5.high+voltage+generation+and+testing+2.0+updated+2012

BEX 44503 / BEK4113BEX 44503 / BEK4113BEX 44503 / BEK4113BEX 44503 / BEK4113High Voltage EngineeringHigh Voltage Engineering

Generation of High VoltageHigh Voltage EngineeringHigh Voltage Engineering

Generation of High Voltageg gg g

BEE 3243 Electric Power Systems – Module 1

Lecture PlanLecture Plan

ObjectivesObjectives

To provide student with the knowledge of the High Voltage generation

Also to introduce required testing techniques on insulation that used the generation equipments

2


Lecture PlanLecture PlanLearning OutcomesLearning Outcomes

At the end of this subject, the student should have the ability to adopt the knowledge as follows:

(1) Understand the HV generation types

(2) Plan high voltage measurement and testing works of high voltage electrical equipment

(3) Perform high voltage experiment setup or project to complete a specific measurement

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(4) Show concern to safety regulations in high voltage measurement and testing works


Lecture PlanLecture Plan

SyllabusSyllabusyy

•• Subject 2Subject 2 – Generation of High Voltages

•• Subject 3Subject 3 High Voltage Measurements•• Subject 3Subject 3 – High Voltage Measurementsand Diagnostic Testing of Insulation

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HV Generations : IntroductionHV Generations : Introduction

• High voltage is a field of engineering linking the scientific principles and industrial practicescientific principles and industrial practice.

• The major use of high voltage is in the area of power systemssystems.

• Increasing demand in power requires modern-system operates at higher voltageoperates at higher voltage.

• DC is used in long transmission lines and equipmentssuch as electron microscope, x-ray units, particlesp , y , paccelerator, electrostatic precipitators and etc.

• Power system equipments (e.g. transformer, circuit

5

y q p ( g ,breaker, insulators, bushings etc.), cables and overhead line with higher ac voltage ratings.


HV Generations: ProblemHV Generations: Problem

• Erecting the higher voltage levels exposingequipment with high voltage / current stressesequipment with high voltage / current stresses

• Major concern in insulating / separating these equipment from the potential and earthedequipment from the potential and earthedstructure / object

• The insulation provided must not only be able to• The insulation provided must not only be able to withstand the normal system voltage (with factor of safety) but also from abnormal overvoltages (i e imp lse stresses)(i.e. impulse stresses)

• In this case, the system exposes with high t it d f f

6

stresses magnitude from power frequency, lightning and switching overvoltages


HV Generations : What to do?HV Generations : What to do?

• Hence, these equipments need to be tested at higher voltages (normal and overvoltages levels) before they can actually put into use.

• It is therefore imperative that there mustbe high voltage generators available tobe high voltage generators available to test an insulation system.

7


**High Voltage**High Voltage GenerationsGenerations****High Voltage High Voltage GenerationsGenerations

8


HV Generations: TypesHV Generations: Types

The common types of High VoltageThe common types of High Voltage generators:

1)1) HVACHVAC GeneratorsGenerators

2)2) HVDCHVDC GeneratorsGenerators

3)3) ImpulseImpulse GeneratorsGenerators3)3) ImpulseImpulse GeneratorsGenerators

9


HVAC GeneratorHVAC Generator

• The principle of the generator is to generate an HVAC voltages. Uses transformer to step up the voltageup the voltage.

• They are three common types of HVACThey are three common types of HVAC generator (transformer) which are:

– Straight transformer– Cascaded transformer

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– Resonant transformer


HVAC Generator: General Characteristic of HVAC Generator: General Characteristic of Testing TransformersTesting TransformersTesting TransformersTesting Transformers

• Equipment used in power system is usually in three-phase type, however, a generated test voltage applied to the testing equipment /voltage applied to the testing equipment / sample is in single-phase.

• Thus, only one phase of the insulation equipment can be tested at a time.

• At same voltage rating, testing transformerstypically have a much lower kVA rating than th t f

11

the power transformers.


HVAC Generator: General Characteristic of HVAC Generator: General Characteristic of Testing Transformers ContinueTesting Transformers ContinueTesting Transformers ContinueTesting Transformers Continue

• This lower kVA rating is due to the ‘usual’ short gduration testing and smaller current produced(varies within tens of mA to few amperes).

• Thus, cooling of the windings will not be a major problem.

• Also the flux density in the testing transformer is kept lower than the power transformer. This to avoid high magnetising current that containavoid high magnetising current that contain harmonics that may distort the output test voltage.

T ti t f l t i d i

12

• Testing transformer also more compact in design with well insulated high-voltage windings.


Construction of Testing TransformersConstruction of Testing Transformers

• Using oil-insulated natural cooling test transformers.

• Usually is constructed as two types:

1) Tank type

2) Insulated enclosure type

13



14Fig: OilFig: Oil--insulated test transformer. (a) Tank type (b) Insulated enclosure type.insulated test transformer. (a) Tank type (b) Insulated enclosure type.


Construction of Testing TransformersConstruction of Testing Transformers1) Tank type:

The core and the windings are enclosed in a metal– The core and the windings are enclosed in a metal container, the surface which provides natural cooling.

– Problem with high voltages due to high cost of bushing and the large space requirement.

– May contain less quantity of oil than the insulated enclosure type, thus have small thermalconstants.

– Heat dissipation through insulated enclosure is high.

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high.

– Require bushing



2) Insulated enclosure type:

‒ The core and the winding are surrounded by an insulating cylinder.

‒ Contain a relatively large quantity of oil and so have large thermal time constants.g

‒ The heat dissipation through the insulated enclosure is small.

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‒ No bushing is required


HVAC GeneratorsHVAC Generators

The principle of the generator is to generate• The principle of the generator is to generate single-phase HVAC voltage. Uses transformer to step up the voltage.

• They are three common types of HVAC generator (transformer) which are:g ( )

1) Straight transformer

2) Cascaded transformer

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3) Resonant transformer


1) Straight Transformer: Concept1) Straight Transformer: Concept

• Concept similar to the normal step-up transformer.

• The transformer is having a single phase HV potential at one-end side winding and usually being earthed on the other end side windingthe other end-side winding.

• The iron-core and the enclosure / tank also being th dearthed.

• The low voltage winding maybe fed to the voltage g g y gregulator (variac) for producing variable high voltagelevel.

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• The rating of the primary winding is usually up to 230 V r.m.s and the high voltage output rating can be up to 200 kV r.m.s.


Straight Transformers: CircuitStraight Transformers: Circuit

(a)

(b) (c)

19Fig: HVAC straight transformer. (a) Single unit circuit (b) Equivalent electrical Fig: HVAC straight transformer. (a) Single unit circuit (b) Equivalent electrical circuit with a capacitive test object (c) Simplified circuit at highcircuit with a capacitive test object (c) Simplified circuit at high--voltagevoltage--sideside


HVAC Single Straight Transformer: Manchester UniversityManchester University

2012 kV HVAC Single Straight Oil12 kV HVAC Single Straight Oil--Immersed Transformer (Insulated Enclosure Type) Immersed Transformer (Insulated Enclosure Type)


100 kV TERCO HVAC & HVDC Set: UTHM

AC GeneratorAC Generator

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1) Straight Transformer: Problem1) Straight Transformer: Problem

• Caution must be taken when using a straight transformer. It is possible the use of the transformer with a particular loads poss b e t e use o t e t a s o e t a pa t cu a oadcapacitance will lead to a resonance condition being developed.

• In general, resonance occurs when the inductive reactanceand the capacitive reactance are of equal magnitude (ωL = 1/ωC), so: 1

• This resonance condition may result in a higher than LC1

required test voltage being applied across the test object with an obvious damaging result.

22

• Also, the resonance condition may not just accentuate the fundamental frequency of the supply voltage, it can also amplify harmonics causing distortion of output waveform.


Resonance Occurs in Straight TransformersResonance Occurs in Straight Transformers

Note:

23Fig: Illustration of resonance condition occurring in straight transformerFig: Illustration of resonance condition occurring in straight transformer

Note: LV resistance R = 0.25ohm, LV inductance L = 50mHLoad capacitance: 1000pF, Vi = 1kV


What happen during resonance?What happen during resonance?

• In RLC series circuit, during resonance XL = XC

• This causes applied V and I in-phase, giving VL=Vc

Giving Z = R (thus circuit totally dependable on R value)• Giving Z = R (thus circuit totally dependable on R value)

• This condition causes current at its maximum as I=V/R

• Since ω = 2πf, resonance occurs at frequency fr; 1

• This condition also causes voltage overshoot or

LCfr 2

1

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This condition also causes voltage overshoot or magnification/ magnified many times greater than the supply voltage (i.e. Vi).


What happen during resonance? What happen during resonance? Calculation Example:Calculation Example:Calculation Example:Calculation Example:

A i l h HVAC RLC i it i t fA single phase HVAC RLC circuit consist of inductance L of 50mH, capacitance C of 1000pF and resistance of R of 0.25ohm. The supplied voltage Vi is at 1kV r.m.s.

a)Determine the maximum current I the voltagea)Determine the maximum current Imax, the voltage overshoot VL and Q factor of the circuit during the resonance frequency fr condition. Neglect any losses in the circ itlosses in the circuit.

b)Find the Q factor of the circuit at 50Hz frequency

25

b)Find the Q factor of the circuit at 50Hz frequency.


What happen during resonance? What happen during resonance? Calculation Solution:Calculation Solution:Calculation Solution:Calculation Solution:

kVVohmRFpFC

HmHL

12501010001000

,10505012

3

resonanceAt1

;_kVVohmR i 1,25.0

HzfrAt 50 f

LCf

r

r

101000105021

21

123

VV

LfILIXIVV

HzfrAt

HCHL

HzrLHzCHzL

1050502104

2

,50_

335050

50_maxmaxmax50_50_

VVresonanceatAlso

kHzfr

;__

51.22

kVkV

VV

VV

Q

kVV

VV

i

o

i

HzLHzfactor

HzL

HzCHzL

83.62183.62

83.62

1050502104

50_50__

50_

50_50_

kAohm

kVRVI

RZVV

i

CL

425.01

max

upQ Hzfactor

ii

.6350__

MVVV

VV

LfILIXIVV

CL

rLCL

292810501051.222104

2333

maxmaxmax

Value similar to the one shown in the graph

26

ukpkV

MVVV

VVQ

MVVV

i

o

i

Lfrfactor

CL

.29.28129.28

29.28

_


Resonance Occurs in Straight TransformersResonance Occurs in Straight Transformers

Note:

27Fig: Illustration of resonance condition occurring in straight transformerFig: Illustration of resonance condition occurring in straight transformer

Note: LV resistance R = 0.25ohm, LV inductance L = 50mHLoad capacitance: 1000pF, Vi = 1kV


2) HVAC Cascaded Transformer: General 2) HVAC Cascaded Transformer: General

• Generating higher voltages requires largeg g g q ginsulation level placing inside the straighttransformer (cost increases rapidly with voltage)voltage).

• Typically to generate a voltage level above 200 kV i i i l d hkV, it is more economical to cascade more than one transformer in series.

• In this case, by insulating the tank of the second (and any subsequent) transformer from earth can significantly reduce the insulation

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earth, can significantly reduce the insulation level in this transformer (thus reduce the cost!).


2) HVAC Cascaded Transformer: Principle2) HVAC Cascaded Transformer: Principle

F h f th i l it t h i th• For each of the single unit stages having threewindings; the low-voltage, high-voltage and exciting windings.g g

• The low-voltage and high-voltage windings are follow the auto transformer concept whileare follow the auto-transformer concept while an exciting winding is used to supplies the low voltage potential to the next stage low-voltage

indingwinding.

• Illustrations of the cascaded transformer

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Illustrations of the cascaded transformerconcept can be seen in the next figures.



30Fig 1: An Illustration of Three Stage HVAC Cascaded Transformer Fig 1: An Illustration of Three Stage HVAC Cascaded Transformer



31Fig 2: Cascade arrangement for 300 kV Transformer Fig 2: Cascade arrangement for 300 kV Transformer


2) Cascaded Transformer Concept: Stage2) Cascaded Transformer Concept: Stage--II

StageStage--II

• The low-voltage winding is connected to the primary of the first transformer and its one-endp a y o t e st t a s o e a d ts o e e dterminal is connected to the earthed transformer tank.

• Also one-end of the high-voltage winding is earthed through the tank. The high-voltage outputfrom this first stage is V (i e 100 kV in Fig 2)from this first stage is V (i.e 100 kV in Fig. 2).

• The exciting winding of this stage supplies (i.e 1 kV i Fi 2) h l l i di f II

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kV in Fig. 2) the low-voltage winding of stage-II.


2) Cascaded Transformer Concept: Stage2) Cascaded Transformer Concept: Stage--IIII

StageStage--IIIIgg

• The high-voltage winding in this stage and that for the previous stage-I are connected in series so that voltage p g g2V is produced / available at the end of the high-voltagewinding of stage-II.

• i e 2 x 100 kV = 200 kV such as shown in Fig 2• i.e. 2 x 100 kV = 200 kV such as shown in Fig. 2

• The tank of the stage-II must be insulated (separated) from earth (bear in mind that tank in stage-1 is earthed)from earth (bear in mind that tank in stage 1 is earthed).

• This condition provides the HV output potential of 2V with respect to earth at one-end of the stage-II

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2V with respect to earth at one-end of the stage-IIhigh-voltage winding.


2) Cascaded Transformer Concept: Stage 2) Cascaded Transformer Concept: Stage -- IIIIII

StageStage--IIIIII

• Similar condition as stage-I, an exciting winding tapped from HV-winding supplies in stage-II tapped o d g supp es stagepotential to the low-winding of stage-III transformer.

• The high-voltage winding in this stage and that for the previous stage-II are connected in series so that voltage 3V is produced / available at the end of the high-voltage winding of stage-III.

• i.e. 3 x 100 kV = 300 kV such as shown in Fig. 2

• Similar as stage-II, the tank of the stage-III must be

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Similar as stage II, the tank of the stage III must be insulated (separated) from earth, to ensure voltage 3Vis obtained.


2) Cascaded Transformer Concept: Stage2) Cascaded Transformer Concept: Stage--IV, V and IV, V and so on (if any)so on (if any)so on (if any)so on (if any)

StageStage––IV, V and so on…(if any)IV, V and so on…(if any)gg ( y)( y)

• Processes / conditions follow the concepts shown in previous stages (stage-II and stage-III)in previous stages (stage-II and stage-III)

• The exciting winding tapped from HV-winding in th i t t f t ti lthe previous stage transformer ensures a potential is supplied to the low-winding of the next transformer.

• Also the tank of the every stage in this case and so onmust be insulated from earth, to ensure voltage increment of 1V in every stages with respect of earth

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increment of 1V in every stages with respect of earth (i.e 4V, 5V and so on) is obtained.


2) HVAC Cascaded Transformer2) HVAC Cascaded Transformer

36HVAC Cascaded Transformer HVAC Cascaded Transformer


3) HVAC Resonant Transformers: General 3) HVAC Resonant Transformers: General

Also kno n as series resonant set or series and• Also known as series resonant set or series and parallel resonant sets.

• These transformers are designed to overcome twoshortcoming straight transformer that are:

− Firstly, resonance is used to reduce the voltage supply demand at the primary winding of the t ftransformer

− Secondly, it also used to ensure that a pure 50 Hz is d li d f t f

37

delivered from a transformer


3) HVAC Resonant Transformers: General 3) HVAC Resonant Transformers: General ContinueContinueContinue Continue

In effect resonant set make used the one that• In effect, resonant set make used the one that previously stated as disadvantages (resonance condition) for the straight transformer set.

• Resonance condition in this transformer allows higher voltage to be produced using significantly lower currentthat would otherwise be neededthat would otherwise be needed.

• The development of fundamental frequency resonance condition in this transformer ensures a waveform low incondition in this transformer ensures a waveform low in total harmonics distortion.

Thi l d th i i t

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• This also reduces the maximum power requirement from the voltage winding of test supply.


3) HVAC Resonant Transformers: General 3) HVAC Resonant Transformers: General ContinueContinueContinue Continue

• Resonant alternating set can be produced using• Resonant alternating set can be produced using series or parallel inductances.

U ll th i bl t (X ) i d d• Usually, the variable reactance (XT) is produced using variable inductance (XLs) rated at low voltage with step-up transformer.

• More recent design, however, used high voltage variable reactors that have been designed withoutvariable reactors that have been designed without the need of step-up transformer.

• Typical circuit of this condition (reactor usage) is

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• Typical circuit of this condition (reactor usage) is shown in Fig. 3.


HVAC Resonant Generator Set: Circuit

(a)

(b)

(a)

40Fig 3: Typical schematic of a HVAC resonance set. (a) series resonance circuit (b) equivalent circuitFig 3: Typical schematic of a HVAC resonance set. (a) series resonance circuit (b) equivalent circuit


HVAC Resonant Generator Set: Manchester UniversityManchester University

41


HVDC GeneratorHVDC Generator

• The principle of the generator is to generate single-phase HVDC voltages Uses HVAC transformer forphase HVDC voltages. Uses HVAC transformer for main/primary supply, the HVAC voltage is then converted to HVDC via rectifier circuit.

• They are four common types of HVDC generatorcircuit which are:

– HalfHalf--periodperiod rectifier circuit–– FullFull--periodperiod rectifier circuit

C k ftC k ft W ltW lt D blD bl i it– CockcroftCockcroft--WaltonWalton Doubler Doubler circuit–– CockcroftCockcroft--WaltonWalton MultiplierMultiplier circuit

42

j|ÄÄ ux w|ávâááxw |Ç à{|á ÄxvàâÜxj|ÄÄ ux w|ávâááxw |Ç à{|á ÄxvàâÜx


1) HVDC: Half1) HVDC: Half--Period Rectifier CircuitPeriod Rectifier Circuit

• The concept similar to the normal single-phase half-wave / bridge rectifier circuit, such as shown in Fig. 4.

• This is a simplest and most basic rectifier circuit. It is adequate for large amount of testing and uses relatively simple component.

ConnectionConnection::A hi h lt t f ith d f th d• A high voltage transformer with one-end of earthed HV winding is connected to the test object via a capacitor.

• This capacitor C is used to reduce the ripple factor. Effect with and without this unit can be seen in Fig. 4.

43• A diode is connected at the high tension of the

terminal of the transformer. RL is the load resistance.


Fig. 4: Typical Half-wave Rectifier Circuit

(a) Half-wave rectifier circuit

(b)

44(b) No smoothing capacitor cause large ripple factor effect seen on load voltage

(c) With smoothing capacitor reduces ripple factor effect seen on load voltage


2) HVDC: Full2) HVDC: Full--Period Rectifier CircuitPeriod Rectifier Circuit

• The concept is similar to single-phase full-wave / bridge rectifier circuit such as shown in Fig 5bridge rectifier circuit, such as shown in Fig. 5.

• This is also among simplest and most basic rectifier circuit It is adequate for large amount of testing andcircuit. It is adequate for large amount of testing and uses relatively simple component.

ConnectionConnection: : • Both ends of high-voltage winding of the transformer

are connected with individual diode D1 and D2.

• Each of diode D1 and D2 conduct one cycle.

• In the circuit the ripple is reduced The ripple

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• In the circuit, the ripple is reduced. The ripple frequency is twice the value of the supply frequency.


Fig. 5: Typical Full-Period Rectifier Circuit

(b)

(a) Full-period rectification (b) Load voltage

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Demo of Rectifier Circuit


TERCO 100 kV r.m.s HVAC & 140 kVpeak HVDC Set: UTHMHVDC Set: UTHM


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3) Cockcroft3) Cockcroft--WaltonWalton Doubler Circuit: GeneralDoubler Circuit: General

• Both of the previously discussed full-periodand half-period rectifier circuits produce a DC voltage less than the AC maximum voltagevoltage less than the AC maximum voltage (peak).

Th d bl i it d DC lt• The doubler circuit produces a DC voltage almost double the peak AC voltage under the loaded condition.

• The circuit has been named after its creator Greinacher or also known as Cockcroft

49

Greinacher or also known as Cockcroft-Walton.


Fig. 6: Typical Cockcroft-Walton Doubler Circuit

(a) Cockcroft(a) Cockcroft--Walton doubler circuitWalton doubler circuit

(a) Simple Cockcroft(a) Simple Cockcroft--Walton doubler circuitWalton doubler circuit

50(b) Simple Cockcroft(b) Simple Cockcroft--Walton doubler circuitWalton doubler circuit



(a) Cockcroft(a) Cockcroft--Walton doubler circuitWalton doubler circuit

51(b) Voltage curve under no load(b) Voltage curve under no load (b) Voltage curve under load(b) Voltage curve under load


CC--WW Doubler Circuit Explanation: Doubler Circuit Explanation: No Loading ConditionNo Loading ConditionNo Loading ConditionNo Loading Condition

• One terminal O’ of high tensionOne terminal O of high tension (secondary) winding is earthed.

• When the potential O becomes• When the potential O becomes negative with respect to O’ (i.e. during ‘first’ negative half cycle of the voltage wave), the diode D’ conducts. ),

• This causes the portion O’-A-O in a half period rectifier circuit statehalf period rectifier circuit state.

• Also at this stage causes the capacitance C charges up to a

52

capacitance C, charges up to a voltage Vmax (i.e. the peak value of AC voltage)


CC--WW Doubler Circuit Explanation: Doubler Circuit Explanation: No Loading Condition Continue INo Loading Condition Continue INo Loading Condition Continue INo Loading Condition Continue I

• By neglecting the diode drop (e.g switch close) the potential of A whichswitch close), the potential of A, which VA is zero during conduction of D’.

The potential of O which is V• The potential of O, which is Voreaches –Vmax when the supply voltage is at its negative peak.

• Also by neglecting any loss of charge from C, its potential (VA to Vo) should always be equal to +V after thealways be equal to +Vmax after the instant X.

Hence V will oscillate between 0 to

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• Hence VA will oscillate between 0 to 2Vmax as Vo oscillates from –Vmax to +Vmax.


CC--WW Doubler Circuit Explanation: Doubler Circuit Explanation: No Loading Condition Continue IINo Loading Condition Continue IINo Loading Condition Continue IINo Loading Condition Continue II

• The diode D start conducting when D’ stops conducting at instant X.

• The capacitor C’ is charged to +2Vmax at the instant of Y of the VAcurve.

• At this instant, D stop conducting. If there is no loading (i.e. R=∞), g ( ),the output voltage will be constant at +2Vmax from the instant Y onwards.

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CC--WW Doubler Circuit Explanation: Doubler Circuit Explanation: Loading ConditionLoading ConditionLoading ConditionLoading Condition

• Vout will be less than +2Vmax when R≠∞ (i.e. circuit having a load). ( g )Reduction of the voltage magnitude based on two reasons/conditions:

• Firstly, due to during non-conduction period, C’ supplies the load current, and load voltage will be less than

2V d t i l+2Vmax due to ripple.

• Secondly, during each cycle, the y g ycapacitor C replenishes the charges lost by C’ in supplying the load.

55

• This results of VA never attains at potential +2Vmax (capacitor C’ is not charges to +2Vmax at all).



(b) Voltage curve under no load(b) Voltage curve under no load

56

(b) Voltage curve under no load(b) Voltage curve under no load



(b) Voltage curve under load(b) Voltage curve under load

57


3) Cockcroft3) Cockcroft--WaltonWalton Doubler CircuitDoubler Circuit

General ConnectionGeneral Connection::

• One input of the HV winding is earthed. The circuit is arranged with position of the diodes D1 and D2 and the capacitance C1 and C2 are arranged such asthe capacitance C1 and C2 are arranged such as illustrated in Fig 6.

I th i it (Fi 6(b)) th di d D1 t th• In the circuit (Fig. 6(b)), the diode D1 prevents the voltage at point A from becoming negative with respect to earth.

• This results in sine wave of 0V to twice the peak of transformer secondary appearing at this location (e.g. if V ac output HV side = 100 kV gives positive

58

if Vpeak ac output HV side = 100 kV gives positive sinusoidal voltage output of +200 kVmax).


3) Cockcroft3) Cockcroft--WaltonWalton Doubler CircuitDoubler Circuit

General Connection ContinueGeneral Connection Continue::

• The changing of the positive and negative half cycles of ac sinusoidal supply voltage allows cyc es o ac s uso da supp y o tage a o s(depending on components position) the conduction / non-conduction of these diodes and charging / discharging process of capacitances.g g g g p p

• These configurations enable the production of a DC waveform with an output voltage of +2VDC waveform with an output voltage of +2Vmax.

• The produced DC waveform voltage having a 2V l di di i i h i Fi

59

+2Vmax at no loading condition is shown in Fig 7(b).


3) Cockcroft3) Cockcroft--WaltonWalton Doubler CircuitDoubler CircuitProblemProblem::

• Voltage drop occurs during loading condition (load resistance) resulting in voltage output less than +2Vmax , such as shown in Fig. 7(c).

Caused by two reasons:

− During the conduction period, the smoothing capacitor C2 supplies the load current will be lessthan +2Vmax due to ripple.max pp

− During each cycle, the capacitor C1 replenishes the charges lost by C2 in supplying the load, causing the

60

g y pp y g , gVA (at node A in Fig. 6) to never attain a potential +2Vmax . So the capacitor C2 is not charge to +2Vmax at all.


4) Cockcroft4) Cockcroft--WaltonWalton Multiplier CircuitMultiplier CircuitGeneral:General:

• Also known as the Cockcroft-Walton Cascade CircuitAlso known as the Cockcroft Walton Cascade Circuit.

• Is an extension of the previously discussed Cockcroft-Walton doubler circuit That circuit is actually beingWalton doubler circuit. That circuit is actually being extended in series to become the voltage multiplier circuit.

• Fig. 8(b) shows the two stages cascade circuit. This configuration produces an output voltage at +4Vmaxduring no loading condition.

• Fig. 9 shows the ‘n’-stage cascade circuit (output = +nVmax under no loading).

61

max g)

• The method of connection of this circuit is will not bediscussed in this subject.


Fig. 8: Two-stage Cockcroft-Walton Multiplier Circuit

(a) Cockcroft(a) Cockcroft--Walton doubler circuitWalton doubler circuit (b) Two(b) Two--stage Cockcroftstage Cockcroft--Walton multiplier circuitWalton multiplier circuit

62


Fig. 9: ‘n’-stage Cockcroft-Walton Multiplier Circuit

63Fig. 9: The ‘n’Fig. 9: The ‘n’--stage Cockcroftstage Cockcroft--Walton Multiplier Circuit. (a) charging of a Walton Multiplier Circuit. (a) charging of a

smoothing column (b) charging of oscillating column smoothing column (b) charging of oscillating column


4) Cockcroft4) Cockcroft--WaltonWalton Multiplier CircuitMultiplier Circuit

Problem:Problem:

1 Similar to previously mentioned for doubler circuit an1. Similar to previously mentioned for doubler circuit, an extended circuit stage also having a problem with a voltage drop during loading condition.

2. Every voltage multiplier stage that used in a DC system adds on an extra voltage drop to the system.

3. The voltage drop is proportional to the load currentand is inversely proportional to the supplied frequencyfrequency.

4. The net result is that it is not economic to use more th d 5 lti li t i i DC t

64

than around 5 multiplier stage in series. DC setswithin HV laboratories also often use higher frequency power supplies to limit this voltage drop.


Impulse GeneratorImpulse Generator

• The principle of the generator is to generate anThe principle of the generator is to generate an impulse voltages/current waveform. Uses combination of capacitors and resistors and spark gaps in generating the impulse waveformspark gaps in generating the impulse waveform.

• Depending upon the capacitors and resistorsDepending upon the capacitors and resistors values and circuit configurations, the +ve and –ve polarity lightning and switching waveforms can be generated from the setcan be generated from the set.

• Unit typically produces standard impulse 1.2/50

65

yp y p pμs for lightning and 250/2500 μs for switching.



• The voltage / current magnitude of the impulse g g pwaveform is depending upon the number of stagescircuit that being used.

• Higher magnitude requires multiple stage circuits (e.g. up to 10 stages circuit is required to generate about 1 5 MV impulse waveform)about 1.5 MV impulse waveform).

• The impulse generator set can be categorised into p g gtwo:

–– SingleSingle stage circuit

66

–– MultipleMultiple stage circuit or Marx Marx Generator



Problem:1 U it i i i ll f lti l i it1. Unit is very expensive especially for multiple circuits

set and handling the test is dangerous. The handling person must be competent enough to set / conduct the unit and test experimentconduct the unit and test experiment.

2. It is difficult to produce the voltage magnitude at required level (accurate at 100 % efficiency)required level (accurate at 100 % efficiency), especially at the higher voltage levels.

3 The efficienc the oltage generated lies from 0 6 to3. The efficiency the voltage generated lies from 0.6 to0.9.

4 Al i ffi i t l t t (l b t )

67

4. Also requires sufficient large test area (laboratory) equipped with good safety components / procedures in-place.


SingleSingle--stage Impulse Voltage Generator: Equivalent stage Impulse Voltage Generator: Equivalent CircuitCircuitCircuitCircuit

• The single-stage circuits are mainly used for generation f l ti l l i l ltof relatively low-impulse voltage.

• The TERCO’s single-stage voltage impulse generator g g g p gat UTHM’s HV Laboratory capable to produced lightning impulse at maximum 140kVpeak.

• The impulse circuit commonly consist of two individual components:

- ‘charging’ component causing the wavefront- ‘discharging’ component causing the wavetail

68

• Simplified circuit such as shown in Fig. 10 and Fig. 11


Impulse Generator: Equivalent CircuitImpulse Generator: Equivalent Circuit

Cs : Charge / Tail capacitorCs : Charge / Tail capacitorCf : Front capacitorCf : Front capacitorRc : Charge resistorRc : Charge resistorRf : Front ResistorRf : Front Resistor

69Fig. 10: Single Fig. 10: Single ––stage impulse generator equivalent circuitstage impulse generator equivalent circuit

Rf : Front ResistorRf : Front Resistor


Impulse Generator: Equivalent CircuitImpulse Generator: Equivalent Circuit

70Fig. 11: Single Fig. 11: Single ––stage impulse generator equivalent circuitstage impulse generator equivalent circuit



Connection:Connection:

• Effectively, two circuit (Fig. 11(a) and Fig. 11(b)) form the overall waveshape (rise and decay lines)

• The stage capacitor Cs is initially charged to predetermined HVDC voltage and is then dischargedvia a switch or spark gap (e.g. spark voltage around 15kV i t d t ll i b tt t d15kV is created externally using battery operated circuit).

• When the switch / spark gap is triggered (closed), immediately the capacitor voltage appears across the ‘tail’ resistance Rt and then gradually start to reduce its magnitude (tail waveform is created)

71

magnitude (tail waveform is created)

• This is a wavetail circuit such as shown in Fig. 11(a)).



Connection continue:Connection continue:

• Upon the conduction of the wavetail circuit, at the sametime, the voltage across the tail resistance Rt causes the voltage to begin to build up across the front

Cf ( f )capacitance Cf (creating rise waveform).

• The Cf is usually the voltage divider. The test object is y g jconnected across the front capacitance and it is due to this part of the circuit that the voltage waveform cannotrise to peak instantaneously.

• The time constant of the front capacitance Cf and front resistance Rf will usually be much lower than the time

t t f th t it Ct d th t il

72

constant of the stage capacitance Ct and the tail resistance Rt (therefore producing rise and tail impulse waveshapes).


Fig. 12: Switching & Lightning Voltage Impulse WaveformsWaveforms

Vp Vp

73


Fig. 22: 140kVpeak Single-Stage Circuit Lightning Impulse Test SetupLightning Impulse Test Setup

74Fig. : The TERCO SingleFig. : The TERCO Single--Stage Circuit Lightning Impulse Test SetupStage Circuit Lightning Impulse Test Setup



75Fig. : The TERCO Single Circuit Lightning Impulse Test SetupFig. : The TERCO Single Circuit Lightning Impulse Test Setup


Fig. 24: 140kVpeak TERCO Lightning Impulse Set: UTHMSet: UTHM

SingleSingle--stage Lightning Impulse Generatorstage Lightning Impulse Generator

Earth StickEarth Stick

76


MultipleMultiple--stage Impulse Voltage Generator: stage Impulse Voltage Generator: Equivalent Circuit (Marx Circuits)Equivalent Circuit (Marx Circuits)Equivalent Circuit (Marx Circuits)Equivalent Circuit (Marx Circuits)

• It is not possible to scale up the capacity of single-stagei l t t t hi h ltimpulse generators to generate higher voltage magnitude (e.g. >500 kVpeak) due to two reasons:

P bl ith th ( t) d diffi lti i– Problem with the expense (cost) and difficulties in supplying high voltage DC.

– Problem with the need to increasing size of high voltage g g gcapacitors.

• Thus the Marx generator is used to generate higherit d i l ltmagnitude impulse voltage.

• This generator uses a number of capacitors that are

77

g pcharged in parallel (thus requiring the a lower DC supply) through high ohmic resistances before being discharge in series through spark gaps.


Fig. 14: FourFig. 14: Four--stage Marx Generator Equivalent Circuitstage Marx Generator Equivalent Circuit

( ) D i h i( ) D i h i(a) During charging(a) During charging

(b) During discharging(b) During discharging( ) g g g( ) g g g

78



Connection:Connection:

• Fig. 14 shows the schematic diagram of a four-stageMarx generator, consisting of four stage capacitors Cs and resistors Rc and four associated tail resistors Rtand damping/front resistors Rand damping/front resistors RD.

• The DC voltage Vc charges the stage capacitors Cs(f b i ll l) th h th hi h l(four number in parallel) through the high value charging resistor Rc as well as through tail resistors Rt, which are smaller value than Rc.

• These charging resistors Rc act in the same way as the tail resistors Rt when the generator discharges but are usually of high enough value to allow the tail resistors to

79

usually of high enough value to allow the tail resistors to dominates.



Connection Continue 1:Connection Continue 1:

• After long time period (e.g. 1 minute charging), a point A,B,C,D will acquire the potential DC source Vc with respect the point G.respect the point G.

• The points H,I,J,K will remain at earth potential, as the voltage drops across the tail resistors Rt are negligiblevoltage drops across the tail resistors Rt are negligibleduring charging time.

• Therefore the load capacitance Cf remains at earth• Therefore, the load capacitance Cf remains at earth potential (0V) during charging of stage capacitors Cs.

• The spark gaps G1 G2 G3 G4 basically are set to spark

80

• The spark gaps G1,G2,G3,G4 basically are set to spark almost simultaneously.




• The discharge of the generator is accomplished by applying an additional impulse voltage around 15 kV (sparking voltage, e.g. battery operated) to the ( p g g g y p )spark gap (triggered gap).

• Since the sphere gap are normally set to just hold-Since the sphere gap are normally set to just holdoff the charging voltage, the presence of this additional impulse spark causes the first gap (G1) to breakdownto breakdown.

• When this occur, the potential at the base of the second stage capacitor (point H) is changed from

81

second stage capacitor (point H) is changed from 0Vc to Vc.




• Due to stray capacitance effects, the second gap (G2) still has earth potential at the tail resistor Rt(G2) still has earth potential at the tail resistor Rt side but the voltage of 2Vc across it causing immediate breakdown.

• This situation continues across all the way to the top of the generator whereas spark gap G3top of the generator whereas spark gap G3 breakdowns and potential across point C is at 3Vc, then spark gap G4 breakdown and potential across point D is at 4Vc.

82

p




• Also contained in the generator design are damping resistors, RD. This actually form part of the front resistance that is install externally to the generator.

• These resistors help in controlling oscillations in the voltage waveform that may arise when inductance is

t i d i t t l d d / t t bj tcontained in test leads and /or test object.

• These damping resistors (or front resistors) RD are p g ( )small in comparison to the tail resistors Rt, (e.g. the RD is about 40ohm in comparison to 540 ohm of Rt).

83

• The front capacitor Cf is used in creating the wavefront impulse and also react as a voltage divider that measure the waveshape produced by the generator.


Fig. 15: 2MV Impulse Generator Set: Manchester UniversityManchester University


Capacitor DividerCapacitor Divider

Prototype to testPrototype to test

84


Fig. 16: 2MV Impulse Generator Set: Big GuyGuy…

85


Part 3Part 3

****TERCO setTERCO set at UTHM Laboratory**at UTHM Laboratory**

86


Fig. 17: HVAC Test Setup (up to 100 kV r.m.s)

87Fig. 9: TERCO set setup circuit for HVAC testFig. 9: TERCO set setup circuit for HVAC test


Fig. 18: HVAC Test Setup (up to 100 kV r.m.s)


88


Fig. 19: 140kV peak Half Period HVDC Test Setup

89Fig. 9: TERCO set setup circuit for HalfFig. 9: TERCO set setup circuit for Half--Period HVDC testPeriod HVDC test


Fig. 20: 140kVpeak Half Period HVDC Test Setup


90


Fig. 21: Cockcroft-Dalton Doubler HVDC Test Setup

91Fig. : The TERCO CockcroftFig. : The TERCO Cockcroft--Walton Doubler Circuit Test SetupWalton Doubler Circuit Test Setup



92Fig. : The TERCO SingleFig. : The TERCO Single--Stage Circuit Lightning Impulse Test SetupStage Circuit Lightning Impulse Test Setup



93Fig. : The TERCO Single Circuit Lightning Impulse Test SetupFig. : The TERCO Single Circuit Lightning Impulse Test Setup


Fig. 24: 140kVpeak TERCO Lightning Impulse Set: UTHMSet: UTHM

SingleSingle--stage Lightning Impulse Generatorstage Lightning Impulse Generator

Earth StickEarth Stick

94


Part 4Part 4

**HV**HV TestingTesting && MeasurementMeasurement****HVHV Testing Testing && MeasurementMeasurement

j|ÄÄ ux w|ávâááxw |Ç à{|á ÄxvàâÜx‹j|ÄÄ ux w|ávâááxw |Ç à{|á ÄxvàâÜx‹

95


Learning Outcome:Learning Outcome:

At the end of the course:

• The student will capable to summarise the concept /method uses in some standard HV testing setup and measurement commonly conducted in the laboratory using the previously discussed HVAC, HVDC and p y ,impulse generator.

96


Purpose of HV Tests:Purpose of HV Tests:

• To locate the manufacturing defects. E.g. AC t t lt 2 3 ftest can use voltage some 2 p.u or 3 p.u of the rated voltage (withstand the TOV).

• To give confidence of operation in–service. Plant is routinely expose to overvoltages when in usewhen in use.

97


Differences Between Lab and InDifferences Between Lab and In--Service:Service:Service:Service:

• Laboratory tests are highly regimented (organised) applying known voltages of given type / shape of a particular timetype / shape of a particular time.

• In-service voltages depend on numerousIn service voltages depend on numerous factors for both their shapes / type and duration.

• Laboratory test are used as equipment has been shown to give good performance in

98

g g ppractice.


HV Tests MethodsHV Tests Methods

The HV tests methods can be divided into two:

1) Non-destructive tests1) Non destructive tests

2) Destructive tests

99


Part 4Part 4--11

****NonNon--destructivedestructive Test**Test**NonNon destructivedestructive TestTest

100


NonNon--destructive Testsdestructive Tests

• The test is conducted on the material / apparatus without destroying their internal and external y gphysical characteristic.

• This test is used to ensure that the apparatus has ppnot deteriorated after a high voltage test even though it has withstood the test successfully (post-durability test).

• Test also provides the information of the quality of insulation before it forms part of an equipment (pre-durability test)durability test).

• The common tests are such like:

101

1. Measurement of the Tan δ of the material2. Partial discharge measurement


NonNon--destructive Test: Measurement of Tan destructive Test: Measurement of Tan δδ

• Also known as a measurement of the dielectric loss or dissipation factor (Tan δ) of the dielectric materialdissipation factor (Tan δ) of the dielectric material.

• On account of the dielectric loss, the current through the capacitor does not lead the voltage across it bythe capacitor does not lead the voltage across it by 90, but by angle (90 – δ).

• In this case, a δ is known as loss angle such as , gshown in phasor diagram of figure below.

102Fig. 25: Phasor diagram and circuit model for non-ideal capacitor


Measurement of Tan Measurement of Tan δδ: Continue: Continue

• A lossy capacitor may be modelled electrically by an ideal capacitor connected in series (also can be inideal capacitor connected in series (also can be in parallel depending on model) with hypothetical resistance such as shown in Fig. 25(b).

• Theoretically, the losses occur due to ionic and dipolar polarization and may be due to partial dischargepolarization and may be due to partial discharge.

• The dissipation factor; tan δ for series and parallel d lmodel:

1tan XXXR

series RCRVV 1tan

103

XXparallel RC

tan

X

Cse ies

CV

1



Th di i ti f t t δ i i di ti f th• The dissipation factor or tan δ is an indication of the state of dielectric.

• The more its value, the worse of insulation condition of insulation will be (prone to conduction) or indication of damage occur (carbonisation void treeing etc ) indamage occur (carbonisation, void, treeing etc.) in material.

A i ti ( l f diff t l ) d dd• Any variation (couple of different values) and sudden change in the tan δ value with applied voltage is an indication of the onset of internal discharge (may

d b b f il h i )

104

caused by above failure mechanisms).



• One of commonly used methods for measurement of capacitance and dissipation factor (tan δ ) is the highcapacitance and dissipation factor (tan δ ) is the high voltage Schering Bridge.

• Fig 26 shows the Schering Bridge circuit• Fig. 26 shows the Schering Bridge circuit.

Connection:F thi i it th l f th di l t i i• From this circuit, the sample of the dielectric is represented by the series model of Cx and Rx, represented as the second arm (Arm-II).

• The first arm is (Arm-1) is a gas filled standard capacitor (Cs), the third arm (Arm-III) is a variable resistor R and the forth arm (Arm IV) is variable

105

resistor R3 and the forth arm (Arm-IV) is variable capacitor C4 in parallel with a resistance R4.


Fig. 26: Schering Bridge Circuit

106

Fig. 26: Schering Bridge CircuitFig. 26: Schering Bridge Circuit



Connection continue:Connection continue:

• In practice, R4 value is constant and C4 is variable.

• This C4 bridge value is calibrated /tuned to obtain a constant value of frequency (may be observed by a d )detector).

• From the circuit, the dissipation factor (tan δ) can be , p ( )determined based on the observed frequency, the C4and R4 values from using an equation below.

107

44tan RCRC XX


NonNon--destructive Test: Measurement destructive Test: Measurement of Partial Dischargeof Partial Dischargeof Partial Dischargeof Partial Discharge

• Also known as the PD measurement. The PD basically takes place through only a part of the dielectrictakes place through only a part of the dielectric material.

• The term partial discharge refers to a discharge that does not completely bridge the space between electrodes (only partial).

• This mechanism is the main reason for ageing and eventual failure of electrical insulation (natural cause)eventual failure of electrical insulation (natural cause).

• How to measure? By electrical pulses for detection ( t th d) ti i i f l ti

108

(most common method), acoustic emission for location (pick up the sound – not practical in the lab).



• PD include internal discharges due to the presence ofPD include internal discharges due to the presence of voids as in solid or liquid insulations, surface discharge at the boundary of insulating materials (e.g. cable) and treeing corona as in gaseous dielectrictreeing corona as in gaseous dielectric.

• These discharges are not breakdowns but have an i t f th lif f i l ti ti l l lid dimpact of the life of an insulation, particularly solid and liquid dielectric.

• For a prolong time, the insulation properties of these dielectrics could deteriorate with discharge, resulting in total breakdown

109

total breakdown.



• Thus, by finding a relationship between a PD and life expectancy of dielectric is

t i t t t i l tiparamount interest to insulation designer.

110


Measurement of Partial Discharge: ProblemMeasurement of Partial Discharge: Problem

Problem:Problem:

• Measurement of PD is not easy as it deals with a noise (discharge) occurring in the void of the dielectric ( g ) gmaterial.

• The measurement result may be disrupted by the• The measurement result may be disrupted by the noises originating from the surrounding, adjoining circuit and the test equipment itself.

• Occurrence of discharges may be self-extinguished due to materials / gas interaction in the void thus producing

111

g p g‘come and go’ results that are not easily predictable.


Partial Discharge: Concept of MeasurementPartial Discharge: Concept of Measurement

• The PD measurement on the dielectric can be expressed as measuring the discharge activityexpressed as measuring the discharge activity occurring in its void / cavity (may contain gas).

• The concept is as shown in Fig. 27.

Theory:Theory:Theory:Theory:• Typically, the cavity will have a relative permittivity

(εr_cavity) of approximately unity since it is a gas, which is lower than the surrounding medium (ε < ε )lower than the surrounding medium (εr_cavity< εr )

• The field strength across the cavity is given by:

112

g y g y

arc EE


Fig. 27: Partial Discharge Concept

CCaa : dielectric capacitance: dielectric capacitanceCCc c : cavity capacitance: cavity capacitanceCCb b : C: Caa--CCccVV lt di l t ilt di l t i

113Fig. 27: Schematic Cavity in DielectricFig. 27: Schematic Cavity in Dielectric

VVaa : voltage across dielectric: voltage across dielectricVVcc : voltage across cavity : voltage across cavity



Theory continue:Theory continue:• In this situation when the field in the cavity (E )• In this situation, when the field in the cavity (Ec)

exceeded the gas in the material’s breakdown strength (Ecb), discharge will occur. Check the ‘Subject 1’ for information of gas breakdowninformation of gas breakdown.

• Thus we can consider the voltage at which this field is greached discharge inception voltage or Vai.

• The breakdown in the cavity will be less than that of• The breakdown in the cavity will be less than that of surrounding condensed matter, thus PD will be seen before bulk failure occur.

114

111

tdtEV

rcbai


Partial Discharge Equation Derivation

115


Fig. 27: Partial Discharge Concept

116



Extra info:Extra info:

H th di h d t th• However, as the discharges occur, damage to the surface of the void / cavity will results the gaseous by-product.

• This gases will increase the pressure in the void and may eventually extinguish discharges until the void i i i diffincreases in size or gasses diffuse away.

• Furthermore, the surface of the void also be chemically changed The production of semiconductor or materialchanged. The production of semiconductor or material with high permittivity on the void surface will change or substantially reduce the electrical fields within the voids.

117

• This may causing the ‘come and go’ discharges and are not easily detectable.


Fig. 28: Conventional Partial Discharge Circuit

118

Fig. 28: Conventional Partial Discharge CircuitFig. 28: Conventional Partial Discharge Circuit


Fig. 29: Data Presentation

(a) Discharge magnitude measurement(a) Discharge magnitude measurement (b) Discharges display on p.f. ellipse(b) Discharges display on p.f. ellipse

119(c) Discharges display on p.f. oscillation(c) Discharges display on p.f. oscillation (d) Discharges distribution on p.f. phase angles(d) Discharges distribution on p.f. phase angles


Part 4Part 4--22

****DestructiveDestructive Tests**Tests**Destructive Destructive TestsTests

120


Destructive TestsDestructive Tests

• The test is conducted on the material /• The test is conducted on the material / apparatus with a possibility destroying / altering their internal and external physical h t i ticharacteristic.

• This test is used to ensure that theThis test is used to ensure that the apparatus has an ability to withstand the normal and abnormal system voltage (meet requirement by standard)requirement by standard).

• Test also provides the information of the

121

pquality of insulation an equipment / material.


Destructive TestsDestructive Tests

• There are many laboratory test types available for y y ypthe destructive tests depending on their application conducted on the gaseous, liquid and solid materials.

• Most of them are already discussed in the beginning of subject 3.

• This section will summarise the HV tests methods for:

1) Salt fog test2) Tracking test3) HVAC d d t ith t d / fl h t t

122

3) HVAC dry and wet withstand / flashover test4) Impulse dry and wet withstand / flashover test


SaltSalt--Fog Test: Working SummaryFog Test: Working Summary--II

• Fig. 30 shows the setup and Fig.31 shows the arrangement of equipment located in the HV supplyarrangement of equipment located in the HV supply room and the salt fog chamber (optional).

• The salt fog was generated by nozzles located inside• The salt fog was generated by nozzles located inside the chamber using a combination of pressurised air and salt water.

• The test object was placed horizontally on vertical blocks in the chamber between the earthed frame and the bushing.the bushing.

• Electrodes were applied to the two ends of the sample and connected to the high voltage and earth

123

sample and connected to the high voltage and earth conductors.


Fig. 30: HVAC Salt Fog Test

124


Fig. 31: HVAC Salt Fog Test: Experiment Setup

125


SaltSalt--Fog Test: Working SummaryFog Test: Working Summary--IIII

• The high voltage was supplied to the conductorThe high voltage was supplied to the conductor from an HVAC transformer set located outside the chamber.

• A ballast current limiting water-based resistor of 1.0 Mohm was used to reduce currents to a maximum of 15x10-3 A at 15 kV AC voltage formaximum of 15x10 3 A at 15 kV AC voltage for intense low current arcs.

• The tests were conducted by applying the HVAC voltage (value and duration depending on the test method) to the specimen that being sprayed

ti l ith t ll d l lt f

126

continuously with controlled-volume salt fog.


SaltSalt--Fog Test: Working SummaryFog Test: Working Summary--IIIIII

• The circuit may also have additional protection scheme in place in case of emergency.

• Data are collected as voltage and leakage current measurement through the data acquisition system (e g Labview) and also from the damage(e.g. Labview) and also from the damage observed on the sample surface.

• As optional, the test may be conducted for a long duration (up to 1000hrs or more) with the combination of the UV lights (repetition exposure) i ti th l td diti

127

in representing the real outdoor conditions.


Tracking Test: Working SummaryTracking Test: Working Summary--II

• Fig. 32 shows the setup and Fig.33 shows the arrangement of equipment (optional).

• A transformer (415 V / 11 kV) was used to supply power to the test circuit and its output voltage was measured using a high voltage probe.

• Each sample was fed from this supply via a high p pp y gvoltage relay and a resistance. This resistance value is depended on the test voltage and the flow rate of the contaminant.

• The test is conducted could lasted up to 6hours (depending on the test method) or to be cut off during

t Th i it l h dditi l

128

overcurrent. The circuit may also have additional protection scheme in place in case of emergency.


Fig. 32: Tracking test: SetupFig. 32: Tracking test: Setup

129


Fig. 33: Equipments Arrangement for Tracking TestFig. 33: Equipments Arrangement for Tracking Test

130


Tracking Test: Working SummaryTracking Test: Working Summary--IIII

• The current flowing across the sample is measured using a resistive shunt. Overcurrent protection g poperated, should the current exceed 60x10-3 A ±6x10-3 A for 2 s to 3 s.

• A peristaltic pump was used to create a water flow across the inclined sample at the flow rate stated in the standard.

• The current and voltage are monitored using a LabVIEW system having an appropriate resolution y g pp p(e.g. 12 bits) and sampling rate (e.g. 10,000 rate per second).

131

• An optional measurement device such as a video or thermal camera can be used to provide visual data.


HVAC Wet and Dry Tests: Working SummaryHVAC Wet and Dry Tests: Working Summary--II

• The test carried out to investigate any possibilityThe test carried out to investigate any possibility visual corona, flashover, survival system voltage, TOV etc.

• Fig. 34 shows the setup and Fig. 35 shows the arrangement of equipment (optional).

• A resonant transformer (415 V / 650 kV) is used to supply power to the test circuit and its output voltage is measured using a capacitance divider. g p

• The volume-controlled artificial rain (with specific conductivity and flow rate according to standard) is

132

y g )generated by nozzles that entirely covered the sample surfaces (e.g. insulation cross-arm in Fig. 34).


Fig. 34: HVAC Test: System Setup

133


Fig. 35: HVAC Test: Equipment Setup

134


HVAC Wet and Dry Tests: Working HVAC Wet and Dry Tests: Working SummarySummary--IIIISummarySummary--IIII

• The HVAC voltages (normal system or TOV level)• The HVAC voltages (normal system or TOV level) are supplied to each test for about 1 – 3minutes duration.

• In dry condition test, no artificial raining is provided to the sample surface (therefore it must be the first one to be conducted)one to be conducted).

• When HVAC voltage is supplied to the specimen (for both dry and wet tests) and data is collected through the current and voltage measurement, visual corona built-up on fittings / surface flashover

135

visual corona built up on fittings / surface, flashover, occurrence of damage etc.


HVAC Wet and Dry Tests: Working HVAC Wet and Dry Tests: Working SummarySummary--IIIIIISummarySummary--IIIIII

• In these tests sufficient clearances of• In these tests, sufficient clearances of the testing area are required to prevent unnecessary flashover to nearbyunnecessary flashover to nearby equipment / object that could cause damage or danger to themselves anddamage or danger to themselves and most importantly, to HUMAN.

136


Impulse Wet and Dry Tests: Working Impulse Wet and Dry Tests: Working SummarySummary--IISummarySummary--II

• The test carried out to investigate any possibility occurrence of an AC visual corona and flashover viaoccurrence of an AC visual corona and flashover via positive / negative impulse voltages.

• Fig. 36 shows the setup and Fig. 37 shows the g p garrangement of equipment (optional).

• An impulse set (set up to produce a LI or SIAn impulse set (set up to produce a LI or SI waveforms) is used to supply the positive / negative impulse voltage to the test circuit and its output voltage is measured using a capacitor divider.

• If required, the volume-controlled artificial rain (with specific conductivity and flow rate according to

137

p y gstandard) is generated by nozzles that entirely covered the sample surfaces (not shown in the figure).


Fig. 36: Switching & Lightning Impulse Test: System SetupSystem Setup

138


Fig. 37: Switching Impulse Test: Experiment Setup

139


Impulse Wet and Dry Tests: Working Impulse Wet and Dry Tests: Working SummarySummary--IIIISummarySummary--IIII

• The specific level impulse voltage (either positive /The specific level impulse voltage (either positive / negative LI or SI waveform) are applied to the test object and its flashover or withstanding ability is monitored.

• In dry condition test, no artificial raining is provided to the sample surface (therefore it must be the first one t b d t d)to be conducted).

• Normally, data is collected through the current and y gvoltage measurement, the condition of ‘pre and post’ impulse waveshape, visual condition, streamers propagation visual flashover occurrence of damage

140

propagation, visual flashover, occurrence of damage etc.


Impulse Wet and Dry Tests: Working Impulse Wet and Dry Tests: Working SummarySummary--IIIIIISummarySummary--IIIIII

• Similar to HVAC tests, also in impulse tests sufficient clearances of the testingtests, sufficient clearances of the testing area are required to prevent unnecessary flashover to nearbyunnecessary flashover to nearby equipment / object that could cause damage or danger to themselves anddamage or danger to themselves and most importantly to HUMAN.

141


Note ReferencesNote References

1. Subir Ray, An Introduction to High Voltage Engineering, Prentice Hall India, 2004

2. E. Kuffel, High Voltage Engineering: Fundamentals, Newness, 2000

3. S.M.Rowland, Dielectric – How They Work and Fail’ MSc. Lecture Note, The University of Manchester, 2006

4 I Cotton An Introduction to High Voltage Testing MSc Lecture4. I. Cotton, An Introduction to High Voltage Testing, MSc. Lecture Note, The University of Manchester, 2006

5. I. Cotton, Partial Discharge, MSc. Lecture Note, The University of Manchester 2006

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Manchester, 2006

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