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C A H B

M D D PDC

F. Huellmandel1, M. Appold1, A. Kuechler1*, R. Krump 2and J. Tite 2

1FHWS Uniersit of Applied Sciences, IgnaSchoenStr. 11, 97421 Scheinfurt, German2HSP Hochspannungsgerte Por GmbH, Kaiserstr. 127, 51145 Kln, German

*Email: akuechler@fhs.de

A: Dielectric properties of bushings ere inestigated b measuring of polarisation and depolarisationcurrents PDC. Netork simulations ere based onspatial discretisation and equialent circuits representing material sstem properties. Different scenarios eresimulated and eemplaril erified b measurements:Conductie surface laers, ater in the bod and in the

surface of the core (oil impregnated paper OIP), ageing,nonuniform distribution of ater, measurements ithsurface bandages. Normall currents from the measuring tap of a bushing are assigned to the insulating core,hich is onl justified if parasitic currents do not haeaccess to free ends of grading foils. With surface bandages either connected to ground or to diagnostic oltage, measured currents gie loer and upper limits.Guard ring bandages are used to find improed estimations of the current through the core. It is proposed tocalculate conductiities ith a ne charge differencemethod CDM. Results: (1) Enironmental influencescan be ecluded b means of bandages. Conductiities

are insensitie against parasitic currents. (2) Selectieinformation about insulation parts can be etracted. (3)Conductiit is related both to moisture and ageing. (4)The analsis of initial polarisation currents is the firstknon dielectric method for identification of aged OIPbushings at room temperature.

1 INODCION

A significant percentage of transformer failures isrelated to bushing insulation defects. In the future anincreasing number of progressiel ageing oilimpregnated OIPbushings ill be subjected to increasing

thermal stresses due to increasing poer flo requirements and oerloading conditions. This can result inaccelerated ageing, enhanced dissipation factors andthermal instabilities. Additionall ater increases dissipation and ageing and reduces dielectric strength. Ifpartial breakdons beteen grading foils occur, theoerall dielectric strength is seriousl affected.

In case of strategicall important bushings there is astrong need for reliable condition assessment b dielectric diagnosis of partial breakdons, ageing conditionand ater content.

Todas dielectric diagnosis of bushings is based onoffline poer frequenc measurements of capacitance Cand dissipation factor tan at the measuring tap hichis connected to the outermost grading foil, Fig. 1. The C is a sensitie quantit for the detectionof partial breakdons beteen grading foils, but it doesnot change significantl during slo ageing processes inoil impregnated paper OIP. During offline measurements, hen tan are measured at

ambient temperature, the alues are lo and insignificant, een for strongl aged or et OIPinsulation. Dissipation factors at serice temperatures aboe 50 Cremain unknon, although the might be high and dangerous. It as found that other dielectric measurementscan gie much better indications, een at ambient temperature.

In general dielectric measurements can be performedas frequenc domain analsis FDA or as time domainanalsis ith polarisation and depolarisation currentsPDC. For linear sstems both methods are mathematicall equialent [1], [2]. The authors decided to usePDC analsis because of the folloing reasons:

Fig. 1: Dielectric measurement at the measuring tap of a bushing ith access of parasitic surface currents to the free edges of thegrading foils in the middle of the grading contour.

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(1) Basicall PDC measurements gie step responses containing the hole sstem information. (2) Timedomain signals can be obsered easil, eplained bphsical models (ion moement, polarisation processes)and described clearl (ion transit times, equialent cir

cuits). (3) Currents at different times are related to different influences [3] (e.g. oil qualit, ageing, moisture).(4) The PDC analsis has successfull been applied totransformer diagnosis [4], [5], [ 6] and bushing diagnosis[5], [7]. (5) PDC allo to calculate d.c. conductiitiescontaining information about moisture and ageing [8].(6) Procedures hae been deeloped to etract releantinformation from er short measurements.

2 MEAEMEN ANDIMLAION

2.1.

D M B

Normall it is assumed that dielectric measurementson bushings can directl be related to the OIP bushingcore. This is not alas true: Sometimes leakage currents hae access to the grading foils, Fig. 1, and can beresponsible for negatie dissipation factor measurements [9]. It is concluded that polarit reersals duringpolarisation current measurements are caused bleakage currents.

For eplanation, a bushing core is basicall described b to capacitances Caand Cband one gradingfoil inbeteen, Fig. 2 and 3. A conductie path is as

sumed from ground or h.. side to the grading foil (leftand right). In the the influence of theconductie path can be described b a phase shiftbeteen measured current Ib and applied oltage Uhich can be bigger or smaller than 90, Fig. 2 (left andright). Thereb the dissipation factor appears to benegatie or positie. Notice: It is an apparent effect, inrealit the ideal capacitance does neither produce nordissipate energ [10] [11], [12]. For the description inthe , additional elements are introduced tothe equialent circuit in order to consider polarisationand conductiit, Fig. 3. The influence of the conductie path can be described analogousl b a reduction or

an enhancement of the measured current b(), Fig. 3(left and right). A conductie path to ground can resultin a temporar discharging of Cb, in a negatie current band in to polarit reersals, Fig. 3 (left).

It is concluded that dielectric measurements onbushings can be disturbed b parasitic currents to anunknon etend.

2.2. B

The behaiour of the bushing ith and ithout parasitic currents as simulated ith a netork model inorder to understand the impact of eternal influencesand in order to look for measuring procedures hich

allo to perform correct dielectric measurements.

Fig. 2: F :A conductie path from a bushinggrading foil to ground or h.. (left and right) results in appar

ent reduction or enhancement of dissipation factor tan . Thebushing core is described b to series capacities Caand Cb.

Fig. 3: T :A conductie path from a bushing grading foil to ground or h.. (left and right) results in reductionor enhancement of polarisation current. Een polarit reersals are possible (left). The bushing is described b Ca, Cbandadditional elements (polarisation and conductiit).

The nodes of the model ere meshed in aial andradial orientation [10]. Each connecting element consisted of a capacitance (replacing permittiit), a resistance (replacing conductiit) and RCelements (replacing different polarisation processes), Fig. 4. Theseequialent circuits, describing the local materials, erederied from real PDC measurements on material samples for all materials used in a bushing [11], [13], [14],[15], Fig. 5. In the case of nonlinear oil, a nonlinearalgorithm as used. The results of simulations are ingood agreement ith measurements [ 10]. The simulation model as used to calculate transient currents dur

ing PDC measurements.

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Fig. 4 : 123 kV transformer bushing ith OIP core (left),spacial discretisation (middle and bottom) and attribution ofequialent circuits representing insulating materials (right).

Fig. 5: Dielectric sstem responses for different insulating

materials at E = 0.1 kV/mm and room temperature (top) andmodelling of linear and nonlinear materials (bottom).

3 INFLENCE OF PAAMEE

The influences of different parameters on dielectricmeasurements on bushings ere inestigated ith PDCmeasurements and ith netork simulations. PDCmeasurements ere performed ith a PDC analserith a high dnamic range from 1 pA to 1 mA [16]. Thesimulation qualit as checked b comparison ithmeasurements. Diagnostic situations, hich could notbe inestigated b measurements, ere simulated.

3.1. M B

Eternal influences as described in Fig. 3 ereinestigated b a set of three measurements at themeasuring tap of a bushing, Fig. 6: (1) The traditionalmeasurement ithout bandages, (2) a measurement itha circumferential bandage in the middle of the gradingcontour at ground potential and (3) a measurement iththe same bandage at diagnostic oltage (h..).

Measurement (1) gies an estimation of the currentthrough the bushings OIP core.

Measurements (2) and (3) impose etreme alues ofparasitic surface currents as assumed in Fig. 3 . Therefore the measured currents can be interpreted as loerand upper limits of the current through the bushingsOIP core, Fig. 6 (top). The bandages can be calledorst (etreme) case bandages shoing the sensitiitof the bushing to parasitic currents.

The behaiour of the bushing ith and ithout bandages as simulated ith a netork model. The resultsof simulations are in good agreement ith measurements, Fig. 6 (top and bottom).

The simulation model as used to optimise the position of the bandage. It as found, that a groundedbandage aboe the edge of the outermost laer on the airside of the bushing collects all releant leakage currentson the air side of the bushing. It can be called guardring bandage protecting the measurement. The currenttaken from the measuring tap is therefore identical iththe current through the bushings core.

Fig. 6: PDC measurements ithout and ith bandages in themiddle of the grading contour. Bandages at h.. and groundgie upper and loer limits for polarisation currents throughthe bushings core (orst case bandages), measurements ()

and simulations ().

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Fig. 7: Polarisation currents measured on a seerel aged420 kV OIP bushing, measured at RT and 1 kV.

Bandages in the middle of the grading contour gieloer and upper limits for the polarisation current

through the bushings core. If the bushing is ne, theselimits are clearl separated, Fig. 6 (top). In the case ofseerel aged bushings, currents are strongl enhancedand leakage currents are negligible, Fig. 7. Upper andloer limits are close together and traditional measurements ithout bandages are considered to be correct.

3.2.

C A

Measurements ith conductie surface laers of increasing length on the air side of the bushing ere realied b conductie bandages, simulations ere madeith increasing lengths of surface resistances, Fig. 8(top and bottom). Surface conductiities result in trans

ient polarisation currents ith polarit reersals, both inmeasurement and simulation. This is in accordance iththeor, Fig. 3. Furthermore, there is a good agreementbeteen measurements and simulations hich shosthat simulations can be used for parameter studies.

It is concluded that transient currents are sensitie toeternal influences, e.g. caused b et and dirt porcelain surfaces, but long term currents and conductiitiesare measured correctl. This means that measurementsshould be handled ith care, if surfaces are not cleanand dr. In these cases the analsis should be based oncurrent end alues. Further improement can be madeb application of bandages as described in chapter 3.1.

3.3.

C

It as further inestigated hether the conductiitof the transformer oil or conductie deposits on thebushings transformer side might cause leakage currentshich hae access to the grading foils and hich caninfluence the measured currents. Measurements ithbandages and simulations ith surface resistances eremade for different lengths of the surface laers, Fig. 9(top and bottom). Both the measurements and thesimulations gie same results: There is no influence forlong term alues because of the high resistie epohousing insulator on the transformer side. For shorter

times, the transient polarisation currents are influenced

b a capacitie coupling from the bushing surface tograding laers on the transformer side.

Fig. 8: Polarisation currents at the measuring tap of a 123 kVOIP bushing ith conductie surface laers on the :M ith conductie bandages (top) and (bottom).

Fig. 9: Polarisation currents at the measuring tap of a 123 kVOIP bushing ith conductie surface laers on the : M ith conductie bandages (top)and(bottom).

Therefore it is adisable to base the analsis on longterm alues, hich as alread concluded in chapter

3.2. Current endalues and conductiities can be cal

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culated ith a ne charge difference method CDMfrom the difference of polarisation and depolarisationcharge [11], [13], also described in an ISH paper [15].

Another result of these inestigations is that poerfrequenc dissipation factor measurements are influ

enced in a similar a, but to a smaller etend [10].Neertheless, frequenc domain analsis FDA of dielectric measurements ill be subject to the same influences as described for PDC measurements.

3.4.

C L

Water in an OIP bushing core increases the polarisation currents, Fig. 10 (top and middle). The relationsere inestigated on material samples and described infurther papers [14], [15]. The influences on dielectricmeasurements on bushings ere inestigated b meansof simulations: If the bushing core is dr, the influenceof parasitic currents is comparatiel remarkable, as it

can be seen from the measurements ith bandages, Fig.10 (top). If the core is etted, the influence of parasiticcurrents is negligible, Fig. 10 (middle).

Fig. 10: Simulation of polarisation currents at the measuringtap of a 123 kV OIP bushing ith and ithout bandages (outerand inner cures). T: Dr OIP core ith a ater content =0.74 %. M: Wetted OIP core ith a ater content =5 %.B: Dr OIP core (= 0.74 %) ith etted surfacepaper laers aboe grading contour (= 5 %).

Furthermore, it as inestigated, if inhomogeneousdistribution of ater in the core could be detected, butunfortunatel significant differences could not be detected. Therefore dielectric measurements gie integralinformation about bushing core properties onl [17].

Neertheless, if the outer paper laers of the core areetted, measurements ith bandages gie a significantl ider separation of the three cures, see Fig. 10(bottom) in comparison ith Fig. 10 (top). This can beeplained b a reduced radial resistance of etted OIPlaers beteen grading laers and bushing surface ithbandages at ground or diagnostic oltage.

It is concluded, that diffusion of ater from the outside into the bushing core can be detected from theseparation of measurements ith bandages at groundand diagnostic oltage. A dr core can be detected fromthe lo polarisation currents during measurementsithout bandage.

3.5.

A

Both ageing and ater result in increasing polarisation currents, Figs. 7 and 10. It as found, that ageingof OIP bushings is mainl related to the ageing of theoil component [11], [14], [15]. Measurements on sericeaged bushings sho, that the seerel aged bushings no.1 and 5 (see Fig. 11) can clearl be detected from highinitial polarisation currents, een in room temperaturemeasurements [10]. All of the bushings had lo poerfrequenc dissipation factors tan at room temperature,but bushings no. 1 and 5 ere thermall instable aboe70 C. The had alread been oerheated in serice

hich resulted in gassing and partial discharges. Furthermore, ageing and etting can be distinguished fromthe gradient of the polarisation current cure.

It is concluded that PDC analsis is the first methodhich is able to detect seerel aged bushings fromdielectric measurements at ambient temperature. Theinfluences of ageing and ater can be distinguished, ifinitial and end alues of polarisation currents are considered.

Fig. 11: PDC measurements on identicall designed 420 kVOIP bushings, differentl aged in serice, measurements at RTand 1 kV.

(( ((

(

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4 CONCLION

The behaiour of bushings during dielectric measurements as inestigated and compared ith the re

sults of netork simulations. The folloing conclusionsare dran:

(1) The PDC analsis is a poerful tool for theealuation of the dielectric properties of bushings, butbushing core properties can not be measured correctl,if parasitic currents hae access to the grading foils.Thereb dissipation factors (frequenc domain) andpolarisation currents (time domain) can be increased orreduced apparentl.

(2) It is proposed to use conductie bandages aboethe grading contour to determine the sensitiit of thebushing against eternal influences. With to additionalmeasurements (bandage at ground and at diagnostic

oltage) loer and upper limits can be found for polarisation currents (orst/ etreme case scenarios). Netork optimisation shoed that the pure bushing corecurrent is measured, if a grounded bandage is situatedaboe the edge of the grounded laer (guard ring effect).

(3) Dielectric measurements on seerel aged oretted OIP bushings are not sensitie to parasitic surface currents, but generall the hae to be considered:

a) Conductie deposits on the air side of the bushingsurface hae a strong influence on transient polarisationcurrents but not on long term end alues and conductiities. It is proposed to clean and dr surfaces, tomeasure ith bandages and to prefer ealuation of current end alues.

b) Parasitic currents on the transformer side areblocked b a high resistie housing insulator, but onlfor long measuring times. Therefore again it is proposedto consider long term current and conductiit alues.

(4) It is possible to distinguish the properties of thecore and the surface laers b PDC measurements ithand ithout bandages from the relatie distance beteen the cures.

(5) Seere ageing of OIP especiall causes high alues of polarisation currents in an earl phase duringsome hundred seconds after beginning of polarisation.Thereb seerel aged OIP bushings (ith critical dielectric losses at serice temperature) could be detectedand distinguished from etted insulation b PDC measurements alread at room temperature.

The PDC analsis is the first method for the identification and separation of aged and etted OIP insulationat ambient temperature.

5 ACKNOLEDGMEN

The authors gratefull acknoledge the financialsupport of this ork b the Deutsche Forschungsgemeinschaft DFG (KU 1384/31 and 2).

6 EFEENCE

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