How to conduct the lightning impulse withstand test on 550kv GIS?

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How to Conduct the Lightning Impulse Withstand Test of

Three Gorges Right Bank Substation 550kV GIS

HuWei, Chen Yong, Wang Qifa, et al

HIMALAYAL - SHANGHAI - CHINA

Abstract: With the rapid development of power grid in China, an increasingnumber of GIS are adopted in new substations. The lightning impulse test isrecommended after the installation of GIS in order to check its insulationperformance and reduce the risk of equipment failure. However, owing to thedesign and production, common lightning impulse generator with largecapacitance load especially GIS cannot generate lightning impulse waveform,which meets the requirement. Therefore, based on lightning impulse withstandtest of Three Gorges Right Bank 550kV GIS, models of lightning impulsegenerator and field equipment are established, and simulation calculation isdone. According to the result, a method of installing the inductor in the impulsecircuit is put forward to generate oscillating lightning impulse which meets therequirement of the test. This method is successfully applied in the test andresults are good. Valuable experience has been accumulated for the lightningimpulse test on GIS of 750kV, 1000kV substations in China.

Key words: Lightning impulse, GIS, wave front time, withstand voltage, field test,inductor

1. Introduction

Owing to many advantages, such assmall volume, small floor area, easy toinstall, high reliability and lessmaintenance, GIS is recommended bya growing number of people. Theimpulse test is needed after theinstallation of GIS on site to checkwhether the equipment is wellinstalled and reduce the risk ofequipment failure, and finally toensure that GIS can operate in safeand reliable mode. Entrancecapacitance of GIS is far greater thanother equipment, so lightning impulse

voltage test with capacitive load ismore difficult. In most cases, loadcapability of lightning impulsegenerator is thousands pF. Whileentrance capacitance of GIS can evenreach 10 thousands, ten times higherthan generator. Large capacitive loadcan lengthen wave front time oflightning impulse, reduce impulsepeak, and cannot generate lightningimpulse waveform, which conforms tothe standard, in the field test. Thatimposes a new challenge for field test.In order to make common lightningimpulse generator applicable to GISlightning impulse test, this paper



comes up with a method of adding theinductor to impulse circuit to generateoscillating lightning impulse whichmeets the requirements of the test bymeans of theoretical calculation andmodel and simulation. This method issuccessfully applied in the ThreeGorges Right Bank 550kV substationGIS field test and the results aresatisfied.

2. Equipment Parameter

2.1 Impulse voltage generator

12-stage bilateral impulse voltagegenerator with high performance isadopted in the test. The body belongsto triangle type. Due to large size, theinfluence of stray inductance andcapacitance on impulse waveformcannot be neglected. Structure ofimpulse generator circuit is shown inFig.1. Main parameters are as follows:2 sets of 100kV main capacitors Cs ateach stage and bipolar charge; serialdischarge stage voltage - 200kV and12 stages in all; the capacitance ofeach main capacitor - 1.5μF, theinductance - 1μH, wave frontresistance per stage rf - 20Ω, residualinductance - 1μH; wave tail resistancert - 100Ω; the length of the lead - 2m,estimate via 1μH/m, the inductanceper stage - 5μH, stay inductance ofdischarge circuit - 80uH or so; arc leadand circuit contact resistance - 10Ω;lead inductance of external circuit isestimated at 20μH. The capacitivevoltage divider is composed of 4impulse capacitors in series. The totalcapacitance of high-voltage arm Cd is500pF.

Fig.1 Lightning impulse generatorcircuit

2.2 GIS

The Three Georges Station consiststwo parts: Left Bank station and RightBank station. The 550kV switchstation adopts 3/2 connection methodwhile unit connection is applied in thegenerator and transformer. Two setsof generator and transformercomprise alliance unit connected into550kV. GIS is adopted ranging frommain transformer high-voltagebushing to outlet disconnectingswitch.

After the installation of GIS, it isnecessary to carry out on-site ACwithstand voltage test and lightningimpulse test. Owing to a large numberof switches, time limitation, 550kV GISfield AC withstand voltage test ofThree Gorges Right Bank station isconducted according to five testintervals. Among them, thecapacitance of longest internal one isabout 23nF. The lightning impulsevoltage test is conducted for each testinterval.

According to standards, in the GISlightning impulse test, front wave timeof lightning impulse wave ≤ 8μswhile wave front time of oscillatinglightning impulse waveform ≤ 15μs.

3. Theoretical Calculation



3.1 Lightning impulse generatorcircuit

In order to facilitate the production,the lightning impulse discharge circuitin Fig.1 can be simplified to equivalentcircuit in Fig.2. According to Fig.2,RLC’s circuit differential equation islisted below:

tf= 3.24 (r+Rf)21

21

CCCC

(1)

tt= 0.693 (r+Rf)(C1+C2) (2)

Where:

tf — wave front time

tt — half peak time

C1 = C8 /n — main capacitance ofimpulse circuit

n — stage of impulse circuit

C2=Cd + Cg — load capacitance

r — arc lead and circuit contactresistance

Rf= nrf — wave front resistance

Rt= nrt — wave tail resistance

Fig.2 Equivalent circuit of lightningimpulse generator

The stray inductance in the dischargecircuit will impose certain effect onimpulse waveform, even cause circuitoscillation. Test results indicate thatcircuit inductance has a significanteffect on wave front time but little

effect on wave tail. Given circuit strayinductance L, equivalent circuit ofwave front time calculation is shownin Fig.3. In order to prevent outputwaveform from oscillating, circuitresistance R should meet therequirements of formula (3). If theoscillation never occurs to the line,wave front time can be obtained viaformula (4).

R≥ 2CL (3)

tf= 2.33RC = 4.66 LC (4)

Where:

R — total resistance of the circuit

L — total inductance of the circuit

C — total capacitance of the circuit

C =C1C2/ (C1+C2)

Fig.3 Equivalent circuit of wave fronttime

According to the formula (4), as loadcapacitance increases, wave front timelengthens. If main capacitance andload capacitance are fixed, wave fronttime can be altered by means ofreducing the circuit resistance. As forimpulse voltage generator withoutconsideration of large capacitive testobject, R value is taken little in orderto restrict wave front time withininternational standards. However,little R value may cause impulse



waveform oscillation and overshootexceeding, and the equipment cannotoutput the waveform which meets therequirement.

3.2 Features of impulse voltage outputwith large capacitive load

The charge voltage of each stagecapacitance should be less than 100kV.Based on equipment parametersprovided by manufacturers, entrancecapacitance of GIS reaches 23nF. Bymeans of PSCAD/EMTDC, simulationcalculation is done for lightningimpulse waveform. The result isshown in Fig.4.

Fig.4 Lightning impulse waveformwith GIS

From the Fig.4, if impulse source isdirectly connected into GIS, due toinsufficient load capability of impulsegenerator with large capacitance,output lightning waveform deformsseriously. Its wave front time is 14μs,which is higher than standardlightning impulse wave front time -8μs. The output efficiency of impulsevoltage is 59%, which cannot meet thetest requirement.

3.3 Influence of inductance onimpulse voltage

According to standards, oscillationlightning impulse wave can beadopted in the GIS test. After the

inductance L is connected betweenimpulse voltage generator body andload in series, the voltage UL of energystorage element L is proportional tocurrent change ratio of circuit charge,which can be represented by UL = L(diL/dt). During the lightning impulseprocess, UL is attenuated oscillationwave of voltage polarity alternatingtransformation. Therefore, UL cangenerate oscillating lightning wave inthe test. At the moment of discharge,UL changes all of a sudden and theinductor starts to store the energy; atthe first peak of discharge current, theinductance energy begins to release.In other words, main capacitance andinductance in series continue tocharge load capacitance, generatingimpulse overshoot. The maximumpeak overshoot appears after the firstcharge. After the oscillationinductance is connected in the testcircuit, higher impulse voltage will begenerated around impulse peak due tothe release of storage energy but largeoscillation inductance will causelonger wave front time. Hence, beforethe test, suitable inductor should beselected.

According to test condition, three2mH inductors are selected andconnected in series to test circuit inorder to generate oscillation lightningwave. The lightning impulse waveformis shown in Fig.5.



Fig.5 Lightning impulse waveformwith oscillating inductor

Compared with the result shown inFig.4, the amplitude of outputwaveform improves a lot after theconnection of the inductor but wavefront time increases as well. In orderto make wave front time meet theinternational standards, Rf should be20Ω in the test. Both the amplitudeand wave front time of impulsevoltage meet the requirement of thetest, which is shown in Fig.6.

Fig.6 Oscillating lightning impulsewaveform via best proposal

By this connection method, wave fronttime of output oscillation lightningimpulse wave is 14μs and outputefficiency reaches 112%, which meetsthe requirement.

4. Field Test

Based on simulation result ofoscillation lightning impulsewaveform described above, onlypartial transformation to availableequipment can meet the requirement.As for Three Gorges 550kV GISsubstation lightning impulsewithstand test, the inductor isinstalled in the impulse voltage circuitto generate oscillation lightning wave.Layout of test equipment is shown inFig.7.

Fig.7 Layout of GIS lightning impulsetest equipment

According to test requirement, it isnecessary to conduct lightningimpulse test for many times. Onepositive and negative polarity test oflightning impulse with 620kV, 990kVand 1120kV peak; three positivepolarity tests and five negativepolarity tests for 1240kV lightningimpulse. The updated equipment fullymeets the requirement. Testwaveform and simulation waveform of1240kV lightning oscillation impulseis shown in Fig.8.

Fig.8 Comparison of test waveformand simulation waveform

(dot lime--simulation waveform



solid line--test waveform)

The peak of lightning oscillationimpulse waveform generated by theequipment in the test is 1240kV andwave front time is 14μs, which meetsthe requirement. In the Fig.8, thesimulation waveform conforms to testwaveform; oscillating frequency ofsimulation waveform is close to testresult; oscillating depth is slightlylower than test result. Hence, impulsegenerator and GIS model establishedin the paper is accurate. In theimpulse tests with different peaks,simulation result conforms to testresult.

The breakdown discharge neveroccurs among many lightning impulsetests of 550kV GIS, which indicatesthat GIS insulation meets therequirement of the design.

5. Conclusions

Some 12-stage bilateral-chargelightning impulse generator with highperformance is analyzed. Based onequivalent circuit, outputperformance of this generator underlarge capacitive load is discussed.Conclusions are as follows:

1) When lightning impulse voltagetest is carried out for test objects withlarge capacitance, such as GIS orhigh-voltage cable, wave front time oflightning impulse wave will increase,and impulse peak will decreasebecause load capacitance exceeds themaximum value bore by impulsevoltage generator.

2) The inductor can be installed inthe lightning impulse circuit. Theinductor’s property of storing energy

can be used to generate lightningoscillation impulse wave whichconforms to international standards,and detect the insulation level of GIS.

3) Simulation result shows that theinductor can be made use of togenerate lightning oscillation impulsewaveform with different amplitudeand frequency by means of basing oncommon lightning impulse generator.

4) The proposal of installing three2mH inductors in series is putforward in this paper according tosimulation result and withstand testof Three Gorges Right Bank 550kV GISsubstation. This proposal issuccessfully applied in the field testand results are good. Moreover,valuable experience has beenaccumulated for the lightning impulsetest on GIS of 750kV, 1000kVsubstations in China.

5) Simulation result and field testresult indicate that installing theinductor is an effective method forgenerating oscillating lightningimpulse, and can meet the specialrequirement.

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