A STUDY OF INSULATION PAIUtRL IN A EHIG VOLTAGE CURRENT TRANISPORMIRV

A Study of Insulation Failure in A High Voltage Current Transformer

A.A. Jlinoh, V.S. Mahlasela and DYV. NicolaeGraduate School of Electrical & Electronic Engineering, Faculty of Engineering.

Tshwane University of Technology,Prctoria, South AfricaTel: +27 82 787 0251;Fax: +27 12 703 4872

e-mails: adisaJimohgctnw.ac.za vusi.mahlasela@eskom.co.za, danaurel(dyebo.co.za

AcknowledgementsThe authors will like to acknowledge the supports received from Eskom South Africa.

Keywords~<Device modelling)>, ((Protection device>, <6Simulatiorn>, <Transformer»>, ((Transmission of electricenergy»>

AbstractThe ageing and deterioration of insulation in high voltage (HV) plants have been a source of concerns toutilities. Breakdown of insulation leads to failures of HV equipment. The ageing and eventual failure ofinsulation in a high voltage curTent transformer (JHVCT) is the subject of investigation in this paper. Alumiiped parameter model ofHVCT is developed. This model is used to study the influlence of deterioratingand failed iinsulation on the state variables of the HVCT. Somie possible scenarios that could lead to a CTfailure are investigated in this paper. For all the scelarios considered, steady alid transient equationsrelating the state variables of the mlodel have been developed and analyzed. The objectives of theseanalyses are to establish the behavioural claracteristics of the state variables, establish the interactionsbetween these variables, and investigate the possible generations of harmonics under the various scenariosof deteriorating and outright failure of insulation in the CT. The paper concludes with a discussion of theresults obtained.

1. IntroductionHigh voltage bushin-gs and current transformiers are among the most vulnerable power systems equipmenttbecause they are subjected to higlh dielectric anld tlherlmal stresses [1-5]. Sudden failuwe of a cutrenttransformer in the transmission substation can cost a u-tility in excess of hund-ed thousanlds of dollars indamage and loss of revenue [1. 2]. This cost alone is for putting the system back to nomal. It excItudescost suffered by customer due to power loss and production lost in some instances.

The failure pattern of current transformers in HV substations has generally been violent, catastrophic andin somiie instanlces near fatal. The natt-e of these failures makes it difficult in most cases for a post-mortemtechnical investigation to determine the root cause of the failtre. Therefore, in general, the phenomenonand processes leadinig to this rather violent and sudden failLre of HVCTs are still not clearly understood[3].In South Africa alone over 95 of this type of failure of HVCT were recorded in the 17 year periodspanning 1982 through to 1999; that is an average of between 5 or 6 current transforlmers per year. Arecent studies conducted on this failure record [1] rev,ealed that the transfonners rated 400, 275 an-d 132k1V

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A STUDY OF INSULATION PAIUtR IN A EHIG VOLTAGE CURRENT TRANSPORIIO

were the most affected. Accordin-g to the chart in Figure 1 that shows the distribution of the failure by themonth of occurrence there is nlotlhig to suggest that weather influenced the failure.

Classification by design type, Figure 2, even tlhough may appear to imply top core design to be more

prone to failuLre, there is really nothing to suggest that design type is a factor either.

Fig. 1: FailuLre distribuLtion by month of occurrence. Fig. 2: Failure distribuLtion by transformer designtype.

Anl investigation of the mlodes of failure revealed, as shown in table 1, that the most comiumlon mode iswhen one of CT's phases has failed. Even then the root or primary cause of failure could be anything. butit is suLspected in most cases to be insulation failurez because the CTs -were usually so badly danaged thiscould not be ascertained.

Table 1: Failure modes of CTs

Failure mode Cause NumberInsulation failure Unspecified 24Single phase faulted Unspecified 58Unkilown Unspecified 6Pre-energization Unspecified 2

Star point fault Unspecified I

Taik rapture Unspecified I

It is believed that the processes leading to a total failure of this equipment is complex [4. 5]. Failure couldhave started w^ith a graduLal buLt progressive deterioration of insuLlation at any part of the device. The failureof insuLlation at a sport in the device couLld then initiate the cascading of other insulation failuresculminating in concuzrent failures of insuLlation at several parts of the device, resuLlting thLs in the completefailure of the equipment.hIn an effo-t to seek solution to this problem ofHVCT failures this paper focuses on investigating the rootageing alnd eventuml failure of insulation at a location in a high voltage current transformer (HVCT). AluLmped parameter model ofIIVCT is developed. This model is sted to stLdy the inifluence of deterioratinigand failed insulation at a particular location on the state variables of the HVCT.

Root deterioration and failure of insulation is possible at several parts of the equipmiient. This paper firstinvestigates insulation deterioration and failure within the primary winding. It then considers deteriorationand failure of insulation betwccn alny sport on the primar winding alnd the groulnd. For all the sccnariosconsidered, steady anld t-ansienlt equations relating the state variables of the mlodel have been developedand analyzed. The objectives of these analyses are to establish the behavioural characteristics of the state

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A S-' DV Ofh IINl,1:;'1.R] 1I\.N A. IG Lo 1S C., R:l 1-'1l RANSI %NA].R Vi@A:. ki:jd,i,

vrlble& vstablsh th interacdns bemwn icse var iml iild kL Sith-e p 'ibt gmeraetions ofha1nnoic maier the vaiious celnalios ofdtrirtna1lid ottnh01lX ure of mulauonin te CT. Thepaper co1i6lu&&,kuth a Lscussion of the res-uls obtained.

2. Current Tmsfor ner Strueture1,igLu Ls4i cm-a.LavU.\ viowk of 4i. 1Spiml cand transfomer slimvng thc, priuiryE XsinhW wrm31N ELichis riDrinaUy ckamected direMyl to thle 14r liuce. Tlle instilamor (uormfall porelain initerial) isolats the higlpotifia [)ruLnanr% cozuolor frm the Promd leveZ1 [61, PrLmirv Txindqg wLbductor wiisists of its iiiainmlultmor andin wnasmesLF-FslatFW od his |ie (o instLiae 'it friz the CT ca (a i is noriiiav-grun&Is.. c-n be sm froin Fio,lo 3)ther is no phi lsicat bet-we t11e pur2y.- cunlmt caffiiconductor cm; soondan cuan%mt ca n cndLotor. Tls liki clocmrgL,4aic throughdi Core

Te-iina head

Prilar- TcrmzLua

Instillato

Primqi-v Par1t 5L5Isuato

S oindar> in L igs

Figilre 3: Secioill NrIew- of aF HV C!rre-mTmfonluer

3. The AIodcl DorclopmentFi,nrc 4 is ihe developed iiodte f-or [he CT hi FigLr- 3, Nxilh a2 quandlie&s and parameters rel-efred to Lhef)rhw. . I11e i1>lsulwr is mo4e2ed as5 aF pal-.Ut coiibhiaion ofXSlrei(iiead aFixicilance <=uset isrQplr&,i-s3 its prucdcal L¢reprx-ntio [4].

1~~ VWPI tP rF v 9Ll1IFrxFI Vw t; wt~~~~~~~~~~~~~~~~~I rFig. 4: N.Iodel ofHN' (-lluenLt TnlsfbrmeZr: (,,t hS1isulaor faiurc- withi rm an wvld'w (b} i'zllsudonfaillue RF5, PrImr wxin m'g nd round .

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A STUDY OF LNSLLTION FIMLURLN A HIGH VOLTAGE CURbNT UAN&SOhua NIOLAHDUM V.cota

A perfect insulation will be an open circuit, whlile a sholt circuit is an indication of failure. The insulationdeterioration anid its eventual failure -within the primary winding is shownll in Figure 4(a); while thedeterioration and eventual failure of instulation between the primary winding and ground is reflected inFigure 4(b). Breakdowvn voltage is a function of the current flowing through the insulation. Therefore, it isdepicted by a current controlLed voltage source.

The current to be transformed is depicted as the current source I ....... in the primary side of the circuitmodel. This is the current flowing through the prinary winding. A fraction n (< 1) of the primary windingZp is separated from the rest by the location of the insulation failure on the primary winding.The magnetising windings which link primary and secondary windings are represented by parallelcombination of magnetizing resistance (R.) and inductance (L,,). The secondaly vinding of the CT isrepresented by a resistance (R) and a leakage imductance (Li). The CT burden in the secondary isrepresented by the resistance (R,,) in series with an inductance (LI).

3.1 Steady State Equations

3.1.1 Insulation Failure within the Primary Winding

The steady state equations governing the model of Fig 4(a) are:

(I - n)Z (I)

(1- fl)ZP + ZisWhere lo; is the current leaking through an imperfect or deteriorating insulation, ITO... is the soturcecurrent and 4, is the prinary winding impedance given by:

Zp, = Rp, + jX1, (2)

where Rp is the primary winding resistance and X,, is the primary winding leakage reactance. Theinsulation impedance may be expressed by:

z. jxRj x j,,s (3)R + jlty

where Rj,,, is the insulation resistance and Cd,, is the insulation capacitance.The current through the secondary winding and the burden may be expressed as:

I.,= Z"' (4):sec = m sotl,,ceZn s6

The magnetizing current may thus be wsritten as:

fHUg = Z JI. (5)iig z" + z AI/O

where ZA, is the magretising impedance, expressed as:

1R±+jxX, (6)

and Z4, is the series combilnation of secondary winding and the burden impedances.

ISBN 90-I7S1S08-krLE 2005 - Dmsdca PA.

A STUDY OF LNSLLTION FIMLURLN A HIGH VOLTAGECUMNT UAN&3Ohua NIOLAHDMU Vg.it.

Zh = R, + Rh+ j(X + XI,) (7)

3.1.2 Primary Winding Insulation Failure to Ground - Breakdown Voltage Ignored

The steady state equations governing the model in Figure 4(b) with the breakdown voltage ignored are:

I =IiZZ,,& + Zq (8)

where Z4 is the equivalent impedance of secondary, burden, magnetising and primary windings given as

Z =nZ, + zz-i-+Z,

zsb =Z + zl/llt,,n

1"~= I',,h +Z,wagup + Z,

I,1 is the curent flow as indicated in Figure 4(b).

I = Inp "'Zsk+4 ..

(9)

(10)

(11)

3.1.3 Primary Winding Insulation Failure to Ground with Breakdown Voltage

Usimg superposition principle the relevant steady state equations for Figure 4(b) can be obtained as below.Subscript I is for contribution when the current source is open-circuited, while subscript 2 denotesquantities contributed when the voltage source is short-circuited.

iitS - IIa + /I2 -Z11,1.111 hrllJZ.+z,

If the high voltage rating ofthe primary winding is VP/ then the breakdownv voltage is

b,,a,mlkao I"I. .-.. X is the maximum insulation current possible, wlichMill be l . ....

Inag i/l + Dnl2 =4,bl,,I,t + 2v1.,1,2

Zs6 + Z",,where 'api and ',4, are the currents through nZp.

Isec = IsecI +I2 =Z,,,'I,,p + Z,,.I,tp2

Z,sh + Z,

3.2 Transient Equations

With thc aid of mesh currcnt analysis, thc transicnt cqu ations can be obtaincd usilng the mcsh currcnts asshowvn in Figure 4.

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ASTUDY OFILNLATION ILLUWIRN A HIGH VOLTAGE CUTMNTTgNS&Ohua NIOLAHDUM Vg.im.

(a) Insulation Failure within the Primary Winding

|If......dt_(i _ )x(I _}RP_(I _1)L , dti(i- , = (16)t~~~~~~~~~~~~i

1

R Xi,,,-, -- Jfmi,,dt=U (17)

LId

illbglR. x 0ba- 8LdwgfRX,nrdtm(18)

(R +R)x i + (LI + Lb ) dti- dt ,=0 (19)

(b) Primary Windinrg Insldation Failure fo Grouind wiftlBreakdown Voltage

f,-jdt=O (20)

nR,2 xi, ±nL, d.±,R xo if (21)dt it _dt V,taa (21)

Vi..'ak = (1PtK/m / b.... ) i)x (22)

Lmdd _RmXi,rngr =0 (23)dt

(3

(R±+ R )Xi±b+ (L, + Lb ) d iS -LI, d i. g- =- (24)dt sc dttia--

4. Analysis and Results

4.1 Steady State Analysis

4.1.1 Insulation Breakdown within Primary Windings onlyWith the aid ofMadlab®Tm [6] the steady-state equations in dte previous section were solved to obtain thevariation of some key quantities with changing state of inter-primary winding insulation as shown inFigures 5(a) to (e). The variation of insulation current with the insulation resistmice and capacitaice are asin Figures 5(a) and 5(b) respectively. For each figure, one of the insulation parameters was kept constantat an assumed value, while the other was varied. The family of curve displayed is as a result of varying n,which depicts the position of deteriorating/failing insulation along the primary winding. Insulationresistance values of 102° to 104 ohms represent pelfect insulation, but for 103 ohm and below the insulationdeteriorate and failed outright at the least value. Similarly, capacitances of 10-14 to 100 Farads presentpelfect insulationi, while 101 to 103F deteriorating and then failed insulation. In both cases the range fordeterioration of insulation, dturing which intelvention could be made before outright failure, is quitelimited.

Figures 5(c) to (e) show 3-dimensional variation of insulation current wvith insulation parameters, or withposition of insulation failure and either of the insulation parameters.

ISBN 90-781S08-5FI'T 2005 - Dli b 1'.6

ASTrDYOF NSULATiONFAILUE NAlUHVOLTACECURENTANSFOhF NICOLfERaVIe da

n=0 9

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'on=006. n=0 5-0 4nO 3

,, n=0 2i. n= 1

ins in ohms

(a)

jO

lo1

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0 7 '- 5=

@03 t~~~0

-O

1 O 1 O 1-O3

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Finoo loton in ohms 100 Cnsol ati on in

(c)

Io0 Cnoulaton in F

(d)

5

Fnsolalion in ohms

(e)Fig. 5: Results ofthe analysis of insulationbreakdown withinprimarywv dinLg only: (a)l6, svs. R1m; (b) 14

vs. C . , (c) I<a, vs. Rj,,, & Cjn; (d) 1>in vs. w and C109,<. (e) Lj/5 vs. n and R,0,

4.1.2 Insulation failure between primary winding and ground

Analysis as in the last section was repeated for the case of insulation deterioration and failure to ground atdifferent locations on the primary windinig, but with the breakdowin voltage ignored. The ensuing results,which are similar to those above, are as shown in Figures 6(a) to (f).

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Rnsulation in ohms

(c)

10 ,0R nsu ationin ohms

(b)

><. .

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Q. ½

(d)

10

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10

R nsu at onin Ohms

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(e)Fig. 6: Results of the analysis of insulation breakdown within primary

voltage ignored: (a) I.,w vs. RzDw; (b) 'meg vs. R.m; (c) Y,, vs. R.n;& Ri,s; (fi I.. vs. C>,., and R1,.

10

1I0 10 Cins

(f)winding to ground, breakdown(d) li,> vs Rjn,, & CQ7s; (e) Iag vs. c

With the breakdown voltage incorporated in the analysis the steady-state results are as in Figure 7, graphs(a) and (b). These reflect the same state change from good insulation to deteriorating and failed insulationas observed so far. Zero to very low voltage breakdown leads to insignificant contribution to currentflowing through the insulation. However, large voltage breakdown results in large negative value ofcurrent contribution to the insulation current. In propoition this contribution of the breakdown voltage isquite significant that it causes proportionally large current to flow in the rest part of the device, thusresulting, on one hand, in the outright bumnt out of the device. On the other hand, the large culTents in thepresence of magnetic fields produce large forces responsible for the violent nature of the failure of thedevice.

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A STUDY OF INSULATION FAILURE IN ASGE VOLTAGE CURRENT TRANSFORMER

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10

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(a) (b)Fig. 7: Results of the analysis of insulation brealkdown within primary winding to ground with

breakdown voltage accounted for: (a) ]>l vs. Rm,; (b) I, vs. Ci,,

4.2 Transient AnalysisWith the aid of Simplorer®(TM) simulation software [7] the result of the transient analysis are as shown inFigures 8(a) to (e). These are waveforms of current quantities at various parts of the device duringinsulation deterioration. Of particular interest is the influence of insulation deterioration on the source,insulation, and the secondary current waveforms. Apart from expected glitches in the insulation currentthrough the insulation capacitances there generally are no significant influences on the main currentwaveforms of the device. This implies that there is no significant production of harmonics due to failinginsulation.

(a) 5X0 10 OmD 20 OO030OO 40 00Om

Time in (seconds)

0 1(0c)O 2000m 30 OOm 4000mDO-

(&) X]0 2

400 0 I..1"

333.33u 33

-8 .I'll ,__

-)l

(b)

Time in (seconds)

F-

2L

(d)

Time in (seconds) Time in (seconds)

a)

F-Q

(e)

Time in (seconds)

Fig. 8: Transient analysis results

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EPE 21005 - Direden ISBN: 90-75815-08-5 P.9

A STUDY OF LNSILJTION FAILUREN AHkGH VOLTAGECUUNT UTAN&Ohua NIOLAHDUM Vgd.ot

The graphs from Figure 8 show the transient analysis resudts: (a) Input cuuTent waveform during failinginsulation within primary winding; (b) Insulation capacitor current waveforn dturing failing instulationwithin primary winding; (c) insulation capacitor current waveform dturing failing insulation withinprimary winding to groLund; (d) Insulation cuuTent waveform during failing insulation within primarywinding to ground vwithout breakdown voltage; (e) Insulation current waveform during failing insulationwithin primary winding to ground with breakdown voltage.

5. Conclusion

A Itumped parameter model suitable for investigating insulation failture in a high voltage currenttranisformer has been proposed in this paper. This model has enabled the study of the influence ofdeteriorating insulation before complete failure in the primary windings of the transformer. Two caseswere investigated: insulation failure within primary winding only, and between primary winding andground. Three main outcomes are worthy of note:* if the current quantities are defined as the state variables of the device, these variables are

significantly influenced by the change in the state of insulator at most points in the device. Theseinfluenices may be obselrvable and could then be monitored for quick intervention before a completefailure of the equipment.

* The band on the insulation parameters scale between die commencement of deterioration andcomplete failure of insulation is quite nalrow.

* The influence of failing insulation on the signal waveforms within the device is insignificant.Consequently, there is no significant generation of harmonics as a result of deteriorating or failure ofinstulation in the device.

References[1]. Evert R, Hoch DA, Comnack It "Condition monitoring for high voltage currelnt transformer in Eskom",

HVCT symposium, Portland, Oregon, 22-24 September 1999.[2]. Cormack R, Naidoo P, Cowan P, "Eskom(RSA) experience in condition monitoring in curewlt tralsfonrer and

transfonner buslings using on-line tan delta measurements', HVCT symposium, Pordand, Oregon, 22-24September 1999.

[3]. Mallasela VS, Jimoh AA, "Condition Monitoring of HV Current Transformers in Esltom Substatioln",Proceedings of die 13"' SAUPEC, 2003.

[4]. Van Bolliuis, Gulski E, Smith JJ, "Monitoring and Diagnostic of Transformer Solid Insulation", IEEETransactions on Power Delivery, Vol. 17, No 2, March 2002.

[5]. Bengtsson C, "Status and Trends in Transformer Monitoring", IEEE Transactions on Power Deliverv, Vol. I1,No 3, July 1996.

[6]. http://-Av.mathworks.com/index.shtmI[7]. http://wvw.ansofl.com/products/em/simplorer/

ISBN 90-7W51S08-1FlIE 2005 - Di 1'.10