Investigation of propagation of partial discharges in power transformers and techniques for locating the discharge

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1 Investigtion of propgtion of prtil dischrges in power trnsformers nd techniques for locting the dischrge S.N. Hettiwtte, Z.D. Wng nd P.A. Crossley Astrct: The loction of prtil dischrges in power trnsformer cn e determined sed on the chrcteristics of the trnsfer function from the dischrge source to the mesuring terminl. Previous studies prtilly vlidted the technique using computer simultion nd prcticl experiments sed on PD clirtor to represent dischrge signl. Rel dischrges produced y insultion defect models were used to study how dischrges propgte in n kv plin-disctype trnsformer winding. A coron model nd floting ojects in oil model produced dischrge signls with different durtions t vrious loctions long the winding. Mesurements were tken t the tp of the ushing cpcitnce through conventionl dischrge detector. The signls were filtered, mplified nd fed into digitl storge oscilloscope. The frequency spectr of the mesured signls showed significnt similrities irrespective of the type of dischrge source. The chrcteristic of the trnsfer functions, i.e. the crests nd troughs in the spectr, could e used for locting the source of the dischrge. Energising the trnsformer incresed the level of electric noise, which did ffect the low frequency end of the spectr, ut did not hve ny impct on the chrcteristics used for loction. Introduction A significnt percentge of the primry plnt used in power networks in the developed world is now operting eyond its predicted design life. Ensuring tht such equipment continues to operte sfely nd relily is of crucil importnce prticulrly when one considers the existing competitive/regultory operting environment. Insultion fults re one of the min cuses of equipment filure nd re often preceded y prtil dischrges. A prtil dischrge (PD) is loclised electricl dischrge tht only prtilly ridges the insultion etween conductors nd which my or my not occur djcent to conductor []. PDctivity inside power trnsformer cn deteriorte the oil or pper insultion, degrde the voltge withstnd cpility nd eventully led to ctstrophic filure. Detecting nd locting dischrges t n erly stge in their evolution cn prevent n incipient insultion fult, fcilitte mintennce nd extend the sfe operting life. Prtil dischrge my occur in solid, liquid nd gseous insultion medi nd re generlly initited y n excessive loclised electric field. The PD-induced current in n externl circuit depends on the nture of the dischrge nd the geometry of the system []. Specificlly, in power trnsformer the most common sources of PD re gseous r IEE, IEE Proceedings online no. 9 doi:.9/ip-smt:9 Pper first received 9th My nd in revised form th My S.N. Hettiwtte nd Z.D. Wng re with the School of Electricl Engineering nd Electronics, University of Mnchester, Mnchester M QD, UK P.A. Crossley is with the School of Electricl nd Electronic Engineering, Queen s University Belfst, Belfst BT9 AH, UK E-mil: zhongdong.wng@mnchester.c.uk dischrges tht occur in ules in oil or voids in pper/ pressord, surfce dischrges long oil/pressord rriers or dischrges cused y floting ojects. The PD signls detected t the terminls of trnsformer will hve different wveshpes due to the different propgtion pths from the dischrge source to the terminls, even if the sme type of dischrge genertes them. Previous simultion studies [] hve suggested tht the PD signls t the mesurement terminls footprint their propgtion pths nd the trnsfer functions from the source of the dischrge to the mesuring terminl hve unique chrcteristics. The mesurements conducted using PD clirtors on severl plin-disc-type windings [, ] vlidted the simultion nd confirmed tht the chrcteristics of the trnsfer function cn e extrcted from the mesurement signls nd used for loction purposes. A conventionl dischrge detector ws used during those mesurements. Studies crried out on the trnsfer function sed PD loction techniques [, ] indicted the importnce of developing these techniques into prcticl pplictions. In this pper experimentl results otined y energising n kv trnsformer winding with vrious externlly connected insultion defect models re presented. The resulting rel dischrge signls help vlidte the trnsfer function sed PD loction technique nd prove it is fesile to comine this loction technique with mesurements mde y conventionl PD detector. Chrcteristics of trnsfer functions If prtil dischrge occurs t position x in the equivlent circuit of winding s shown in Fig., the currents t the ushing tp i nd the erthed neutrl i g cn e descried IEE Proc.-Sci. Mes. Technol., Vol., No., Jnury

2 y () nd () [] where C B n+ K K K. K C C C C C B i ig Fig. x l Equivlent circuit of single, uniform trnsformer winding i ðjoþ¼ C B C r sinhðrðl x ÞÞ coshðrlþþ C i pd ðjoþ B C r sinhðrlþ i g ðjoþ¼ coshðrx Þþ C B C r sinhðrx Þ coshðrlþþ C i pd ðjoþ B C r sinhðrlþ r ¼ ðþ ðþ Co Ko ðþ is the ushing cpcitnce, C,, nd K re the cpcitnce to erth, inductnce nd series cpcitnce of the winding per unit length nd l is the totl length of winding. These equtions show tht the trnsfer functions hve fixed poles ut different zeros, since the denomintors re only relted to the physicl prmeters of winding, i.e. C B,C,l nd r, while the numertors re lso relted to the loction of the dischrge source, i.e. x. Consequently the frequency positions of these zeros re relted to the loction of the dischrge source nd the positions of poles re determined y the overll winding construction nd therefore re t fixed frequencies. Trnsformer winding nd defect models The kv trnsformer winding used in this study is plin disc-type winding with 7 sections. Sections nd 7 hd 7+/ turns per section nd ech turn consisted of doule conductors of.. mm. The remining sections hd +/ turns per section nd ech turn consisted of doule conductors of.. mm. The thickness of insultion on ll the conductors is. mm. Two defect models were used in the experiments; one ws coron model nd the other floting ojects in oil model. They were connected t vrious loctions on the winding nd when energised used to study the dischrge propgtion chrcteristics. The coron model ws used when the winding ws energised directly to test the possile noise prolem due to the opertion of trnsformers. Figure shows the normlised rel dischrge signls generted y the coron nd floting ojects models. The dischrge wveform of the floting ojects in oil hs rise nd fll time of roughly ns, while the coron hs similr rise time, pproximtely ns, nd significntly slower fll time, roughly ns for negtive coron nd ns for positive coron. Figure shows the mesured impednce of the circuit ( kv winding + ushing cpcitnce) otined using n impednce nlyser. At high frequencies the trnsformer winding ehved s RCG network comprised of inductnce, cpcitnce C, nd resistive R nd dielectric G losses. Therefore the winding impednce incresed or decresed, depending on whether the impednce ws inductive or cpcitive t tht prticulr frequency. The I pd, ma c time, μs Fig. Normlised dischrge wveforms Floting ojects in oil Negtive coron c Positive coron voltge, db ngle, deg 9 7 frequencies of impednce crests vry ccording to the type of winding nd its physicl construction. Tle gives the frequencies of crests of mesured impednce in Fig.. Experiment results. Two types of dischrges: coron nd floting ojects in oil Figure shows the test set-up used for investigting the propgtion of rel dischrges long the winding. The dischrges were produced using insultion defect models energised t high voltges. The winding ws connected in prllel with dischrge-free coupling cpcitor C k ¼ pf, chosen to simulte the ushing cpcitnce. Fig. Mesured impednce of circuit ( kv winding + ushing cpcitnce) Tle : Frequencies in khz of crests of mesured impednce in Fig. r r r r 9 7 IEE Proc.-Sci. Mes. Technol., Vol., No., Jnury

3 Z C k HV winding erthed V winding dischrge detector DSO Fig. Experimentl test set-up nd Set-up Set-up coron model The mesuring impednce Z ws connected in series with C k to mesure the line-end signl of the winding. Signls picked up y Z were fed into the dischrge detector which hd pssnd of B khz. The filtered nd mplified signls were not only oserved on the ellipse of the detector, ut were lso rought out through coxil cle to digitl storge oscilloscope (DSO) used for recording the wveforms. In Fig. s the coron model ws moved to different discs on the winding, the ir gp etween the point electrode nd the outer insulted conductor ws mintined t mm. The effect of coron in the ir gp ws then mesured t the tp of the ushing cpcitnce. Figures nd show the dischrge signls mesured t the line-end of the winding using the DSO. The loction of the dischrge is expressed in terms of the numer of disc pirs from the line-end terminl. It cn e seen from Fig. tht when the PD is locted close to the line-end the shpe of the mesured signl is the response of the filter used within the detector. If the PD is locted further wy from the line-end the mesured signl ecme more oscilltory due to the propgtion pth of the winding. The difference in the shpe of the mesured signl cn e used to determine whether the dischrge source is close to the lineend or deep down into the winding. A more precise loction cn e chieved y extrcting the fetures tht chrcterise the dischrge loction from the frequency spectrum of the mesured signl. ~ () floting ojects in oil model () ~kv ~ The frequency spectr of the mesured signls in Fig. were computed using n FFT nd corrected using the inverse frequency chrcteristic of the nd pss filter. ThesereshowninFig. nd. The crests of frequency spectr occurred t fixed frequencies, s given in Tle. The troughs chnged with the loction of the PD source, for exmple the first trough for PD t disc pir ws t khz nd for disc pir it moved to khz. The frequencies of the troughs incresed s the loction of the PD moved wy from the mesuring terminl t the lineend, s given in Tle. It cn e seen tht the frequency of the first trough incresed s the loction of the PD moved up to disc pir, fter which it disppered. The region eyond disc pir, where the troughs re no longer oservle ws clled the lind region. Within this region it is not possile to locte the precise position of PD. The lind re for the trnsfer function sed loction method cn e explined y () nd (). sinhðrðl x Þ¼ Co Ko ðl x ðþ Þ ¼k p k f zk ¼ q ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ; k ¼ ; ; ::::n Cðl x Þ þ Kk p f zk p p ffiffiffiffiffiffi ¼ f c ; when x pproches l K ðþ As the position of dischrge x pproches the neutrl end of the winding (disc pir ), i.e. l, the frequencies of the zeros of the trnsfer functions pproch single frequency s given in (), which is clled the criticl frequency of the winding. The test set-up shown in Fig. corresponds to floting ojects in oil dischrge. The winding ws kept t erth potentil nd n oil vessel contining point/pressord/ plte electrode with floting metl ojects ws energised t high voltge. The floting ojects, pproximtely mm in dimeter, were mde using luminium foil. The metl plte inside the oil vessel ws connected to different tps long the winding. The line-end signls were then mesured through the dischrge detector. Although the dischrge source of floting ojects in oil is significntly different to the coron model, the mesured signls were significntly similr to those shown in Fig.. For resons of revity, they re not included in the pper. Figure 7 nd show the frequency spectr of these signls. voltge, V 7 9 time, μs Fig. ine-end mesured signls for test set-up 7 9 time, μs IEE Proc.-Sci. Mes. Technol., Vol., No., Jnury 7

4 voltge, dbv Fig. Frequency spectr of line-end signls for test set-up Tle : Frequencies of crests in mesured spectr shown in Fig. (poles of trnsfer functions) p p p p 7 7 Tle : Frequencies in khz of troughs in mesured spectr shown in Fig. (zeros in trnsfer functions) Disc pir z z z F F 9 F F F Although the originl dischrge signls otined from coron nd floting ojects in oil hd different durtions nd shpes, conventionl PD detector cn only pick up their low-frequency components nd cnnot differentite etween them. This ws regrded s the min deficiency in chrcterising dischrge source since durtions nd shpes of dischrge signls were relted to dischrge mechnisms nd were the min fetures for identifiction. An extrwidend mesurement system would e essentil for dischrge chrcteristion. However, it is n dvntge in locting the dischrge since the conventionl dischrge detector is only interested in the low frequency rnge of the dischrge signls nd within this rnge the footprint of the propgtion pth would e dominnt, irrespective of the type of dischrge source. By compring Fig. 7 with Fig. it cneseenthespectrresulting from floting ojects in oil is similr to coron. Frequencies of troughs in these frequency spectr re given in Tle nd they re gretly similr to those given in Tle.. Energising winding with defect models The winding ws energised using n kv distriution trnsformer controlled y vric. In Fig., point electrode ws directly connected to different tps long the winding with mm ir gp to the erthed plte electrode. The model is designed to simulte dischrges etween shrp conductive extrusion from the winding nd the tnk. voltge, dbv Fig. 7 Frequency spectr of line-end signls for test set-up IEE Proc.-Sci. Mes. Technol., Vol., No., Jnury

5 When the point plne gp ws connected to disc pir coron incepted t supplied voltge of. kv. In comprison when it ws connected to disc pir, the coron incepted t 9. kv. Mesurement of dischrge ctivity could only e recorded for point electrode connected to disc pir etween nd. Between nd it ws not possile to crete dischrge using n kv supply. Tle : Frequencies in khz of troughs in mesured spectr shown in Fig. 7 (zeros in trnsfer functions) Disc pir z z z F 7 7 F F F F F F F F F HV winding When the winding ws energised there ws significnt udile nd electricl noise due to core virtion, which ws then detected y the monitoring circuit. The result ws n increse in the noise level seen in the time domin signls. The corresponding frequency spectr in Fig. 9 nd 9 show tht the noise only ffects the spectr in the lower frequency rnge, i.e. elow khz. The fetures used for locting the dischrge, i.e. the crests nd troughs within the frequency nd to khz were not ffected y the noise from the trnsformer. Tle lso gives the frequencies of troughs in the spectr. In Fig. n erthed point electrode ws used to simulte the effect of shrp protrusion t erth potentil. With this set-up, the dischrge current generted ner the point electrode hd to couple through the min insultion nd trvel through the winding efore reching the line-end mesuring terminl. Figure nd show the frequency spectr, which hs similr chrcteristics to those presented in Figs., 7 nd 9. Tle gives the frequencies of troughs in the spectr of Fig. for comprision with Tles, nd. They show good greement nd indicte the zeros of trnsfer functions re the good fetures for loction of dischrge sources. Tle : Frequencies in khz of troughs in mesured spectr shown in Fig. 9 (zeros in trnsfer functions) Disc pir z z z ~kv ~ Z C k erthed V winding DSO dischrge detector Fig. Experimentl test set-up nd Set-up Set-up () point electrode t erth potentil () point plne gp F 7 F F 7 F 7 F F voltge, dbv Fig. 9 Frequency spectr of line-end signls for test set-up IEE Proc.-Sci. Mes. Technol., Vol., No., Jnury 9

6 voltge, dbv Fig. Frequency spectr of line-end signls for test set-up Tle : Frequencies in khz of troughs in mesured spectr shown in Fig. (zeros in trnsfer functions) Disc pir z z z F F F 79 F F 7 99 F F F 9 F F F F F F F Conclusions Rel dischrges produced when energising coron model nd floting ojects in n oil model connected to n kv plin disc-type trnsformer winding were used to study the propgtion chrcteristics of prtil dischrges. Irrespective of the durtion of the dischrge signl generted y different types of dischrges, the frequency spectr of the signls mesured t the tp of ushing cpcitnce showed significnt similirites. The crests of the spectr lwys occurred t fixed frequencies, while the troughs chnged in frequency with the position of the dischrge source. As the dischrge source ws moved wy from the mesuring terminl, the frequencies of the troughs increse. Although energising the trnsformer incresed the level of electric noise, the noise only ffected the low-frequency spectr, it did not ffect the frequency chrcteristics used for loction. Using conventionl dischrge detector tht extrcts the low-frequency signls of the dischrge, the trnsfer functions tht descrie the dischrge propgtion pths dominte the frequency rnge of the mesured signls, irrespective of the type of dischrge source. The proposed trnsfer function sed loction technique could e implemented using conventionl PD detector nd digitl storge oscilloscope. Acknowledgments This work ws finncilly nd techniclly supported y Ntionl Grid, Alstom Trnsformers nd Edison Mission First Hydro. The uthors would especilly like to thnk Mr. P. Jrmn, Mr. A. Drwin nd Mr. G Edwrds for their helpful comments nd encourgement. The first uthor would like to thnk the Open University, Sri nk in llowing him to study for PhD degree t UMIST. 7 References IEC7: High-voltge test techniques - Prtil dischrge mesurements rd Edition, Decemer Boggs, S.A.: Prtil dischrge: overview nd signl genertion, IEEE Electr. Insul. Mg., 99,, (), pp. 9 Wng, Z.D.: oction of prtil dischrges in power trnsformers. PhD Thesis, UMIST, Mnchester, UK, 999 Wng, Z.D., Crossley, P.A., Cornick, K.J., nd Zhu, D.H.: Prtil dischrge loction in power trnsformers, IEE Proc. Sci., Mesur. Technol.,, 7, (), pp. 9 Hettiwtte, S.N., Wng, Z.D., Crossley, P.A., Drwin, A., nd Edwrds, G.: Experimentl investigtion into the propgtion of prtil dischrge pulses in trnsformers, Presented t the IEEE Power Engineering Society Winter Meeting, New York, USA, Jnury Deruyne, H., nd esint, O.: A method to locte PD source in winding y processing signls t its terminls. Proc. th Int. Symp. on High Voltge Engineering (ISH ), Bnglore, Indi, August 7 Akri, A., Werle, P., Borsi, H., nd Gockench, E.: Trnsfer function-sed prtil dischrge locliztion in power trnsformers: fesiility study, IEEE Electr. Insul. Mg.,,, (), pp. IEE Proc.-Sci. Mes. Technol., Vol., No., Jnury