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Field Test and Analysis of Partial Discharge

on GIS Under Impulse Voltage

HIMALAYAL - SHANGHAI - CHINA

Sun Qiang, Dong Ming

Abstract: It is shown that many GIS devices still have insulating faults even afterthey have passed partial discharge test under power frequency voltage. Hence, itis necessary to experimentally study the partial discharge in GIS under impulsevoltage so as to timely identify internal incipient faults and to insure stable andsafe operations of equipment. We designed a set of partial discharge detectiondevices which could be conveniently applied in the field experiment consideringthe conditions of impulse voltage experiment on the basis of investigating thenecessity of partial discharge of GIS under impulse voltage, and applied thedevice in partial detection on several 800kV breakers and GIS devices of 750kVsubstation. Moreover, we discussed anti-interference measures in fieldexperiment. The results indicate that the test device is effective.Key words: impulse voltage, partial discharge, GIS, SF6 switchgear, PD detection,field test technology.

Introduction

GIS (gas-insulated metal-enclosedswitchgear) is one of the mostimportant electric equipment in thegrid power. Its operation reliability isdirectly related to safety and stabilityof the whole power grid. However,with large-scale construction ofhigh-voltage power gird and wideapplication of GIS equipment, themanufacturer is forced to delay thedelivery of equipment because ofproblems of design, production,technologies, materials and mainaccessories etc. Many faults occurduring the transportation, installation,debugging and operation. Causes ofequipment faults are divided intoseveral aspects, such as productdesign, production technologies,outsourcing parts, on-site installation

quality, which may expose in the typetest, field hand-over test andequipment operation. In order toavoid GIS faults and ensure the safetyof electric system, people in theindustry are searching for morereliable methods by trial and error toguarantee GIS quality and improve thediagnosis level of GIS insulationcondition.

The GIS field handover test codestipulates that on-site withstandvoltage test is an important project. Asfor newly-installed equipment, thetest can effectively detect insulationdirt in the inner GIS, incorrectinstallation, physical dimensiondeviation, damages, and foreign objectleft during the package,transportation and installation. Hence,it is the most strict and effective test



method, playing a decisive role indetermining whether the equipmentcan be put into use. It also can ensureinsulation level and avoid insulationfaults. According to relevant statistics,two thirds of GIS equipment withoutfield withstand voltage test haveinsulation faults. Therefore, the fieldwithstand voltage test must beconducted after the installation of GIS.

Research has shown that hiddentroubles in the equipment havedifferent effects on gas insulationunder different voltage waveforms. Allthe time, there is a consensus in theindustry that switching impulsevoltage test is of special significanceonly above super high voltage. So,among field handover test, theimpulse voltage test have not beenconducted for many years. However,with large-scale construction andoperation of super-high voltage powergrid in recent years, there have beenan increasing probability of faultsbefore being put into use and duringthe operation, which illustrates thatonly conducting the power frequencyvoltage test cannot meet therequirement. The surface flashoverhappens many times after 363kV GISequipment of Li Jiaxia HydropowerStation was put into use. As for theThree Gorges Hydropower Station,even though AC voltage test isqualified, flashover occur at threepositions over the impulse voltage test,which shows the limitation ofsuper-high voltage equipment and thenecessity of impulse voltage test.

In addition, as for some defects, eventhough power frequency can stimulateand expose the defects, it also

worsens the defect owing to itspersistence at the same time, causinggreater damage to the equipment. Theimpulse voltage cannot cause thedefect expansion because of its“transient” characteristics. Therefore,field impulse voltage test of GIS is ofpractical significance to detect GISequipment problems and furtherensure its safe operation.

Please refer to reference [20] for thesensitive relation between test voltagewaveform and GIS insulation defect.

To sum up, in view of the necessityand practical demand of field impulsevoltage test and inconvenience ofdouble-index impulse voltagewaveform, IEC issued IEC 60060-3standards in 2005. China also adoptedGB/T 1927.3 High Voltage TestTechniques Part 3 Definition andRequirements of On-site Test in 2010.This standard recommends oscillatinglightning impulse (OLI) and oscillatingswitching impulse (OSI) as thewaveform of filed impulse voltage test.The oscillating impulse waveform hasmany advantages, such as highefficiency, suitable for on-site usageand close to actual waveform of theequipment.

This paper presents a set of partialdischarge (PD) detection systemunder field impulse, which is appliedin the PD detection on 800 kV breakerunder standard lightning impulsevoltage test and 750kV substationunder GIS on-site oscillating lightningimpulse handover test. Theanti-interference capability of PDdetection system is also discussed,which is of great importance to reducethe rate of fault and improve



operation reliability.

1. PD Detection System under FieldAvailable Impulse

As shown in Figure 1, PD dischargedetection system is mainly composedof voltage generator, voltage divider,GIS group, Rogowski coil, attenuator,transient voltage suppressors (TVS)and oscilloscope. In order not to affectsmooth implementation of routinedelivery test, impulse voltage test andpartial discharge detection must beconducted on site at the same time.However, the signal of partialdischarge is weak. Hence, Rogowskicoil is used as signal sensor (which isshown in Figure 2) to avoid bringingharm to insulation performance ofelectric equipment. The Rogowski coilis wrapped around the grounding lineof breaker. One line is connected tooscilloscope through attenuator bymeans of cables while the other isconnected to oscilloscope throughTVS. Given that the amplitude ofdisplacement current passing throughthe grounding line of breaker is great(1-3kA), TVS can shield the signalabove threshold to avoid PD impulsebeing covered by displacementcurrent signal. Compared with theattenuator, TVS can better capture PDimpulse without digital filter; whilethe attenuator can effectively collectthe signal of displacement current,which passes through the groundingline, and measure and analyze thesignal. Figure 3 shows the untreatedsignal detected by TVS in the actualmeasurement.

Fig.1 Circuit diagram of partialdischarge test on site

Frequency/MHz

(a) Characteristics of amplitudefrequency response

Frequency/MHz

(b) Characteristics of phasefrequency response

Fig.2 Characteristics of dynamicresponse for Rogowski coil

Fig.3 Untreated signal of partialdischarge using TVS under positivepolarity lightning impulse voltage

2. PD Detection of 800kV Breakerunder Lightning Impulse Voltage



Test of Delivery Standards

The double-index lightning impulsevoltage test circuit of 800kV breaker isshown in Figure 4. The impulse sourceis 4800kV Marx impulse generator.The output waveform of power-supplyvoltage is shown in Figure 5 (Upk ispeak voltage; β is lightning over-shoot;t1 is front time and t2 is tail time ). ThePD measuring system (Fig 1)is utilizedto detect the signal of breaker groundline.

Fig. 4 Experiment system underimpulse voltage

Fig. 5 Outputting wave of thegenerator of impulse voltage

First, 1 min power frequency voltagetest is conducted to 3 breakers (peakof test voltage is 960kV) and at the

same time, Haefely PD 561 instrumentis used to observe the PD condition.The test goes well and any abnormalcondition is not found; Secondly,conduct 80% positive polarity and100% negative polarity standardlightning impulse voltage test forthree breakers 2 times under openand closed switch condition;meanwhile, partial discharge is alsomeasured. The 100% lightningimpulse voltage of 800kV breaker is2200kV. The test is conducted in thestandard high-voltage test hall. Theelectromagnetic shieldingeffectiveness S ＞ 60dB andinterference level of power frequencyPD is below 3pC, which conforms tothe requirement of GB/T 12190-1990.The test found that B-phase breakerdetects current impulse under bothopen and closed switch condition andhas high repeatability. The momentand impulse characteristics areconsistent with the signal of partialdischarge obtained from lab research.Because the level of electromagneticnoise in the test is very low and thereis no impulse interference source, wecan determine that this impulse signalis PD current signal (as shown inFigure 6). In the test, A and C phasebreakers do not have PD signal.



(a) Open-switch condition 80%(1680kV) PD impulse underpositive-polarity lightning impulse

(b) closed-switch condition 100%(2100kV) PD impulse undernegative-polarity lightning impulse

Fig. 6 Partial discharge test onB-phase breaker under lightningimpulse voltage

Based on Figure 6, there exitsinterference impulse with greatamplitude at the initial momentbecause of circuit interference causedby sphere gap discharge. The PDsignal mostly appears within 3-10μsshortly after the peak. The sourceinterference does not have an impacton PD measurement. The magnitudeof PD impulse can even reach about20V. In addition, the base line in theFigure 6 is not 0V. The base line in

Figure 6(a) reaches around -4V, whichillustrates that under the impulsevoltage there is obvious capacitivecurrent in the GIS cavity, especiallywhen the breaker is in open-switchcondition. However, because of therole of voltage attenuator, the currentvalue remains at the low level.

This example proves that the samedefect differently reacts to differentkinds of voltage. Even though thedefect is not found under powerfrequency, it may be exposed underthe impulse voltage of higher filedstrength. Hence, the detection ofpartial discharge under impulsevoltage is conducted as diagnosis test.That can better play a part ofdiagnosis, detect and avoid the fault intime, and cover the shortage of powerfrequency voltage test.

3. PD Detection of 750kVSubstation under GIS on-siteOscillatory Lightning ImpulseHandover Test

The test circuit of 750kV substationGIS on-site oscillatory lightningimpulse handover test is shown inFigure 7. The impulse source is2400kV Marx impulse generator.Output waveform of oscillatorylightning is shown in Figure 8. Pleaserefer to Figure 1 for PD measuringsystem under oscillatory lightningimpulse. Since GIS group has manygrounding points on site, the sensor isconnected to one of them to detect thesignal.



Fig. 7 Equipment and test object offield experiment

(a) Oscillatory lightning wave ofpositive polarity (Upk = 1547.53kV, t1 =2.18μs, t2 = 17.12μs )

(b) Oscillatory lightning wave ofnegative polarity (Upk = 1523.57kV, t1

= 2.30μs, t2 = 17.24μs )

Fig.8 Oscillatory lightning impulsevoltage wave created by the generatorof impulse voltage

Now conduct 75% positive polarityand 100% negative polarityoscillatory lightning impulse voltagetest 3 times under closed-switchcondition to A, B and C Phase. Theresult is good. There is no breakdownfound in any phase; measurementresults of impulse voltage PDdetection system show that there is nopartial discharge. Take C-Phase as an

example, the detection result isillustrated in Figure 9. There is sourceinterference with great amplitude atthe initial moment.

(a) 75% (1680kV) oscillatorylightning wave of negative polarity

(b) 75% (1680kV) oscillatorylightning wave of positive polarity

Fig.9 Partial discharge test on C-phaseGIS group under oscillatory lightningimpulse voltage

4. Discussion on PD detectionAnti-interference Measures

The basic idea of partial dischargedetection under impulse voltage is todetect high-frequency current impulse,which flows through GIS test objectinsulation structure. In the actualdetection, it is likely that theinterference generated by test systemor the outside environment generateshigh-frequency current impulsesimilar to PD signal. Themeasurement results of Figure 6 and 9also prove that. Therefore, testpersonnel should distinguish betweenoutside interference signal and actual



PD signal, and prevent incorrectjudgment from causing seriousconsequences. In addition, on-site testenvironment is complicated, and eachinterference signal can reducedetection sensitivity and increasesmallest measurable level. So, theability of distinguishing interferencewaveform must be improved, andmeanwhile, anti-interferencecapability of test system needs to beenhanced.

Based on some materials at home andabroad, interference sources of PDdetection on site are summarized asfollows:

1) Space electromagneticinterference generated by theoperation of external electricequipment and electromagneticinterference by sphere gap double hitand defect model discharge;

2) Direct interference generated byeach kind of high-frequency signal inthe power distribution network;

3) Neutral point potential of powersource led by the power-source line ofmeasurement instrument andelectromagnetic interference;

4) Interference caused by transientcurrent flowing through outershielding layer of cables.

Against the above interference, pleaserefer to the following anti-interferencemeasures:

1) One-point grounding of test circuit.Lay the copper belt as grounding wirebetween voltage divider andmeasurement instrument; the voltagedivider should approach thegrounding electrode as close as

possible; lay the measurement cablealong the copper belt to reduce thecircuit area as much as possible; usetin paper to wrap the cable joint.

2) The power source for detecting thesystem should be separated byisolation transformer and adoptsoptical and photoelectricity totransmit the detected signal.

3) Adopt digital filter. The digitalfilter can utilize the discrete system tochange the waveform or spectrum ofinput digital signal, allowing usefulsignal frequency to pass and restrainuseless signal output. In the test, thefilter must ensure that PD signal bearthe characteristics of constantamplitude-frequency response andlinear phase. Compared with IIR(infinite impulse response) digitalfilter, FIR (finite impulse response)digital filter not only ensures straightamplitude frequency feature, but alsogains strict linear phase feature,avoiding phase distortion. Therefore,the FIR is recommended. The effectdrawing of digital filter filters out thedisplacement currents flowingthrough GIS (Figure 10). Based on thedrawing, displacement current of theoscillating is filtered out.

Fig. 10 Digital filter filters out thedisplacement currents flowingthrough GIS

5. Conclusions



Safe operation of super-high voltageGIS equipment is directly related tothe reliability of main power gird. Inrecent years, many researchers athome and abroad have been focusingon utilizing impulse voltage test andits PD detection to conduct GISinsulation breakdown diagnosis andanalysis. We designed a set of PDdetection and analysis device underGIS impulse and gained theexperience about on-site diagnosis ofGIS equipment fault.

1) The PD detection device underimpulse voltage can effectively extractPD signal without affecting normaldelivery test. This new trial can be asupplementary to PD and impulsevoltage test and is of practicalsignificance to the evaluation ofequipment safety.

2) Hidden troubles in the GIS canhave different effects on gas insulationunder different kinds of voltage. PDtest under the impulse can cover theshortage of available field test and caneffectively find fault defect; in theactual test of 800kV GIS breaker, it isfound that if the quantity of powerfrequency PD is not exceeding thelimits, partial discharge occur at 80%positive polarity lightning impulseunder open-switch condition and100% negative polarity lightningimpulse under closed switchcondition.

3) This paper utilizes TVS to detectPD signal in the strong displacementcurrent and obtains good effect.However, the current condition cannotbe detected as a whole during thewhole process of applied voltage.Hence, the PD detection system under

GIS impulse on site needs to befurther improved and practicalized.Various methods are utilized toimprove anti-interference ability ofmeasuring system to effectivelydistinguish and eliminate interferencesource.

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onGISUnderImpulseVoltage HIMALAYAL-SHANGHAI-CHINA ... under GIS on-site oscillating lightning impulse handover test. The