. T. Boyd, "Circuit breaker performance on overhead line disconnection," School of

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Appendices

Appendix A: Site Measurement Procedure

Site Measurement Step by Step – Procedure

Purpose:

This document is to describe the procedure in carrying out measurement on background

signals and transient signals in Blackwall Substation. Allowable number of close and

opening operation to be finalised with Powerlink.

Reference:

Powerlink Safety Rules Oscilloscope Operation Manual

Related Document:

Antenna Details Measuring Equipment Layout Measurement settings

Responsibility:

QUT operator

General Requirement:

1.All measurement works are to be carried out with the presence of safety personnel from Powerlink.

2.All measuring equipment will be transported in the vehicle. 3.Vehicle is to be parked as close as possible to the equipment to be measured. 4.No equipment to be carry at a height more than 1.5 meter from the ground. 5.Operator to wear suitable Personal Protective Equipment (PPE) e.g. safety helmet and safety shoes.

6.Insulated rubber glove to be used during setting up. 7.To ensure equipment are earthed (temporarily/permanently) during measurement set up.




Procedures:

Setting up measuring equipment

1.0Connect the earthing connections. 1.1From Substation grid to the car 1.2From Car to UPS equipment. 1.3From UPS equipment to Measuring equipment via power cord. 1.4Connect the measuring co-ax cables to measuring equipment.

2.0Setting up Active Antenna 2.1Place the tripod at the measuring point. 2.2Mount the active antenna on the tripod 2.3Adjust the height of the tripod to give 40cm from the ground. 2.4No grounding connection.

3.0Setting up Passive Antenna 3.1Carefully lift the antenna base and place it at the measuring point. 3.2Connect the earth connection from the substation grid to the base plate. 3.3Carefully assemble the Passive antenna assembly to the base plate.

4.0Setting up and connecting to Measuring equipment in vehicle. 4.1The portable power supply unit is powered through 24V DC Battery. 4.2Connect the power supply cord of each oscilloscope to the Portable power supply unit.

4.3Run the antenna cable from Oscilloscope and connect to the Active Antenna.

4.4Switch on the Active Antenna. 4.5Repeat steps 4.3-4.4 for Passive antenna.

5.0Visual and physical inspection on connection.

Performing site measurement

1.0Measuring background signal in substation 1.1Switch on the Active antenna and oscilloscopes before carrying out any measurement.

1.2Measurements to be carried out in the switchyard with the HV equipment live.

1.3The operator will make the measurement from inside the vehicle. 1.4Record the background waveform.

2.0Measuring transient signal on 1st - closing operation of circuit breaker 2.1Switch on the Active antenna, oscilloscopes before carrying out any measurement.

2.2The operator shall go into the control room. 2.3Upon confirmation from safety personnel, Circuit Breaker shall be CLOSED.

2.4After the closing operation and confirmation from safety personnel, the operator shall go to the measuring vehicle and check the recorded waveform data.

2.5The operator download the data, make adjustment and then prepare for the next measurement.

3.0Measuring transient signal on 1st�±�RSHQLQJ�RSHUDWLRQ�RI�FLUFXLW�EUHDNHU��3.1Repeat steps 2.1 to 2.5 for Opening operation.




Records

Installation details – substation single line diagram, physical dimension and layout.Waveforms data. List of waveforms Tests forms – oscilloscope settings, triggering settings. Photos




Appendix B : Sample of forms used for site measurement

FORM 1 :




FORM 2:




Appendix C : Matlab Program for Capacitive Divider Model (In Chapter 7)

%*******************************%% 3-phase Voltage Monitor %% %%% File: VoltageMonitor02.m %% %% By: S.Ramli %% with assistance by T.Tang % % 26 May 2007 %%*******************************%

% This program calculates the signals picked up by 3 capacitive% sensors located at the base of each powerline phase%% The program then reconstruct the phase waveforms from the % received signals.

% A user can change the following parameters:% coupling capacitances: C1, C2 and C3 ASSUME CP = 1PF AND CAPACITANCE% INVERSELY PROP TO DISTANCE. I.E LARGER DISTANCE, SMALLER CAPACITANCE% distance PA1 : s% sensor loading resistance: R1% frequency of operation: f% system voltage: Vrms%

%

%**** BEGINNING OF PROGRAM ***clear;clc; % clear Command windowformat compact;

%%

% Define coupling capacitances & termination resistanceCp = 1.000e-12 % capacitance for phase directly above sensorR1 = 0.5e6; % 0.5 M-ohms

% Dimension for antenna locationl =4;h =3;s =2; % moving PA1, s=0 for symmetrical

% Define phase voltagesf = 50; % frequency is 50Hzw = 2*pi*f; % pre-calculated constantwR1 = w*R1; % pre-calculated constantVrms = 275e3/sqrt(3) % phase voltage (Vrms)VA = 1.4142*Vrms; % phase A voltage (volts)VB = 1.4142*Vrms; % phase B voltage (volts)VC = 1.4142*Vrms; % phase C voltage (volts)phiA = 0; % phase angle of Phase AphiB = phiA-2*pi/3; % phase angle of Phase BphiC = phiA+2*pi/3; % phase angle of Phase C




%% Antenna at Location 1

% For Middle Phase BB1= sqrt(h^2+(l+s)^2); % overall distance C1b = Cp*(h/B1);

% For Left Phase AA1 = sqrt(h^2+s^2); % overall distance C1a = Cp*(h/A1); % capacitance

% For Right Phase CC1 = sqrt(h^2 + (2*l+s)^2); % overall distance C1c = Cp*(h/C1); % capacitance


% For Middle Phase BB2=sqrt(h^2); % overall distance C2b=Cp*(h/B2);

% For Left Phase AA2 = sqrt(h^2+l^2); % overall distance C2a= Cp*(h/A2); % capacitance

% For Right Phase CC2 = sqrt(h^2+l^2); % overall distance C2c=Cp*(h/C2); % capacitance


% For Middle Phase BB3=sqrt(h^2+l^2); % overall distance C3b=Cp*(h/B3);

% For Left Phase AA3 = sqrt(h^2+(2*l)^2); % overall distance C3a= Cp*(h/A3); % capacitance

% For Right Phase CC3 = sqrt(h^2); % overall distance C3c=Cp*(h/C3); % capacitance

%% Generate original waveformststep = 0.10e-3; % step in time scale (s)tstop = 30e-3-tstep; % last pointN = tstop/tstep+1 % number of points (start with 0)t = (0:tstep:tstop); % simulation time rangevA = VA*exp(complex(0,w*t+phiA)); % vA(t)vB = VB*exp(complex(0,w*t+phiB)); % vB(t)vC = VC*exp(complex(0,w*t+phiC)); % vC(t)

%% Calculate sensor voltages v1, v2 and v3, due to C1, C2 and C3v1 = complex(0,wR1)*(C1a*vA+C1b*vB+C1c*vC);v2 = complex(0,wR1)*(C2a*vA+C2b*vB+C2c*vC);v3 = complex(0,wR1)*(C3a*vA+C3b*vB+C3c*vC);




%% Reconstruct phases A,B,C waveforms%***introducing errors to selected capacitance e.g. C1a, C1b, C1cC1a =C1a*0.9;C1b =C1b*0.9;C1c =C1c*0.9;C = [C1a C1b C1c; C2a C2b C2c; C3a C3b C3c] % Set up [C] matrix%Ce = C*0.95 % introducing errors to allCi = inv(C)vAA = (Ci(1,1)*v1+Ci(1,2)*v2+Ci(1,3)*v3)/complex(0,wR1);vBB = (Ci(2,1)*v1+Ci(2,2)*v2+Ci(2,3)*v3)/complex(0,wR1);vCC = (Ci(3,1)*v1+Ci(3,2)*v2+Ci(3,3)*v3)/complex(0,wR1);

%% Plotting waveforms

% PLOT OVERALL USING SUBPLOT%*** Plot original waveforms of phases A,B,Csubplot(3,1,1);plot(t,real(vA),'ro',t,real(vB),'bx',t,real(vC),'k*')grid%xlabel ('time(s)')ylabel ('volts')title ('Original Waveforms of Phases A,B,C')

%*** plot waveforms of sensors 1,2,3 (v1, v2 and v3)subplot(3,1,2);plot(t,real(v1),'ro',t,real(v2),'bx',t,real(v3),'k*')grid%xlabel ('time(s)')ylabel ('volts')title ('Waveforms of Passive Antenna 1,2,3')

%*** plot reconstructed waveforms of Phase A,B,Csubplot(3,1,3);plot(t,real(vAA),'ro',t,real(vBB),'bx',t,real(vCC),'k*')gridxlabel ('time(s)')ylabel ('volts')title ('Reconstructed Waveforms of Phases A,B,C')

%% ANALYSING EFFECT OF DISTANCE FOR CAPACITANCE

%*** Plot original waveforms of phases A,B,Cfiguresubplot(2,1,1);plot(t,real(vA),'ro',t,real(vB),'bx',t,real(vC),'k*')grid%xlabel ('time(s)')ylabel ('volts')title ('Original Waveforms of Phases A,B,C')

%*** plot waveforms of sensors 1,2,3 (v1, v2 and v3)subplot(2,1,2);plot(t,real(v1),'ro',t,real(v2),'bx',t,real(v3),'k*')gridxlabel ('time(s)')ylabel ('volts')title ('Waveforms of Passive Antenna 1,2,3 with S=2m')

%% ANALYSING RECONSTRUCTION OF PHASE VOLTAGE%*** Plot original waveforms of phases A,B,Cfiguresubplot(2,1,1);plot(t,real(vA),'ro',t,real(vB),'bx',t,real(vC),'k*')




grid%xlabel ('time(s)')ylabel ('volts')title ('Original Waveforms of Phases A,B,C')

%*** plot reconstructed waveforms of Phase A,B,C with 10% error on selected%capacitancesubplot(2,1,2);plot(t,real(vAA),'ro',t,real(vBB),'bx',t,real(vCC),'k*')gridxlabel ('time(s)')ylabel ('volts')title ('Reconstructed Waveforms of Phases A,B,C with 5% error in capacitance')

%%

%*** locate zero-crossings for reconstructed voltagefor i=2:N

if real(vAA(i-1))<0if real(vAA(i))>=0

ZeroA=i, real(vAA(i-1)), real(vAA(i))end

endif real(vBB(i-1))<0

if real(vBB(i))>=0 ZeroB=i, real(vBB(i-1)), real(vBB(i))

endendif real(vCC(i-1))<0

if real(vCC(i))>=0 ZeroC=i, real(vCC(i-1)), real(vCC(i))

endend

end

%%

%*** locate zero-crossings for sensor voltagefor i=2:N

if real(v1(i-1))<0if real(v1(i))>=0

Zero1P=i, real(v1(i-1)), real(v1(i))end

endif real(v1(i-1))>0

if real(v1(i))<=0 Zero1N=i, real(v1(i-1)), real(v1(i))

endendif real(v2(i-1))<0

if real(v2(i))>=0 Zero2P=i, real(v2(i-1)), real(v2(i))

endendif real(v2(i-1))>0


endendif real(v3(i-1))<0

if real(v3(i))>=0 Zero3P=i, real(v3(i-1)), real(v3(i))




endendif real(v3(i-1))>0


endend

end



thesis capacitor bank switching

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