A 6.6 kV continuous disc type winding of a distribution transformer is used to investigate the propagation of partial discharges (PD) with the aim of location. The winding was modelled, as multiconductor transmission lines with each turn represented by a transmission line. This approach results in the model being valid up to a few MHz in frequency. The validity of the model was confirmed by impedance measurements on the winding. The transfer functions calculated between probable PD source locations to winding terminals showed that the troughs (or zeros) change in frequency with the location of PD source and hence can be used for the location of PD. Transfer functions obtained experimentally using a discharge calibrator as the PD source, showed very good agreement with the calculations

Crossley, P.A.

Darwin, A.

Edwards, G.

Hettiwatte, S.N.

Jarman, P.

Wang, Z.D.

English

Research Repository

   MURDOCH RESEARCH REPOSITORY   http://dx.doi.org/10.1109/ICPADM.2003.1218453         Hettiwatte, S.N., Wang, Z.D., Crossley, P.A., Jarman, P., Edwards, G. and Darwin, A. (2003) An electrical PD location method applied to a continuous disc type transformer winding. In: Proceedings of the 7th International Conference on Properties and Applications of Dielectric Materials, 1 - 5 June, Nagoya, Japan, pp 471-474.    http://researchrepository.murdoch.edu.au/14156/          Copyright © 2003 IEEE  Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.   Proceedings  of  the  7th  International  Conference  on Properties  and  Applications  of  Dielectric  Materials lune  1-5  2003  Nagoya  P2-43 An Electrical PD Location Method Applied to B Continuous Disc Type Transformer Winding S. N.  Hettiwatte",  2.  D. Wangl, P. A. Crosslef,  P. Jarman',  G. Edwards4 & k Darwin' I UMIST, Manchester, UK,  'Queen's  University, Belfast, UK 'National  Grid Company, Coventry, UK,  4Edison Mission, Dinonvig Power Station, Llanheris, Gwynedd, UK 5Alstom Transmission & Distribution Transformers, Stafford, UK 'E-mail:  s.hettiwatte@student.umist.ac.uk Abstract:  A  6.6kV  continuous  disc  type winding  of  a distribution transformer is used to investigate the propagation of  partial  discharges  (PD)  with  the  aim  of  location.  The winding was  modelled,  as multicondunor transmission  lines with  each  turn  represented  by  a  transmission  line.  fis approach results in the model  being valid up to a few MHz in frequency.  The  validity  of  the  model  was  confirmed  hy impedance  measurements  on  the  winding.  The  msfer functions calculated Ween  probable PD source locations to winding terminals showed that the troughs (or zeros) change in frequency with the location of PD source and hence can be used  for  the  location  of  PD.  Transfer  functions  obtained experimentally using a discharge calibrator as the PD source, showed very good agreement with the calculations. INTRODUCTION Dielectric  breakdown  in  transformers  are  most frequently preceded  by  partial discharge  (PD).  There can  be  many  causes of  partial  discharge:  defects in insulation, loose connections,  floating metallic objects arcing  under  oil,  are  a  few  [I].  In  operating transformers, online condition monitoring is @armed to  measure the PD. Unless, action  is taken  to detect, quantify and remedy the situation at an  early stage of PD  development,  catastrophic  failures  can  occur resulting  in  outages  and  further  secondary  losses. Depending upon  the severity of the PD, diagnosis and maintenance of the transformer may be required and for the  maintenance,  the  location  of  the  PD  source  is important. The PD signals, which originate inside a transformer, can only be measured at a limited number of places due to the  structural  design  of  the transformer.  The  PD signal gets distorted as it havels from the site of origin to the measuring terminals. Therefore, it is imperative to understand the propagation mechanism of the PD for the location of its site of origin. A recent  study [Z]  has shown that  by  modelling each double  section  of  a  winding  as a  lumped  element  a model  valid  up  to  a  few  hundreds  of  kHz can  be obtained. With  this model, the troughs of the transfer functions calculated between  probable PD sites to the measuring twinals  could be used to locate the PD. The aim of this  pap^ is to devise a transformer model valid for even higher f?equencies  (a few MHz), and use it to locate  the  PD.  Hence  each  turn of  the winding  was modelled  by  a  transmission  line  with  distributed parameters. The validity of this model up to lOMHz was confirmed by impedance measurements on the winding. The papa presents an extension of the work described in  a  previous paper  [3],  which  was  not  convincingly validated due to the impact of noise and the low  A/D resolution  of  the signal  conditioning  equipment  used [4].  The experimental  results  shown  in  this  paper, obtained  using a  higher  A/D resolution  oscilloscope, agrees well with the simulation results from the model. This  paper  also  presents  some  features  of  signals measured at a transformer windig terminals that can be used to locate a PD. COMPUTER SIMULATIONS The design data of the 6.6kV transformer used in the model are listed below: Transformer Design Data Number of sections  = 22 Number of turns per section  = 13 Conductor dimensions  =2.5mmx 13.75mm Singlesided inter tum  insulation thichess  = O.2mm Inter-section distance  = 4.5" Outer radius of the LV winding  = 203.3mm Inner radius of the HV winding  = 230" Outer radius of the HV winding  = 267.7mm Transformer Winding Model The model used for the transformer winding,  based on  the  multiconductor  transmission  line  theory  [SI assumes  the  parameters  to  be  uniformly  distributed along the line. Each transmission line can be regarded as made up of a cascade of identical cells of infinitesimal length,  as shown in  Fig.  1. L,  R, C and G denote the series inductance, series resistance, parallel capacitance and conductance, all per unit length and calculated per turn basis so that each turn of the winding is considered as equivalent to a 'transmission line'.  The winding can then he represented as in  Fig. 2, where the subscript 's' denotes sending end and  denotes the receiving end. The number of transmission lies  in the model, n equals to 286 (22 sections x 13 turns). ..................... Ax Fig. I. One cell ofdiaibuted parameter model -  471 - Fig.  2 shows that the transmission line model has the following terminal conditions:  ~ ..............  In'  4-  -..  Val +  Is, ....  ..................  b2  *  5 .._.  VSI VS2 .............  ..........  ........... (9) ._  -. Is(i+l)=-f,(f)  for i=l ton-l  ....... -  .............. t  vu  .  ..  ............. -  ..............  -  ......  V,(i+l)=V,(r]  for  i=l ton-l  (10)  .........  ......... ~ ............ &  [Wj  IW-1) VI41  ......-.  .............  +-VW ..  .  ,-.....  ............  ..... rsd.-.---.....""' v..  fv, Fig. 2. Timsfomer winding represented by multiconductor bansmission line made1 The  voltage  and  current  distribution  along  these transmission lines can  be described by  wave equations [61, (1)  and (2): [Z][Y]V  = [P2]Y  d'V dr2 -= [U[ZII = [P,'lI  (2) d21 dx2 where  [Z]  and  [Yl are the  impedance and  admittance matrices for the winding given by -= [Zl=  [RI+  jo[Ll  (3) [Yl= [cl+  jo[Cl  (4) and [PI  = [Z][Y]  and [P?]  = [Yl[Z]. The solution of (1)  and (2) can be expressed in classical form as a summation of two waves one travelling in the positive (or  forward) direction and the other travelling in the negative (or reverse) direction. (5) 2  )  (6) Yo  in (6) is the characteristic admittance matrix given by [Yo] = [z]-'[P] = [Yl[P]-'. v, = vle(-LPI=)  + v*e(lPII) I, = yo(yle(-vl=)  -  y  e([pl  r) With  terminal  conditions  applied  at  the  sending and receiving ends it is possible to express the currents in Fig.  2 in  terms of  voltages  as given  by  (7). Further simplification of  (7) is possible  using the eigenvalues (y)  and  eigenvectors  (Q)  of  [PI.  Equation  (7)  then simplifies into the form of (S),  where [A]  = M[Q][y]- 'coth([rll)[QI-'  and  PI  = M[Ql[ul~'~~h([ull)[Ql~' are n x n matrices, and 'n'  is the numbex of conductors in  the model.  Equation  (8)  expresses 2n  equations of sending and receiving end currents in terms of sending- end and receiving-end voltages. coth([ PI[)  cos ech([P]I)  V,  [  = "[cos  ech([P]I)  coth([ PI[) ][ Y,) (7) Using these terminal conditioos, the matrix in (8) can be reduced 60m (2n x 219 to a (n+l) x (n+l) matrix. PD injection:  If a PD current signal, IpIh is injected to the km  turn of the winding, (9) will get modified for i = k-l as I,  (k)  +  I, (k  -  1)  = I,  (11) The voltagecurrent distribution along the winding can now be expressed by (12). The  terminal  conditions  of  the  winding  have  to  be incorporated  into the  calculations. If  the  line-end  is connected to a bushing (as is the usual case) having a capacitance Ce,  the line-end current can be expressed as I,  (1)  = -W,V,(U  (13) If the neutral-end is solidly earthed, V,(n)= 0  (14) Transfer functions can now be calculated between  the PD  current  and  the  he  (ls(l)flpD) and  the  neutral currents (IR(n)/Im). Fig. 3 and Fig. 4 show such bnnsfer functions  calculated  for  various  positions  of  the  PD current  along the winding. The ranges of  fiequencies used  were  60m  1 !&z  to 2 h4Hz.  Calculations  were performed for every other section  starting 60m the 2"d section. Hence the position of the PD signal source is in multiples of 26 turns (2 sections  ~2x13  tums). Some salient features of these transfer functions are: (i)  The  crests  (or  poles)  always  occur  at  fixed 6equencies. (ii)  Ihe troughs (or zeros) change in  frequency with the position of the PD current source. The troughs at  the  limeend  transfer  functions  increase  in 6equency as the PD source is moved  away 6om that  end  whereas the troughs  at  the neutral  end transfer functions decrease in  6equency as the PD source is moved towards that end. ................... 1-  " "  ''E??E! - I/ A- Y  ,  V  .  ,  .,  ,.' .  ,  i I  ..................... t,,  .-I.,  .-.  ,l a,  .................. mansformer bushing was represented by a capacitor (C,) with a value of 22OpF. Two 50 R resistors were used to measure the line-end and neutral-end signals generated across the winding terminals. The oscilloscope @SO) used has a  12-bit vertical resolution  with  a maximum sampling rate of 2GS/s on  a single channel. CS S -v  ............ ;  .......... ..  :*  ;.  ;,  :.  :  ,',  :.  ,',  ,'. i '.Y.IIII> Fig. 3. Transfer Iimctions (in dB) between PD  soum  and line-end t,  ,  ,  ,  .  ,  1 *'#  I,  I.  I.  .I  I  (1  I.  I.  I.  I Fig. 4. Transfer tinctions (in dB)  been  PD sourcc  and neutmal-end EXPERIMENTAL MVESTIGATION The experimental test setup used to investigate the PD propagation along the winding is shown  in  Fig. 5. The PD signal was generated using a calibrator consisting of a  step  wave  voltage  generator  (S)  and  a  coupling capacitor  (Cc)  of  IOpF.  The  capacitance  of  the S =  Rep wave generator, Cc =coupling capacitor (  IOpF) Ce = bushing capacitance (ZZOpF),  R  = m&og  resistors (SO  C2) Fig. 5. Experimental test set up The calibrator was used to inject a PD signal pulse of 20OpC  to altanate sections along the winding starting kom  the 2"  section. This included sections 2, 4,  6,  8, 10,  12, 14,16, 18 and 20 kom the line-end. In terms of turn numbers these were: 26,52,78,104,  130, 156, 182, 208,234 and 260 turns. The responses at the terminals as  well  as  the  injected  signal  were  measured simultaneously using three channels of the DSO. Each signal measured by the DSO had a memory  length of 500 data points. In the DSO, the vertical resolution was  set to  12 bits and the samplmg rate to IOMSls and the bandwidth to 20MHz.  The  spectra of  the terminal signals were obtained aller augmenting the data points with  zeros up to a total of  8000 points to inuease the kequency resolution, and then applying the Fast Fourier Transform (FFT) algorithm in  Matlab@.  These spectra are shown  in Fig. 6 and Fig. 7. In comparing the spectra f?om  the line-wd responses in Fig.  6  with  the  corresponding  results  6om  the simulations in Fig. 3, one can see  the similarity of pole- zero pattern clearly. There are two poles before the first zero  for a PD signal in the top half of the winding (i. e. up to turn numbers 130), and this pattern diminishes as the  PD  source  is moved  fivtha  down  the winding. When  the PD source is closer to the neutral-end, the zeros at the line-end terminal spectra are undetectable for this range of kequencies  (1 kHz -2  MHz). The  spectra  of  the  neutral-end  responses  60m  the measurement and the simulations (Fig.  7 and  Fig.  4) show that there is only one pole before the first zero  and as the  PD  signal  moves  away  kom  the  neutral-end terminal, apart kom the first zero, other zeros become undetectable. -  473 - The  two measuring  terminals,  line and  neutral  ends, only  differ  by  the  capacitance  of  the  bushing,  CB, connected at the line-end terminal. That is, if C, were not thme line-end and neutral-end spectra would have given  similar  characteristics for  PD  signals at  equal distances 6om  their measuring terminals. 40 .P I  o,  0.  me  0.  1  <x  I.  I.  I#  2 Im"l.i,  ,I*., Fig. 6. Frequency spefw of measured linesnd signals (in dBj P  I m>  OI  .II  0s  I  ,2  I.  (6  ,I  1 FI..YCI,  ,"*1, Flg  7  Frequency spem  of measured oeuml-end signnls (an dB) Another  feature  noticeable  in  the  measured  signal spectra is that the crests and troughs are not as sharp as in  the simulations.  This is due to the high  damping effect  of  the  losses  in  the  winding,  which  is  not adequately represented by the simulation model. In the simulation  model  the  calculated  resistance  and  the conductance  only  consider  the  skin  effect  and'the dissipation factor. PD LOCATION To locate the position of a PD along the winding, one has to obtain the 6equency  spectra of  the measured terminal signals and compare them with  the simulation results of transfer functions. The 6equmcy of the first- zero 60m measurements has to be  compared with the first-zero  6om  simulations.  If  fiquencies  match closely, the location of the PD source along the winding can be approximated by the location of the PD source in the simulation. CONCLUSION The  6equency  spectra of  signals measured  at  the terminals of a continuous-disc type transformer-winding can give useful information with regard to the location of any partial discharge. The spectra of terminal signals show  that  troughs  increase  in  fiequmcy  as  the  PD source  is moved  away  from the  measuring terminal. Simulated transfer functions from probable PD source locations to the terminals can  be  compared with  the measured  tenninal  signal  spectra  to  match  the 6equencies of the fist-zero. If a close match is found, the location  of  the PD  can be  approximated by  the location of the simulated PD. ACKNOWLEDGEMENT The  authors  would  like  to  thank  Alstom,  Edison Mission,  and  the  National  Grid  Company.  The  first author  is  grateful  for  the  scholarship  provided  by UMIST and the Open University of Sri Lanka. REFERENCES 111  Ryan, M.  H.,  'High  Voltage  Enginkng and  Testing',  2.d Edition, IEE, Londq  United Kingdom, 2001, pp. 635 - 653. Wang Z. D., Crossley, P. A., and Comick, K. J., 'An Algorithm for  Partial  Discharge Location  in  Distribvtion Transformers', IEEE/PES Winter Meeting, Singapore,  January  ZOOO. Hettiwatte, S. N., Crossley, P. A., Wan& Z. D., Darwin, A., and Edwards, C.,  'Simulation of a Transformer  winding for Partial Discharge  Propgation  Studies'  IEEE  Power  Engineering Society Winte Meeting 2W2, New York,  laouary 2002. Heniwane, S.  N., Wang Z. D., Crossley, P. A., Damin, A., and Edwards, G.,  'Experimental Investigation into Ihe Propagation of Partial  Discharge  Pulses  in  Transformers'  EEE  Power Engi-ing  Society Winta Meeting 2001. New Yd,  lanuary 2002. Paul,  C. R.,  'Analysis of Multiconductor Transmission Liner', John  Wiley &t Sons, 1994. Guardado,  J.  L.,  and Comick,  K  I., 'A Computer Madel  for Calculating  Stnphnled  Surge  Distribution  in  Macbe Windings',  IEEE  Trsnractions on  EneIgy Conversion, Vol. 4, No. I, March 1989. [?] 131 141 [SI 161 474 - 

A Computer Madel for Calculating Stnphnled Surge Distribution in Macbe Windings',

Analysis of Multiconductor Transmission Liner',

Experimental Investigation into Ihe Propagation of Partial Discharge Pulses in Transformers'

High Voltage Enginkng and Testing', 2.d Edition, IEE, Londq United Kingdom,

Simulation of a Transformer winding for Partial Discharge Propgation Studies'

An electrical PD location method applied to a continuous disc type transformer winding

http://researchrepository.murdoch.edu.au/14159/

An electrical PD location method applied to a continuous disc type transformer winding

Abstract

Similar works

Full text

Available Versions

Research Repository