Electrical Power Engineering

Power Lab II Experiment: Partial discharges

Room : S3/09/21

Summer Semester 2011 University of Technology Darmstadt Tutor: M.Sc. MH. Nazemi High Voltage Department nazemi@hst.tu-darmstadt.de

Partial discharges 2 Phone: 16-4495 Building: S3|10/306

Partial discharges 1 Partial discharge measurement Contents directory

1 Introduction ........................................................................................................................................... 2

2 Basics ..................................................................................................................................................... 3 2.1 Internal partial discharge ................................................................................................................... 3 2.2 External partial discharge .................................................................................................................. 6 2.3 Creeping discharge .......................................................................................................................... 14 2.4 Appearance of PD ............................................................................................................................ 16

3 Measurement technological coverage ................................................................................................ 17 3.1 Classification of PD measuring devices for measurement of the apparent charge q .................... 19 3.2 Electrical method ............................................................................................................................. 20

4 Overview ............................................................................................................................................. 23 5 Description of the used measurement technique ............................................................................... 23 5.1 ICMsystem-digital partial discharge detector ........................................................................ 23 5.2 PC Software ............................................................................................................................ 25 5.3 Calibration .............................................................................................................................. 26 5.4 Special options ........................................................................................................................ 26

6 Carrying out the test ............................................................................................................................ 27 6.1 Internal partial discharges at a cast resign body ............................................................................. 27 6.2 External partial discharges .............................................................................................................. 27 6.3 Creeping discharges at the Toepler sliding arrangement ............................................................... 28

7 Composition ........................................................................................................................................ 28

Partial discharges 2 1 Introduction Partial discharges (PD) are imperfect breakdowns, i.e. they bridge a part of the insulation between the electrodes. These discharges result at a local exceeting of the breakdown ratings of an insulation arrangement. In opposition to a total breakdown at a partial discharge didnt form any breakdown channel with low resistance between the electrodes. It only amounts to a short discharge impulse. If the dielectric medium has self-healing properties at the overloaded position and if the electric field builds up again, pulse shaped partial discharges will built up. Partial discharges can also be without a pulse if they appear in connection with a stationary gas discharge. Pulse shaped partial discharges and partial discharges without a pulse can appear at every form of voltage. Considering the practical meaning only partial discharges at a.c. voltage are looked at in this experimental description, even though the executions over external partial discharges particularly have validity for d.c. voltages, too. Examples for the location of so-called internal PD are empty spaces (cavities) in insulators, which are emerged because of the production process, or screws sticking out at a transformer for so-called external PD. Partial discharge measurements have become an important test method of the high-voltage technology because they can give advices on dimensioning and manufactoring defects of operating resources or the state of ageing of an insulation. The goal of this test is to interfere acquaintances about different types of partial discharge events and their measurement technological coverage.

Partial discharges 3 2 Basics PD attract attention to themselves at a specimen to be tested by short-time voltage break-in causing high-frequency interference fields. In practice partial discharges dont only cause losses but also disturbances of the radio and tv transmission as well as noises. If partial discharges occur inside an insulating body it is called internal PD. Accordingly external PD characterises activities, which cause partial discharges on the surface. Creeping discharge means processes of partial discharge at interfaces of different materials. 2.1 Internal partial discharge While production of insulators with solid or fluid dielectric mediums gas-filled empty spaces can emerge. Already at a relative low applied voltage internal discharges occur inside these empty spaces because of the increased field displacement in there. In the course of time these discharges reduce the surface resistance of the empty space. At last the empty space becomes conductive and shortens the isolation distance, so that it can come to a total breakdown caused by an erosion machinery or the life expectation of the isolation becomes shorter. To demonstrate these activities, an insulation with solid dielectric medium containing a gaseous empty space, is shown in an equivalent circuit (figure 1) for pulse shaped internal partial discharges.

Partial discharges 4

C1 empty space capacity C2 - in series to the empty space lying capacity of the error free dielectric medium C3 - parallel to the empty space lying capacity of the error free dielectric medium F - spark gap figure 1: Arrangement with internal PD and equivalent circuit a) specimen with empty space b) equivalent circuit

Gas blebbs show higher field intensity than the surrounding medium because of the lower relative permittivity r compared to solid. In many cases the pressure in these empty spaces is very low. Thus the breakdown rating is additionally decreased. If the voltage that is situated at the empty space exceeds the sparking voltage Uz, the spark gap F will break down. It amounts to an imperfect breakdown of the arrangement and C1 will be completely discharged. (note: that doesnt completely come up to reality because the conductivity can only adopt finite values and consequently a residual voltage is always left.) In figure 2 is the behaviour of voltage at this equivalent circuit drafted when applying an a.c. voltage.

figure 2: Behaviour of voltage in the equivalent circuit for pulse shaped internal PD

Partial discharges 5 u(t) is the voltage that is applied to the specimen. u10 shows the behaviour of the no-load voltage that would result from the capacitance divider of empty space and the in series situated error free dielectric medium.

The voltage u1 at the empty space is increased over this voltage divider as far as the voltage achieves the sparking voltage Uz and breaks down. This event recurs as long as the empty space capacity is recharged. The crest magnitude of the test voltage u(t), where first partial discharges emerge, is denoted as onset voltage Ue. In practice is to pay attention at the PD-test, that the test voltage is increased when detecting Ue. In opposition to the onset voltage the output voltage Ua is the voltage, where no PD emerge at decreasing test voltage (keyword: hysteresis). The applied voltage achieves Ue when the no-load voltage (crest magnitude) just equals Uz. Out there the following relation results:

If the test voltage sits above the onset voltage, a repeated charging of C1 occurs and with it an new achievement of Uz. The pulse sketched in figure 2 arises. The sketch of the voltage behaviour shows, that the biggest frequency of discharge emerges at crossover of the test voltage, thus in the range of the biggest voltage change du/dt. The sign of the PD-pulses doesnt depend on the polarity of the voltage, but from their change. The dependence of the frequency of impulse n (per second) of is given in figure 3. The straight line illustrates a close approximation for high frequencies of the impulse:

2 210

1 2 1 2

(t) sinC C = u = u tu + + C C C C (1)

1 2e z

2

+ C C = U UC

(2)

Partial discharges 6

e

e

mit als Frequenz -n = 4f f

(3)

figure 3: Frequency of impulse of PD At every single discharge the charge quantity Q1 is compensated. But only the "apparent charge" Q1s is supplied to the capacity C2.

At arrangements with internal partial discharges and unknown partial capacities it is impossible to measure the real charge Q1, because only the amount of charge Q1s can be captured by measurement technique. Recapitulatory can be said, that the behaviour of certain partial discharge characteristic parameters as charge, frequency, energy and temporal behaviour of PD-pulses allows a rating of the effect of internal processes of ionization upon the insulation. 2.2 External partial discharge At electrodes with sufficient high curve collision ionization constitutes in gases or liquids when exceeding the onset voltage. Electron avalanches and photoionization lead to imperfect breakdown channels in a highly inhomogeneous field. At a.c. voltage these breakdown channels must ignite again at zero voltage after the lapse of the partial discharges. This phenomenon is named external partial discharge or corona discharge. It has high practical importance for high-voltage overhead lines e.g., because maintaining the discharge dissipates energy

ers z ers 1 21 mit = = + Q C U C C C 2 z1s = Q C U (4)

with

with f as frequency

Partial discharges 7 (corona losses) and the occurring current impulses create electromagnetic waves (radio disturbances). Furthermore undesirable chemical reactions can occur because of partial discharges, as creation of ozone O3 in the air e.g. External partial discharges can long-term lead to a weakening of the insulation, that later possibly causes an entire breakdown. A tip-vane-arrangement (figure 4) in air is a typical example of an arrangement with external partial discharges. Figure 5 shows the accompanying behaviours of voltage for pulse shaped discharges.

Figure 4: Arrangement with external PD and equivalent circuit

a) Arrangement tip-vane b) equivalent circuit

Figure 5: Behaviours of voltage in the equivalent circuit for pulse shaped external PD

Partial discharges 8 As at the internal partial discharges for the pulse frequency the relation shown in figure 3 is essential, too. In contrast to internal PD it is shown in the drawn behaviours of voltage of the external PD, that partial discharge pulses predominantly occur on the top of the test voltage. An equivalent circuit for pulse shaped external partial discharges is subsequently shown. C1 describes one of each breaking down gas line allocated capacity, that will completely be discharged when achieving the breakdown voltage Uz of the spark gap F. The charge carriers, which are generated from the tip, move to the field room and lead to a certain conductivity there. This conductivity is described by R2 in the equivalent circuit. C3 is a shunt capacity given by the arrangement. Under the assumption, that R2 >> 1/C1 , the current through R2 is:

This current also appears in C1 and here creates (in steady state) the no-load voltage

When the voltage at C1 achieves the breakdown voltage Uz, C1 is abruptly discharged (= PD). Referring to the test voltage, this means that the peak of the test voltage has the height of the onset voltage:

22 2

(t) sin bei sinusfrmiger Prfspannungu 1 = = u t iR R

(5)

10 22 21 1 1

(t) sinj j 2

1 u u = = = ( t - )u iC C CR R

(6)

2e 1 z = U C UR (7)

at sine test voltage

Partial discharges 9 At a rising test voltage C1 will be charged afterwards with a behaviour of voltage in parallel to u10 until Uz is achieved again and so on. At every single discharge the charge quantity

is compensated in F. This charge is supplied to C1 over R2 from the voltage source again and is ascertainable by measurement technique. In opposite to the measurement of the charge quantity at internal PD, it is possible at external partial discharges to detect the compensating charge: Q = Q1 ( means, that it is about a size, which is allocated to a complete, but compared to the period of the test voltage very short pulse.) Only (continuous) pulse shaped discharges without consideration of polarity were regarded so far. Other forms of discharge actually occur, too. In figure 6 the experimentally, with oscilloscopes, detected behaviours of voltage are schematically shown.

figure 6: Forms of phenomenons and phase position of partial discharges

Behaviour of voltage 1 schematically shows, that at inclination of the applied voltage pulses occur at crest of the negative semi-oscillation at first. Their behaviour and temporal distance are quasi constant. They are the also at negative d.c. voltage arising "Trichel pulses". In 1938 G. W. Trichel verified the pulse shaped character of corona discharge at their occurrence. The endurance of the pulses is about a few 10 ns, their pulse frequency may amount up to 105 s-1. If the voltage is more increased, pulses occur at crest of the negative semi-oscillation, too. However, they are irregular (behaviour of voltage 2).

e e1 z 11

2 21

U U = = = Q C U CC R R (8)

Partial discharges 10 At both polarities, it can also come to long-lasting partial discharges in the range of the crest with increasing voltage. These discharges are called "permanent corona" (behaviours of voltage 2 and 3). This kind of discharge is not to detect as clear as the Trichel pulses on the oscilloscope picture. In fact, a voltage crest without pulses refers to corona discharges. But this voltage crest is surrounded by pulse shaped discharges on both slopes. The last typical form of discharge before the total breakdown in the positive semi-oscillation are intense brush discharges in the positive crest (behaviour of voltage 3). The becoming of pulse shaped behaviour of predischarge is explained at Trichel pulses as example: In front of an electrode with highly bend (e.g. at a tip) it amounts to collision ionization in air when exceeding the onset voltage. The negative eletrons, which are much more mobile than the positive ions, quickly leave the ionization area. The more slowly ions stay in front of the tip and generate a positive space charge there. This space charge changes the potential distribution shown in figure 7 as a dashed line.

Figure 7: Polarity effect at a tip-vane spark gap

a) tip negative b) tip positive

Partial discharges 11 At negative tip the electrons migrate to the vane. The remaining ions directly lead to very high field intensities at the tip while the remaining field space has only low potential differences. Because of this highly declining field intensity the velocity of the electrons highly decreases and it amounts to a generation of negative ions by adsorption of electrons. The thereby arising space charge decreases the field intensity at the cathod tip. Thus a continuing generation of electron avalanches will be prevented. Not until the abolishment of the space charge by recombination and drifting a new electron avalanche can arise by the cathod. At positive tip the electrons migrate to the tip. The remaining ions directly decrease the field intensity in front of the tip. Because through this a high field intensity arises in the middle of the arrangement, the increasing of discharge channels will be advantaged. This polarity effect is shown therein, that at negative tip higher breakdown voltages are achieved. Thus the configuration of technical devices must happen after the breakdown voltage at a positive tip. Annotation: supplementary informations are to find in High Voltage Technology II, SS 2003 lecture notes,

chapter 9, page 36 et sqq. Equivalent circuits are meant for a better understanding of the processes. On this account they

describe the simplified reality. The given equivalent circuit can be conformed to the physical behaviour of a real arrangement

with external partial discharges by additional switching elements. For instance is to mind that the height of the breakdown voltage depends on the polarity in many cases. The additional installation of a demodulator parallel to C1 allows that periodic appearing discharges of only one polarity are regarded. In those half periods, where no partial discharges appear, the demodulator discharges the capacitor C1.

As mentioned before, the external partial discharges and the through this arisen (corona-) losses have a high practical relevance, in particular for high-voltage transmissions. The corona behaviour of open-wire cables is important for the technical properties and the economic efficiency of a high voltage transmission. Corona measurements can be carried out in a laboratory, too, if the conductor arrangement to analyse is chosen as internal electrode of an arrangement of coaxial cylinder (figure 8). In such a "corona weir" the field behaviour nearby the conductor differs

Partial discharges 12 only a bit from the field behaviour at a real open-wire, because one can expect that at this open-wire the conductor space is very large compared to the conductor radius. Thus the field nearby the conductor proceeds cylindrical symmetricly (as in the test arrangement).

Figure 8: Corona weir (1 inner conductor, 2 external cylinders)

This corona weir can be used for experiments with sine-shaped a.c. voltages up to 25 kV. Conductor 1, which is to be analyzed, is clamped in the axis of external cylinder 2. An a.c. voltage u(t) is applied at conductor 1. For picking up the PD-level a PC measuring system, which displays the current PD-level, is available. A rapid rise of the PD-level from a few pC (basic noise level due to environmental effects) up to a couple of hundred pC signals the achievement of the onset voltage Ue. With Ue the breakdown field intensity of the inner conductor can be calculated:

Furthermore the current i in the earth connection of the insulated erected external cylinder can be measured. It can be expected that the current is approximately equal to the current which comes from the high voltage conductor. Figure 9 shows the behaviour of a current with applied voltage u(t) which was measured in such a way:

ed

ai

iln

U = Er rr

(9)

Partial discharges 13

Figure 9: Behaviours of current and voltage at a corona weir

Herefrom is to detect that the current i consists of a sine-shaped current and a "hump" per half period. The sine-shaped current part arises through the capacity conductor-cylinder, which can be expected as constant. The humps present the corona current ik (and thus the interesting current part here!). The formula is given by: Experiments make clear that the corona current rapidly increases with the voltages momentary value after exceeding the onset voltage Ue. The current develops through migration of ions generated at the discharge of the prior or same half period. The corona current is an active current and corresponds to the corona losses, which are caused by the necessary power for adhering the collision ionization. The corona losses of open-wire lines highly depend on the absolute humidity and thus on the weather; the losses are remarkable higher at clammy weather than at sunshine and they can add up to 10 kW/km. To achieve an adequate height of the corona onset voltage of an open-wire line the conductor diameter has to be chosen adequately large. To hold the field intensity low at the open-wire cables, bundle conductors are used. Three-phase lines are dimensioned for a mean effective field intensity at the conductor at nominal voltage of about 17 kV/cm.

kddui = C + it

(10)

Partial discharges 14 2.3 Creeping discharge Creeping discharges arise at the threshold of two insulating materials with different state of aggregation, e.g. at the surface of a solid insulator at air, predominantly at surface normal field stress. Typical technical sliding arrangements are bushings, ends of cables or the emission position of winding bars of electric machnines out of the plate package. A classical model arrangement for analysis of creeping discharges is the Toepler sliding arrangement. It consists of a simple latern slide with an attached bar electrode. The backplate electrode is built by a metal surface superimposed at the other side of the latern slide, compare figure 10.

It is Distinguishing for all sliding arrangements, that a direct breakdown cant develop due to the dielectric medium with high breakdown field intensity, which is arranged between the electrodes. The breakdown can only be effected crabwise along the insulator surface. Material to the development of the breakdown is the specific surface capacity of the dielectric medium. Only if the surface capacity has a certain minimum amount, creeping discharges can develop. Another precondition is, that it actually amounts to predischarges at the electrode. Their prevention represents the most effective medium to prevent creeping discharges.

The onset voltage of the above shown Toepler sliding arrangement has the following dependency on the layer thickness s:

er

sU K Ue in kV; s in cm

Figure 10: Sliding arrangement according to Toepler: test arrangement (left) and sectional view (right) with presentation of surface charges and surface capacities

Glasplatte

Knopf - oder Stabelektrode

earthed metal foilon the back side

Latern slide

Button or bar electrode

Partial discharges 15 Factor K can be computed theoretically, but the knowledge of many additional parameters as the development of the electrode or the surface resistance of the insulation is necessary. So it is convenient to schedule empirically found values for K:

Arrangement K

metal border at air 8

in SF6 21

metal or graphite border in oil 30

graphite border at air 12

When exceeding the onset voltage, predischarges, which come from the electrode, charge the surface capacity. The charging current, which is carried on through the insulator as displacement current, depends on the surface resistances, surface capacities and consequently on the insulant thickness and the applied voltage. Because the surface capacity C0 is much bigger than C1, almost the full voltage is applied at C1. That leads to a spreading of the high voltage potential on the surface, without the appearance of a significant decrease of the field intensity at the particular head of discharge. A further increase is advantaged.

First pulse shaped partial discharges happen when applying a 50 Hz a.c. voltage with u(t) > Ue, which dont depend on polarity at creeping discharges. If the voltage is further enhanced after exceeding the onset voltage, creeping brush discharges (streamer) develop. In the Toepler sliding arrangement they spread out astrally as a sliding corona arround the sliding pole. At a coaxial arrangement they spread out along axis over the bulk of the dielectric material; compare the following figures (11).

Figure 11: Creeping discharges at the Toepler sliding arrangement (top left and

right) and at coaxial arrangement (below) Because the charging current highly depends on the surface finish of the insulator, dirt and salt

Partial discharges 16 deposits can decrease the onset voltage in conjunction with humidity (dew, fog). At impulse voltage fast voltage changes lead to eminently high displacement currents. Thus creeping discharges are very rich of energy here. Exceeding the brush onset voltage leads to a permanent damage of the insulant surface already after a short while. 2.4 Appearance of PD One can gather from the appearance of PD to the place in the whole insulation arragement, where this PD happens. Five typical appearances are listed in the table below, in which the PD are presented regarding to the phase postion of voltage (note: this kind of representation can actually be found in some commercialized PD measuring systems).

Type x

Oscilloscope picture

Description

Form of discharge

A

00 _

+

Equal pulses on a half wave, symmetric to voltage crest. At increase of voltage, quantity not until amplutide rises. Pulses on the other half wave not until a higher voltage.

Sharp peaks towards extensive backplate electrode in gas. Charge pulses in negative half wave: peak on high voltage. Charge pulses in positive half wave: peak on earth.

B 00_

+

Pulses in both half waves symmetric to voltage crest. Smaller pulses with equal amplitude. At increase of voltage rising quantity.

Sharp peaks towards extensive backplate electrode in liquids. High charge pulses in positive half wave: peak on high voltage. High charge pulses in negative half wave: peak on high voltage.

C 00_

+

Pulses between crossover and crest on both half waves. On average pulses on bath half waves equal height.

Empty spaces in solid insulant. Gas bubbles in liquid insulant. Contacted insulated conductors. Creeping discharges on surfaces without contact with metal. Not deflected metal parts.

Partial discharges 17

Type x

Oscilloscope picture

Description

Form of discharge

D 00_

+

Pulses between crossover an crest on both half waves. On average pulses on half wave higher.

Empty spaces in insulant at the electrodes. Creeping discharges on surfaces at the electrodes. High charge pulses on positive half wave: discharge on voltage side. High charge pulses in negative half wave: discharge on earth side.

E 00_

+Pulses symmetric to both crossovers.

Bad contact between metal parts or between semiconducting umbrellas.

Further information: lecture notes to lecture high voltage technology I from Professor Hinrichsen, chapter 10, p. 2-5. 3 Measurement technological coverage In general PD inside an operating resource cannot directly be measured at the place of their appearance. Different methods of measurement exist to detect possibly existing PD-activities. The detection of PD happens through

- acoustical PD-measurement - optical PD-measurement - chemical analysises (e.g. gas-in-oil-analysis) - electrical PD-measurement

In fact external partial discharges and creeping discharges are visible to the naked eye, if the test room is shaded. The low-light-level ampilfier is a camera, which needs minimal light conditions for good exposures. Onset voltages for brush discharges can be determined by acoustic cognition: When it begins to crackle heavily, the value of the test voltage is to be read off from the measuring device to get the onset voltage. The just described methods are very simple and effective. But they are not appropriate to a

Partial discharges 18 quantitative measurement. Therefore partial discharges are determined with electrical measuring devices. In the following this method is presented in general. Afterwards the measuring arrangement used at the TUD is presented. Basics of every PD testing and measuring circuit are devices with negligible PD. Outer disturbances can be reduced by the use of disconnecting transformers, filters and screened measuring cabins (keyword: Faraday cage). There the measuring devices are located beyond the high-voltage danger area. Partial discharge processes can be traced out with an oscilloscope or a PD measurement system. In this experiment pulse shaped partial discharges are to be analyzed with an electric PD measurement system. The apparent charge q makes for designation of PD at apparatusses regarding the determination of the insulation condition. It is defined as the short-term supplied charge of an calibration pulse at the specimen clamps, which changes the clamp voltage temporarily about the same value as the PD itself.

Partial discharges 19 3.1 Classification of PD measuring devices for measurement of the apparent charge q

Figure 12: Classification of PD measuring devices

The different kinds of PD charge measuring devices are composed according to the classification in figure 12, whereat the given bandwidths are approximate benchmarks. Devices, which are available on the market, possess additional components in many cases, e.g. oscillographic presentations of PD pulses over an elliptic base line etc. According to the application range and the measurement conditions, the different devices types can have advantages.

PD measuring devices

integration in frequency range )( 0fFq

wide-band 1 MHzf

restricted wide-band 100 kHz 500 kHzf

narrow banded 10 kHzf

integration in time range dq i t

oscilloscope, transient recorder and integration

radio interference meter (with assessment circuit)

Partial discharges 20 3.2 Electrical method The three basic testing circuits of partial discharge measurement are shown in Figure 13, as they are intended in the VDE-guideline 0434 (IEC Publ. 60270). A very often used circuit is given in figure 13a. Therein the specimen is displayed simplified as the capacity Ca. The PD pulses attain from Ca over a coupling capacitor Ck to the measuring impedance Zmi. The filter Z between specimen and high voltage supply blocks disturbances out of the region of the voltage source and prevents a run-off of the PD pulses through the voltage source. In figure 13b a testing circuit is shown, in which Zmi is directly in the ground circuit of the specimen. This implies, that the specimen can be run insulated from earth potential. The conclusion of PD pulses takes place over Ck. This one can casually drop out, if the leakage capacitance connected to earth is big enough compared to the specimen capacity Ca. To achieve a high measuring sensitivity it is more opportune that Z drops out. Therefore the transformer leakage capacitance completely acts. Outer disturbances at PD measurements can be eliminated to a certain extent in the bridge circuit in figure 13c. The PD measuring device is placed in the bridge diagonal. At bridge balance, which is achieved by adjustment of the measuring impedances Zmi and Zmi1, the display of the PD measuring devices is more or less unaffected from outer disturbances. So the bridge circuit is suited for PD tests in unscreened measuring rooms. [1]

Partial discharges 21

Ca - specimen Ck - coupling capacitor MI measuring instrument Zmi - measuring impedance Z - impedance or filter, to prevent a bypass of the discharge pulses at the high voltage supply and to reduce

disturbances caused by the voltage supply CC coaxial connection cable OL optical cable CD coupling four-pole network

Figure 13: Basic PD test circuit according to IEC 60270 a) series circuit of measuring impedance and coupling capacitor

b) series circuit of measuring impedance and specimen c) bridge circuit

Partial discharges 22 Before working reliably with a test and measuring circuit, it must be calibrated (figure 14). For that purpose a known voltage jump U0 and a known charge alternation Q0 is generated at the specimen (so at a parallel wiring). This is related to the display of the measuring device.

G - impulse generator

Figure 14: Calibration

The circuit of the test board of TUD and the used devices are given in figure 15.

L1 - phase N - neutral conductor C0 - high-tension capacity St Tr. - adjustable transformer 400 V/0-550 V 32 kVA Tr. - testing transformer TEO 250/60 0,38 kV/250 kV 60 kVA Rd - damping resistance 2 k MI - partial discharge measuring device Zmi - measuring impedance

Figure 15: Experimental plant of TUD The measuring plant conforms to the measuring circuit a (figure 13a) according to IEC 60270. The damping resistance has to prevent an overload of the transformer on case of a short circuit.

Partial discharges 23 4 Overview Teilentladungen (TE) sind Durchschlge. Je nach Ort ihres Auftretens wird unter und TE und der Sonderform der Gleitentladung unterschieden. Beim Hochfahren der anliegenden Prfspannung an einen TE-Prfling ergeben sich bei berschreiten der Einsetzspannung zunchst . Wird die Spannung weiter gesteigert, kommt es erst zu und dann zu . Zum besseren Verstndnis der TE-Prozesse werden die Prflinge modellhaft in nachgebildet. So knnen impulsfrmige Entladungen dadurch erklrt werden, dass ein beim Erreichen der einer paralellgeschalteten schlagartig entladen wird. Teilentladungsmessungen knnen auer mit elektrischen Messgerten auch mit und Methoden durchgefhrt werden. Anhand des Oszilloskopbildes knnen innere und uere TE durch die zur - Spannung unterschieden werden. Eigens dafr entwickelte TE-Messgerte erfassen die , die in der Einheit gemessen wird. 5 Description of the used measurement technique 5.1 ICMsystem-digital partial discharge detector1 The ICMsystem is part of the Power Diagnostix ICM Series of digital partial discharge detectors.

The ICMsystem is a powerful, versatile instrument for the evaluating the condition of medium

and high voltage insulation. The ICMsystem is usable over a rage of frequency of applied

voltage, including power system frequency (50/60 Hz) and VLF (0.1 Hz).

The power Diagnostix ICMsystem provides high-resolution digital PD patterns for

characterization of defects in:

generators and motors gas insulated switchgear(GIS) transformers, bushings

1 Text cited from the ICMsystem instruction manual

Partial discharges 24 cables joints, terminations

power electronics capacitors other low, medium or high voltage insulation

A wide range of external preamplifiers provides control of the frequency range in which PD

activity is detected, form 40 Hz up to 2 GHz.

Assorted coupling devices, including quadrupoles coupling capacitors and current transformers

are available to sense the PD signal in the object under test. The ICMsystem provide effective

noise gating that blocks phasestable noise as well as noise independent of the applied voltage

cycle, allowing the ICMsystem to be used in noisy environments without losing significant PD

data.

The circuit test setup contain the device under test (DUT) connected to the high voltage source

and a coupling capacitor connected in parallel as in the figure bellow. The quadrupole serves to

separate the high frequency current of the partial discharge signals from the power frequency

current of the capacitor.

The test setup follows the given standards, IEC60270. For a standard laboratory application a

high voltage source and a coupling capacitor are available.

Partial discharges 25

5.2 PC Software

The operating parameters of the ICMsystem are fully computer controlled, making it simple to

use with standard Power Dagnostix software. The actual recording of PD patterns is independent

of the PC, so the ICMsystems performance is unaffected by speed limitation of the PC or

communications. The ICMsystems PC software includes convenient options for in-depth

analysis and printing of stored PD patterns.

Partial discharges 26 5.3 Calibration

Partial discharge measurements are relative measurements and therefore require a calibration.

Also the entire signal path from the discharging site to the instrument and some instruments

properties as filters are introducing an overall attenuation which is not known exactly.

To calibrate a system, connect to the test object a pulse source which is injecting a certain

amount of charge, according to the relevant standards. Subsequently, the instrument is set to

calculate correction factors causing the displays to read the charge amount injection.

With the partial discharge pattern display chosen, a cluster of pixels or a line of pixels at an even

height will appear. Double clicking to the location (with respect to the Y axis) of the cluster or

line of pixels brings up the calibration sub-panel as shown in the figure above.

The peak charge indicator shows now the inserted value.

5.4 Special options

For applications such as DC testing or stepped high-voltage testing, the ICMsystem allows

recording PD activity versus time (sequentially) instead of versus phase angle.

Partial discharges 27 6 Carrying out the test Before every measurement the testing arrangement has to be calibrated. For that purpose the impulse gerenator is attached to the non-earthed arrangement and a defined PD-level, e.g. 100 pC, is set. The line received on the monitor is defined as a value of 100 pC. Also when the arrangement is stress-free, a low PD-level is measurable: The basic noise level, that is caused by environmental effects. This level must be recorded before the measurement. 6.1 Internal partial discharges at a cast resin body A cast resin body with an artificial error is to be attached as specimen. It should be detected, at which voltage and at which shown charge amount a certain number of applications per half period appears. Therefore the testing voltage must be increased slowly to U 20 kV and decreased again, so that both onset voltages and output voltages can be written down. 6.2 External partial discharges 6.2.1 Needle electrode at air Incorporate the needle-plate spark gap should as specimen and adjust a sparking distance of s = 100 mm. Connect the PD measuring device according to circuit diagram a (figure 13a). In figure. 6 schematically shown forms of discharge should be followed and the respective onset voltages as well as the charges should be written down. Pick-up the complete diagram. 6.2.2 Measurements at a corona weir Incorporate the corona weir into the test circuit. Write down the diameters of the inner and outer conductor (at least two different diameters should be used!). At first pick-up the charge amount depending on the test voltage, which is displayed at the PD measuring device (approximately five measured values). Write down the onset voltage Ue, the output voltage Ua as well as the appendant PD levels. Follow the forms of discharge, which are schematically shown in figure 6, write down the respective onset voltages and charges and pick up the complete diagram.

Partial discharges 28 6.3 Creeping discharges at the Toepler sliding arrangement Pick up the corona starting voltage and the handle brush starting voltages at the Toepler sliding arrangement with the electric and acoustic method. Continue until breakover. 7 Composition Diagram the behaviour of Ue and Ua = f(n/f) for increasing and descreasing test voltage of the cast resin body by means of the measured values from 6.1. Calculate the breakdown rating Ed for the wire, which is used as inner conductor of the corona weir by means of the measurement from 6.2.2. Calculate U/s from 6.3.

Partial discharges 29 Bibliographies /1/ Knig, Dieter/ Rao, Y. Narayama: Teilentladungen in Betriebsmitteln der Energietechnik,

VDE Verlag 1993 /2/ Hinrichsen, Volker, Skript Hochspannungstechnik II /3/ Kchler, Andreas , Hochspannungstechnik : Grundlagen - Technologie Anwendungen,

Springer 2005 /4/ Hilgarth, Gnter: Hochspannungstechnik, Teubner-Verlag Stuttgart 1992 /5/ Kind, Dieter: Einfhrung in die Hochspannung-Versuchstechnik, Vieweg-Verlag Braun-

schweig 1987 /6/ Schwab, Adolf J.: Hochspannungsmetechnik, Springer-Verlag Berlin, New York 1981 /7/ Beyer, Manfred/ Boeck, Wolfram/ Mller, Klaus/ Zaengl, Walter: Hochspannungstechnik,

Springer-Verlag Berlin, Heidelberg 1986 /8/ Kind, Dieter/ Krner, Hermann: Hochspannungs-Isoliertechnik, Vieweg-Verlag Braun-

schweig 1982 Standards IEC 60270, High-voltage test techniques Partial discharge measurement, 2000 DIN EN 60270 (VDE 0434):2001-08, Hochspannungs-Prftechnik Teilentladungsmessungen DIN EN 60270 Berichtigung 1 (VDE 0434 Berichtigung 1):2002-11 DIN VDE 0434 DIN 57434 (VDE 0434):1983-05 (Z) Hochspannungs-Prftechnik Teilentladungsmessungen