partial discharges and xlpe cable insulation

7/27/2019 PArtial Discharges and XLPE Cable Insulation

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Partial discharges andthe electrical aging ofXLPE cable insulationbyG.G. WolzakA.M.F.J. van de LaarE.F. Steennis

EUT Report 86-E-160ISBN 90-6144-160-9ISSN 0167-9708Novem ber 1986


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Eindhoven University of Technology Research ReportsEINDHOVEN UNIVERSITY OF TECHNOLOGY

Department of Electrical EngineeringEindhoven The Netherlands

PARTIAL DISCHARGES AND THE ELECTRICALAGING OF XLPE CABLE INSULATION

by

G.G. WolzakA.M.F.J. van de Laar

andE.F. Steennis

EUT Report 86-E-160ISBN 90-6144-160-9ISSN 0167-9708Coden: TEUEDE

EindhovenNovember 1986


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CIP-GEGEVENS KONINKLIJKE BIBLIOTHEEK, DEN HAAGWolzak, G.G.Partial discharges and the electrical aging of XLPE cableinsulation / by G.G. Wolzak, A.M.F.J. van de Laar andE.F. Steennis. - Eindhoven: University of Technology. -Fig., tab. - (Eindhoven University of Technology researchreports / Department of Electrical Engineering,ISSN 0167-9708; 86-E-160)Met l i t . opg., reg.ISBN 90-6144-160-9SISO 663.6 UDC 621.317.333 NUGI 832Trefw.: elektrisch isolatiemateriaal; metingen.


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- I I I -

AbstractThis report descr ibes a search for a poss ib le relat ionship betweenpart ia l discharges and e l e c t r i c a l aging o f XLPE cable insu la t ion .I t i s w el l known that part ia l discharges ca n deter iorate polymericinsu la t ing mater ia l s . Recently several theories on the re la t ionsh ipbetween p a r t ia l discharges an d aging have been publ ished.In order to explore this relat ionship by experiments a method for thedetect ion and local i zat ion of small part ia l discharges ( l es s than 1 pC)in high vol tage cab les has been developed.A number o f t e s t cables has been aged during s ix months at a s tress o f15 kV/mm rms at the conductor; three cables were held at roomtemperature and three cables were heated to 9ifc conductor temperature .The fo l lowing measurements were carr ied out: part ia l dischargemeasurements, tan 0 measurements, breakdown voltage measurements andmeasurements on phys ica l an d chemical propert ies .The resu l t s show no s ig n i f i c a n t aging an d therefore no correlat ionbetween aging and p a r t i a l discharge ac t iv i ty could be es t ab l i s hed .

Wolzak, G.G., A.M.F.J. van de Laar and E.F. SteennisPARTIAL DISCHARGES AND THE ELECTRICAL AGING OF XLPE CABLE INSULATION.Department of Elec tr ica l Engineering, Eindhoven Univers i ty o fTechnology, 1986.EUT Report 86-E-160

Addresses o f the authorsD r . i r . G.G. WolzakHigh-Voltage LaboratoryDepartment o f E le c t r i c a l EngineeringEindhoven Univers ity o f TechnologyP.O. Box 5135600 MB EindhovenTh e Nether landsIr . A.M.F.J. va n de LaarResearch an d Development DepartmentNKF Kabel B.V.P.O. Box 262600 MC Del f tThe Nether landsIr . E.F. SteennisElectr ica l Research DepartmentN.V. KEMAP.O. Box 90356800 ET ArnhemThe Netherlands


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1 .1.1

2 .

2.12 .22 .32.42 .53.3.13.23.2 .1

3 .2 .23 .2 .33.2.43.33.3 .13 .3 .23 .3 .33 .3 .44.

- IV -CONTENTS

AbstractIntroduction

Scope o f the workA method for detect ion and localization of par t ia ldischarges in high voltage c a b l e s ~State of the a r tPrinciple of the detection methodMeasuring systemResul tsConclusionAccelerated aging and measurementsIntroductionTest programType of cableTes t condi t ions , t e s t parametersTest set-upTest proceduresResultsElect r ical characterist icsPhysical characterist icsChemical character i s t icsBreakdown voltage dis t r ibut ionConclusions, further outlookAcknowledgementsReferences

PAGE

III

112

236789999

911121212141515161718


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1 . Introduct ionCross- l inked polyethene (XLPE) insu la ted power cable has a numberadvantages with respect to o i l - f i l l e d paper cab le . However whi lelong-term r e l i ab i l i t y of o i l - f i l l ed cable has been proven by aperiod of favourable experience, the long-term behaviour of theinsulated cable i s still somewhat uncertain .

o fthelongXLPE

In th i s repor t we discuss the aging of XLPE insu la ted cable under theinf luence o f e l e c t r i c a l an d thermal s t r e s s under dry condi t ions .The e lec t r ica l aging and breakdown of polymeric insu la t ion have beeninves t igated for more than 30 years . A f a c t of ten reported i s thatp a r t ia l discharges are responsible for the i n i t ia t i o n o f e l e c t r i c a lbreakdown. Th e corre la t ion between e l e c t r i c a l aging an d p a r t ia ldischarges i s however not f u l l y understood.For polymeric insu la ted cable the qual i ty of the extruded d ie l ec t r i c hasbeen great ly improved over the years , resu l t ing in a reduced number o fvoids an d contaminants in the insu la t ion , thus reducing the number andmagnitude o f the p a r t i a l discharges . As a consequence o f th i s one couldimagine a di e l ec t r i c ( e .g . a cab le ) , free o f p a r t i a l discharges in whichno aging would occur .

An in terest ing quest ion i s whether the aging process could s ta r t withvery small pa r t i a l discharges , which would escape detec t ion by c l a s s i c a lequipment. One would l ike to have a detect ion method, su i tab le for useon f u l l sca le cable l engths .The hypothesis t h a t very smal l pa r t i a l discharges play a ro le is a lsofound in the l i t e r a tu r e [Er 84, Ra 84, Ly 81, Ok 77].1 .1 Scope of the workThe scope o f the work reported here i s to search fo r a re la t ionsh ipbetween p a r t i a l discharges and aging o f XLPE insulated power cab les . Toinvest igate th is re la t ionsh ip , f i r s t a method wa s developed for thedetect ion an d loca l iza t ion o f smal l p a r t i a l discharges in medium lengthsof power cable. The development of t h i s method i s reported in Chapter 2.Next an at tempt wa s made to cause aging of a number of cables under thein f luence o f e l e c t r i c a l an d thermal s t r e s s , during a s ix month p e r i o d ~During t h i s per iod the phys ica l an d chemical aging processes in thedi e l ec t r i c o f th e cable were monitored in order to corre la te the changesin chemical an d physical character i s t i cs with the appearance o f p a r t ia ldischarges . The resu l t s o f the measurements are reported in Chapter 3.


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2. A method for detect ion and loca l iza t ion of par t i a l discharges inhigh vol tage cables .

2.1 State of the a r t .The detect ion of par t i a l discharges in high-vol tage cables has receivedconsiderable i n teres t during the l a s t tw o decades. A number o f detect ionmethods has been developed, but only those methods t ha t allowl ocal i zat ion wi l l be treated here. The measuring techniques forloca l iza t ion can be divided in tw o groups: 1 . scanning methods2. trave l l ing wave detec t ion methods.Th e common aspect o f a l l scanning methods i s the movement o f detec torand cable with respec t to each other . The detec t ion can take place bymeans o f e l ec t r i ca l contacts [Kr 64 ] or an acous t ic transducer [An 82] .General ly, these methods are applied to unfinished cables and aretherefore res tr ic ted to factory te s t s . The development of the trave l l ingwave detection methods has shown considerable progress since th e in i t ia lwork, performed by Kreuger [Kr 64] an d Tempelaar [Te 60] . The pr inc ip leo f the trave l l ing wave detect ion is given in Figure 2 .1 . A par t ia ldischarge in the cable insulation generates travel l ing waves whichpropagate in tw o direct ions along the cable .

A { "-tJ J\J

v / X- I..-

II~IIIc:pI,

_I

L

'\ BJ

Figure 2.1 Travell ing waves generated by a part ia l discharges in acable .The t ravel l ing waves can be detected a t the cable ends A and/or B. Asthe waves propagate both on the centra l conductor and the outer sheath ,detec t ion i s poss ib le by means of a coupl ing capac i tor connected to theconductor or by an interruption of the outer sheath.Detection with a coupling capacitor i s described in numerous papers [L e79], [Be 82], [Wi 85] . These methods can be employed fo r cable lengthsin excess o f 20 mi the bandwidth of the detection system i s in the orderof 50 MHz ( for longer cables even 10 or 5 MHz). The smal les t detectablepar t i a l discharge i s about 1 pC. Reeves [Re Bl] has descr ibed anaccurate local izat ion method for cables shorter than 20 m. This methodemploys two coupling capacitors (one at each end) and a dual beamosc i l loscope . The bandwidth of the detection system i s better than 100MHz. However, the smal les t detectable discharge i s 10 pC.


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I t i s in terest ing a t t h i s point to mention two rather pecu l iar detect ionmethods. Weeks [We 82] describes a specia l ly developed correlat ionmethod and Anderson [An 82 ] investigated the poss ib i l i t i es of timedomain ref lectometry (TDR). Both methods have not been used extens ive lyup to da te .Fo r the detect ion and local i zat ion of very small part ia l dischargesduring the ear ly stage o f cable aging, a detect ion method should be usedwhich meets the fol lowing demands:1. The method must be able to de tec t pa r t i a l discharges smaller than 0.1pC , independent o f cable l ength .2. An accurate loca l iza t ion o f the discharge s i t e must be poss ib le ; theaimed inaccuracy i s 0 .1 m on a cable length o f 20 m or l e s s .3. It must be poss ib le to perform waveform analys i s of the detec tedp a r t i a l discharge pulses : the waveform o f the pul se might change duringthe aging process o f the insu la t ion .4. Th e method must al low p a r t i a l discharge detect ion on hv an d ehvcab les .I t i s obvious from the foregoing t hat none o f the ex is t ing detect ionmethods i s able to meet a l l these demands.Therefore a new detect ion method has been developed, basedprinc ip le o f ul tra wide band detect ion o f the s igna l s over ain t errupt ion .2.2 Pr inc ip le of the detec t ion method

on thesheath

The required detect ion s ens i t i v i t y can be obtained with an ul tra wideband (UWB) detec t ion method. Boggs an d Stone [Bo 82] have shown t ha tth is method i s super ior to other methods in a number o f aspects . The UWBmethod employs a detec t ion system with a bandwidth as high as poss ib le ;the bandwidth can reach up to 1 GHz for Gas Insulated Switchgear.I t has been shown however, t hat high frequency s igna l s are attenuatedwhile propagating over a power cab le . This e f f e c t , mainly due to thesemiconduct ive l ayer s (see [St 81] , [St 82] and [Wo 83]) l imi t s theusable bandwidth of a detec t ion system to about 200 MHz. A bandwidth o f200 MHz makes it impossible to use ext ernal coupl ing capacitors (asdescribed in sec . 2.1) a t the input en d o f the detect ion system. Thisleaves tw o other options [St 81] [Wo 83]:- coax ia l coupl ing capac i tors , as used in GIS, are impract ical for h igh

vol tage cab le ,- a measuring r e s i s to r across an interrupt ion o f the outher sheath o f

the cable.The l a t t e r method has been chosen fo r th i s detec t ion method. A pic tureo f the inc ident , re f l ec ted and transmitted t rave l l ing waves - generatedby the p a r t i a l discharge - a t a sheath interrupt ion i s given in Figure2 .2 . Important for detect ion i s the re la t ionsh ip between the vol tageacross the measuring impedance (resistor) Zm and the or ig ina l waveampli tude e :


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2222222221 PIZ??????? sheath1711 r*i dielectric,r_...l.!O.J.....E..JL-__ " ' -"-__ . conductor

er -f {ZZ2222?2fL_2i _r - i - ,__ _ ? ~ i 2 2 2 2 2 2 2 2

c:::JZm

=--.L...---!.

Figure 24 2 Signa l s a t a sheath interrupt ion

V 2e

Th e re la t ionsh ip between the apparentdischarge and the voltage e i s given by :

charge, q, of the part ia l

To) ed t

2q

owhere To i s th e duration o f the d ischarge pulse and Zo i s th echaracter i s t i c impedance of th e cab le .I f we include th e 'ground impedance' Zg ' the wave impedance o f th ecab le shea th w i th respect to the nearby ground plane , t hen theexpression fo r V becomes: (see Fig . 2.3)

V 2e ______+

v

Figure 2 .3 Equivalent c i r cu i t o f sheath interruption.

The measuringresistance of impedance Zm can bethe semiconducting formed by a discrete re s i s t or , thelayer and the input impedance of themeasuring system.


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With two sheath in te r rupt ions in adetect ion c i r cu i t s are possible : cable , on e a t each end, the following- s t r a igh t detect ion a t one in te r rupt ionpd pulse shape. For local izat ion onehas to be used.

can be used for analysis of theof the two following ci rcui t s- detect ion with two in te r rupt ions in para l le l , see Figure 2.4a . Theequivalent c i rcu i t i s given in Figure 2.4h.- balanced detection, see Figure 2.4c. The measuring cable with

cha r ac te r i s t i c impedance Zm i s connected between a and b. A balun(balanced-unbalanced transformer) has to be used to match thedi f f e r en t ia l vol tage Va - Vb to the asymmetric measuring cable andoscilloscope amplif ier .

Figure 2.4. Possible measuring c i r cu i t s :(a) cable and sheath interrupt ions(b) unbalanced detec t ion , equivalent c i rcu i t(e) balanced detect ion, equivalent c i rcu i t .The vol tage sources in (b) an d (c ) represent one of the twoincident wave f ronts which may ar r ive a t di f feren t moments.

The balanced c i rcu i t can be used for the suppression of externalin te r fe rence . However, t h i s advantage i s only fu l ly exploi ted when theremaining cable l eng ths beyond the in te r rupt ions are equal and the highvol tage connection (point d. in Figure 2.4.a) i s located halfway betweenthe tw o terminat ions ; only then the t r a ns i t t imes of the in te r f e r ings igna ls to the in te r rupt ions are equal so t ha t they do not show up inVa - Vb-


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Local izat ion o f a discharge s i t e in the cable between th e interruptionsi s possible by the measurement of the t ime difference of the pulses ,arr iving a t the interruptions . In two cases t h i s procedure can give somedi f f i cu l t i e s :- a pd located exact ly in the middle o f the cable does not r e g i s t e r in

the balanced detec t ion se t -up , because the tw o s ignal s arr ive atthe interruptions s imultaneously . The b e s t so lut ion to th i s problemi s to measure with tw o s ignals and a dual beam osc i l loscope orwith the balanced se t -up .

- a discharge s i t e in the immediate v i c i n i t y o f a sheath interruptionl eads to more complicated s ignal s because r e f l e c t io n s from theterminations arr ive quickly. In the in terpre tat ion one should note thatthe polar i ty of the measured pulses depends on the di rect ion inwhich the wave propagates .

The length of the cable between the in te r rupt ions can eas i ly bedetermined by the in ject ion o f a s imulated discharge pulse a t onein te r rupt ion which i s then measured a t both in te r rupt ions . The r e f l ec t eds ignal s from the terminations should not in terfere with the directs igna ls . This can be achieved when the sheath interruptions are a t asu f f i c i en t ly large dis tance from the terminat ions . A length of 2 to 3meters cable i s adequate .2 .3 Measuring systemThe measuring system cons is t s o f thethe measuring cable , the preampl i f ier ,2 .5 .

sheath interruptions , the balun,and the osc i l loscope , see Figure

1-------,I II 0 I~ = = = = = = ~ ~ :

II :L _______ J

Figure 2 .5 . Schematic diagram of the measuring system.The sheath interruption was constructed by removing the outer PVC jacketand the stranded copper wires over a l ength o f about 11 em. A layer o finsulat ing paper tape i s wrapped over the semiconducting layer and thecopper wires are r e in s ta l l e d , leav ing an opening o f about 1 cm, seeFigure 2 .6 .


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measuring cable central conductorcopper shield wires semi-conducting layerbraid wrapping ...c insulationinsulating paper semi-conducting layerPVC jacketr-l., ~I , II I I III ! I II I I . i! I ., . . - c-I

10mrrl 100mm .1Figure 2.6. Sheath in terrupt ion.In th i s way the res is tance of the sheath interrupt ion is more than 200 Q,while the perturbat ion of the coaxial geometry i s kept to a minimum.Different types of baluns were used, a l l with a small fe r r i t e core and 7to 13 turns of wire , const ructed according to Ruthroff [Ru 59] . Thebandwidth o f a l l ba luns was ove r 400 MHz. The measur ing cab le , an RG 214 /U coaxial cable was la id in a copper pipe to el iminate externalinterference.For the ampl i f i ca t ion of smal l pa r t i a l discharge s igna l s , tw o types ofpreamplif iers were used. Both were Avantek preamps, with 28 dB gain; the402 type with a bandwidth of 2 MHz - 550 MHz, the 462 model with abandwidth of 200 Hz - 550 MHz.The s igna ls were recorded with a Tektronix 7844, 400and camera. The osc i l loscope with the pre-ampl i f ie rsmall shie lded room.

MHz osc i l loscopewas loca ted in a

A ll t e s t s were car r ied ou t in a shielded high-vol tageprovided 80 dB damping of externa l in te r fe rence .2.4 Resul ts

ha l l which

Th e method has been f i r s t t e s ted on two t e s t lengths o f XLPE cable andl a t e r on the s ix lengths of XLPE cable of our aging t e s t (see Chapter 3 ).However, those cables showed very little pa r t i a l discharge ac t iv i ty .During the pre tes t s an d the aging t e s t s , 12 pa r t i a l discharge have beenmeasured. The magnitude of the discharges var ied from 0 .3 pC till 10 pC.In a l l cases the discharges d id not occur in the unperturbed cable.The sens i t iv i ty of the method depends on the length of the pd pulse . Thetheore t ica l sens i t iv i ty fo r t h i s type of cable has been ca lcu la ted [Wo83]; a graph i s shown in Figure 2.7 . A number of measured pulses hasa lso been marked in th i s Figure .


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10)

1.;'''''/ ,,-0 -"

V ,,-1-- theoreticalsensitivityQl

om1

....

,,",f-""

10

,.

100 T(ns) 1000

Fig. 2.7 Theoret ical sensi t iv i ty and observed discharges (small c i rc les ) .Only little exper ience has been gained with the loca l iza t ion of p a r t i a ldischarge s i t e s . In the 10 cases of discharges in the t e rminat ions , thelocal izat ion was re la t ive ly easy.In one case of an in te r na l p a r t i a l discharge loca l iza t ion was poss ib lewith an inaccuracy of 1% of the cable length, 30 em. The other case of anin ternal p a r t i a l discharge was a discharge in between a sheathin te r rup t ion and the termination; on t h i s shor t length (5 m)localization waS possible within 20 em.2.5 ConclusionA method fo r detec t ion and loca l iza t ion of pa r t i a l discharges in mediumlength of high vo l tage cables (about 30 rn) has been developed. Thesens i t i v i ty of the method i s l imi ted by the usefu l bandwidth, which i sdetermined by the a t tenua t ion of the pd pulses as they propagate alongthe high-vol tage cable .In cables without semi-conduct ing l ayer s (such as RG 214/U) much smal l e rpa r t i a l discharges (down to 0.04 pC) have been observed [S t 61) .A pre requ i s i t e for the measurements i sin te r fe rence to have thermal noise asmeasurements. A shielded high vol tagecondi t ion for t h i s type of measurements.

a su f f i c i en t a t tenua t ion ofthe l imi t ing fac tor for the

laboratory i s a necessary


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3. Accelerated aging an d measurements3.1 IntroductionTo es tab l i sh the correlat ion between part ia l discharges an d aging o f thecable insu lat ion , it was decided to conduct an aging t e s t . Before an dafter the aging period , the breakdown s trength o f the cable was to bedetermined by t e s t s with a stepwise increasing voltage. By comparison o fthe breakdown s trength dis tr ibut ions before an d after the aging period ,poss ib le aging can be detected an d correlated with other phenomena,observed during the t e s t s .Because uncertainty ex i s ted about the aging p h e n o m ~ n a i t was decided tomeasure not only part ia l discharges but also tan 0 an d a number o fphysical an d chemical propert ies o f the cable insu lat ion . The e l e c t r i c a lquant i t i es were to be measured on the ent ire cables , while the phys i ca lan d chemical propert ies were to be determined from samples, taken a tregular in terva l s during the aging period.3 .2 Test program3.2.1 Type of cable.A ll t e s t s were carried out with cables , manufactured by NKF. Therelevant cable data are shown in table I .

Type - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - YMvKrvc 240mm' Al 30/50 kVYear of production - - - - - - - - - - - - - - - - - - - - - - - 1980Conductor mater ial an d construction - - - - - - sol id aluminumConductor diameter - - - - - - - - - - - - - - - - - - - - - - - 17.3 mmThickness inner semiconducting layer - - - - - 1.1 mm (average)Insulat ion materia l ; production process - - - Cross- l inked PE; long-dieInsulat ion th ickness - - - - - - - - - - - - - - - - - - - - 11.3 mm (average)Thickness outer semiconducting layer - - - - - 1 .2 mm (average)Earth screen - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 72 Cu wires , 0 .8 mm diameter

Cu fo i l 25 x 0.1 mmOuter jacket - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 5.3 mm PVCTable I . Data for the cables tes ted .The design nominalthe nominal voltagekV!mm rrns.

voltage ofthe design thecable

electr ical

3 .2 .2 Test condi t ions , t e s t parameters.

is 30 kV (phase to ground); a ts t ress a t the conductor is 4.1

The t e s t s were conducted on 6 lengths o f cable o f the above spec i f i edtype, each length about 30 m.Three cables (referred hereaf ter as cables 11, 12 and 13) were kept a troom temperature. The other three cables (21, 22 an d 23) were heated to aconductor temperature o f 90C by res i s t ive heat ing.


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Each se t o f three cables wa s on a separate drum. To acce le ra te thee l ec tr i ca l aging, the t e s t stress was chosen to be 15 kV/mm rms a t theconductor . This meant a t e s t vol tage of 11 0 kV (3.67 t imes th e nominalvo l t age ) . Because a number o f character i s t i c propert ies cannot bemeasured continol lsly , i t was decided to perform a l l measurements a tregular interval s . The period between the measurements was chosen to betw o months. During these periods of two months the vol tage was to beappl ied cont inuously an d three cables (21, 22 23) were kept a t e levatedtemperature .All measurements had to be performed with the cab le s a t room temperature .The e lec t r ic breakdown strength o f the cable has been determined beforethe t e s t s (on other lengths of cable) .The fol lowing propert ies were measured every two months:- e l e c t r i c a l pa r t i a l discharges by means of the wide band method

part ia l discharges by means of the narrow bandmethod

- phys ica lloss tangent ( tan 6)the melting pointYoung's modulus (log E)the mechanical l o s s tangentthe alpha peak, the temperature a t which a maximummechanical los s tangent i s observed

- chemical

the degree of crys ta l l i za t i onthe thermal s t a b i l i t ythe modulus o f shear ing ( log G)carbonyl contentcurnylalcohol con ten tacetophenon content

The wide band part ia l dischargemethod, descr ibed in Chaptermeasurements were performed withunit nr. 3.

measurements were2. The narrow banda Robinson (ERA-3)

performed with thepa r t i a l dischargedetector with input

The measurement o f the e l ec tr i c loss tangent (tan 0 ) was carried outwith a Schering bridge (H & B) with a lock- in ampl i f i e r (EG & G 128) asnul l ind ica tor . The standard capac i tor (H & B) was 99.98 pF.A ll measurements o f physical and chemical propert ies were carried out atthe KEMA l aborator ies . A detai led descript ion of the resul t s and themethods used can be found in (Ge 84J and the r e ferences quoted the re in .A b r i e f summary o f the measuring methods i s given here . The methods werechosen to give an optimal character izat ion of the materia l .The melting point of the insulat ion i s determined by the DSC(d i f f e ren t i a l scanning calor imetry) method.Young's modulus and the mechanical los s tangent can be determined fromthe DMTA-curve. The mechanical los s tangent, determined as a function oftemperature always has a peak between 40C and 70C, the so-cal l ed alphapeak.


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The degree o f c r y s ta l l i z a t io n can be determined from the resu l t so f density measurements. An indicat ion for the thermal s tab i l i ty o f theinsu la t ion can be obtained from the resu l t s o f the TG (thermalgravimetric) measurements. The modulus of shearing ( log G) has beenmeasured with a DMTA (d i f ferent ia l thermo mechanical analys i s ) method.The chemical analysis has been l imi ted to in fra-red spectroscopy (forthe determination o f the carbonyl content) an d gaschromatography ( forthe cumylalcohol and acetophenon content ) .3.2 .3 Test se t -upAs mentioned before 6 lengths o f cableterminations o f the cables cons is tedshaf t and a surrounding plas t ic bag,external f lashover .

were ins ta l led on tw o drums. Theo f a s t ress -cone , an elongatedf i l led with SF6 gas to prevent

The high vol tage was suppl ied by an SF6 insula ted t ransformer 500 vi250 kV. The high voltage t ransformer was connected to the mainsthrough a regulat ing transformer. The high voltage wa s measured with aprecis ion capaci t ive div ider an d reg is tered with a recorder, v ia aprecis ion ad-dc converter . The inaccuracy o f the measuring system wasl ess than 0.5%.The s tab i l i ty o f the mains proved to be s uf f i c i ent , no further voltageregulat ion was necessary . During the aging t e s t the voltage remainedwithin 2 kV o f the s e t value. The capacitance o f each cable was about5 nF. This means that the s ix cables would require a capac i t ive currento f about 1 A. This wa s considered too much for the transformer, thereforetw o variable inductance units (Hipotronics) were in s ta l l e d p a r a l l e l tothe cab les . A c i rcu i t diagram o f the high voltage set-up i s given inFigure 3.1 .

regulatingtransformer

dampingresistor

high voltagetransformer

11 12 13cables

variableinductor

Figure 3 .1 Circu i t diagram o f the hv t e s t se t -up .

21 22 23cables

Three cables were heated to a conductor temperature o f 90C. Temperaturemeasurements were performed on 5 spots on the cables (2 on cable 21, 2 oncable 22, 1 on cable 23) with PT 100 elements and a temperaturerecorder . The s ize of the PT 100 elements (2 mm diameter, 20 mm long)made d i r e c t temperature measurements at the conductor (of a dummy cable)unpract ical . Therefore the sheath temperatures were reg is tered an d theconductor temperatures ca lcu la ted , according to lEC 287. The parts o fthe cables ly ing on the drum or c lose to it were a l l a t near ly the sametemperature (within 4Q C).


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- 12 -

The samples for the measurements o f the phys i ca l and chemicalproper t i e s were a l l taken from parts o f the cables c lose to the drums.3 .2 .4 Test proceduresThe aging per iod wa s dividedthree periods of continuousperiods) .

into avol tagenumber o f measuring periods an d

application (accelerated agingTable II g ives a summary o f these per iods .

per iod type o f remarksnumber period

1 measuring in it ial measurements2 aging 2 months3 measuring4 aging 1 month, breakdown5 measuring6 aging 1 month7 measuring8 aging 2 month9 measuring

Table I I . Summary of the l i f e t e s t .Period 4 was terminated unexpectedly by a breakdown in cable 22. Toinves t iga te a l l poss ib le fac tors which might have inf luenced th i sbreakdown, an extra measuring period (period nr. 5) was added. Afterperiods nr. 2 , 4,6 and 8 samples were taken from different cables forinvest igat ion o f physical and chemical propert ies .After the f ina l measuring per iod (nr. 9) the cables were subjected to astepwise increase voltage te s t to determine the breakdown voltagedis tr ibut ion . These t e s t s were carried out during 1984.

3.3 Resul ts3.3 .1 Elec t r i c a l charac te r i s t i c s .Figure 3.2 summarizes the results of a l l loss-tangent measurements. Thein i t i a l value and the value after 6 months of aging are given for eachcable . The loss-tangent was measured as a function of voltage in stepsof 12 kV bu t did hardly depend on the measuring vol tage . Data i s fo rth i s reason only given fo r on e vol tage leve l (60 kV).


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loss-angle.0006.0005

.0004

.0003

.0002

.0001o 11 12 13

- 13 -

aged:omonths60kV valueI}}}",,""'j

21cable

aged:6 months60kV valueI!IIIi!!!BI

22 23

Fig. 3.2 Summary of the loss - tangent measurements fo r a l l cables .

The tan 0 of cables 21, 22 and 23 i s very cons tant and near ly the same(2 x 10-4 ) fo r a l l cables . This might be a t t r ibu ted to the heat ingof the cables ( the cables were also heated before the l i f e t e s t tover i fy the proper temperature dis tr ibut ion) which causes the evapo-rat ion o f a number o f chemical addit ives to the insulat ion which areknown to increase the lo ss tangent .The resu l t s o f the p.d. measurements wil l be summarized by the measuringperiods ra ther than by cable number. Only in te rna l discharges in thecables exceeding the noise l e ve l wi l l be discussed.Period 1No dischargesPeriod 3Cable 22 showed a discharge between the sheath interruptions , see Figure3.3 .

2 ns

IV"'V """ . / ..... ~ /'" IV5mV

Fiqure 3 .3 Part ia l discharge in cable 22.


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- 14 -

From t r a n s i t time measurements it became c lear that th i s discharge (0.4pC , Vi 45 kV) was in or very c lose to a sheath interruption.Per iod 5No dischargesPer iod 7No dischargesPeriod 9Cable 13 had a small discharge (0.5 pC , Vi = 100 kV) in one of thes tress cones .Cable 21 had a discharge ofinternal discharge, see F ig .

3 pC (Vi3.4 .

= 80 kV) which resembled an

200mV 20 ns

..i"'- \tv VI /1\V " V -'if Iv

Figure 3.4 . P a r t i a l discharge in cable 21However t ainterruptionshowed, t h a t

very carefu l examination, for which an extra sheathwas made between the termination an d the o r ig in a l one

t h i s s igna l wa s caused by a discharge in a s tress cone.3.3 .2 . Physica l cha r ac te r i s t i c s- From the r e s u l t s o f the DSC measurements, the melt ing-points

insulat ion can be determined as a funct ion o f t ime. A s l i ghto f the melt ing point i s expected. Figure 3 .5 shows, that thepoin t hard ly changes .

of theincrease

melt ing


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107,-- - - - - - - - - - ,

105 " '"cahles 21,22,23/'

.......

" ------ .......-' ....- ........cables 11,12,13103\;-_ _+-_ _+ __o 2 4 6--time months)

Figure 3.5 Results of theDSC measurements

- 15 -

Ii0.45,------------ ,~~;;;;;; 0.44::-v

043~"I .42

,./

...... ,, /......... "

....... _----../cables 11,12,13

~ I ~ _ _ _ _ - - ~o 2 4 6_ time (months)Figure 3.6 Results of the

densi ty measurements- The results of the DMTA measurements, show a large and unexplained

scattering in Young's modulus.- Figure 3 .6 gives the

The increase of thehas been aged a t 90Cnot that clear .

degree of crysta l l iza t ion as a function o f t ime .degree o f c r y s ta l l i z a t io n o f the sample which

was expected. However resul ts on other cables are- A ll other measurements o f physical character i s t i cs

tangent , alpha peak, thermal s tab i l i ty an d modulusnot show s i g n i f i c a n t r e s u l t s .

3 . 3 . 3 . Chemical c h a r a c t e r i s t i c s

(mechanical l o s sof shearing) do

The measurements o f the carbonyl-content show no s ign i f icant changes asa function o f t ime , except for those samples that were c lose to theconductor. The measurements on the contents o f cumylalcohol an dacetophenon are not showing systematic changes as a function o f t ime.3.3.4. Breakdown voltage distr ibut ionThe in i t i a l breakdown voltage distr ibut ion has been determined with a20% stepwise increase vol tage t e s t . The resu l t s are analysed by using asensored maximum l ikel ihood method to determine the Weibull parameters.The breakdown dis tr ibut ion i s shown in Fig. 3 .7 . After the 6 month l i f et e s t the remaining cable samples are also subjected to the above mentionedbreakdown t e s t . Because the number o f samples i s l imited only terminationf lashovers occurred. These values are also given in Fig . 3 .7 .No s ig n i f i c a n t dif ference between the i n i t i a l breakdown values an d thoseafter aging can be observed.


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- 16 -

0.9 NKF Imc~ u

6x~~ 0.7t I.5 I

0.3 I!0.2

0.1

20 25 30 40 50 60 Stress (kV/mm)

Fig. 3.7 Weibull plot ofa t f lashover for

or ig ina l d is tr ibut ion . Aaged cable s .

4. Cons lus ions , further out l ook .

voltage level

Improvements on a part ia l discharge measuring method have made i tpossible to detect and localize small part ia l discharges in a length ofabout 30 meter of high-voltage cable. Since a wide-band detection systemis used the actual waveform can be observed. This waveform might beusefu l for diagnos t ic purposes .A number of t es t cables has been aged during six month a t a s t ress of 15kVjmm rms at the conductor; three cables were held at room temperatureand three cables were heated to 90C conductor temperature.Th e fo l lowing measurements were carried out: p a r t i a l di s chargemeasurements, tan 0 measurements, breakdown voltage measurements andmeasurements on phy s ica l and chemical proper t ie s .The r e su l t s show no s i g n i f i c a n t aging an d therefore no corre lat ionbetween aging and p a r t i a l discharge a c t i v i t y could be e s t a b l i s h e d .


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- 17 -

AcknowledgementsThe research, described in this report, has been carried out at theHigh-Voltage Laboratory of the Eindhoven Universi ty of Technology underthe supervis ion o f Prof .dr . ir . P.C.T. van der Laan. The technica l s t a f fof th i s laboratory i s gra te fu l ly acknowledged for the i r valuabletechnical ass i s tance . The research has been made poss ib l e by f inancia land mater ial support o f KEMA Laborator ies , Arnhem, Netherlands. Th eauthors appreciate the many fru i t fu l discussions with dr. ir . J.H.Blom,ir . W. Boone, ira A.W. van Hamel and drs . W.S.M. Geurts o f theselabora tor ies . The cables and other equipment have been supplied by NKFKabel, Delf t , Nether lands . I r . M. Bal of th i s company contr ibuted bymany valuable comments.


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THE DETECTION OF INCIPIENT FAULTS IN TRANSMISSION CABLESUSING TIME DOMAIN REFLECTOMETRY TECHNIQUES: Technicalcha l lenges .IEEE Trans . Power Appar. & Sys t . , Vol. PAS-I01(1982) ,p . 1928-1934.

[Sa 82] Bahder, G. an d T. Garri ty , M. Sosnowski, R. Eaton, C. KatzPHYSICAL MODEL OF ELECTRIC AGING AND BREAKDOWN OF EXTRUDEDPOLYMERIC INSULATED POWER CABLES.IEEE Trans . Power Appar. & Sys t . , Vol. PAS-I01(1982) ,p . 1379-1390.

[Sa 83] Bahder, G. and M. Sosnowski, C. Katz, R. Eaton, K. KleinELECTRICAL BREAKDOWN CHARACTERISTICS AND TESTING OF HIGHIV O L T ~ G E XLPE AND EPR INSULATED CABLES.IEEE Trans . Power Appar. & Sys t . , Vol. PAS-102(1983) ,p. 2173-2185.

[B e 82] Beyer, M. an d W. Kamm, H. Sors i , K. FeserA NEW METHOD FOR DETECTION AND LOCATION OF DISTRIBUTEDPARTIAL DISCHARGES (CABLE FAULTS) IN HIGH VOLTAGE CABLESUNDER EXTERNAL INTERFERENCE.IEEE Trans . Power Appar. & Syst . , Vol. PAS-101(1982) ,p . 3431-3438.

[BO B2 ) Boggs, S.A. and G.C. StoneFUNDAMENTAL LIMITATIONS IN THE MEASUREMENT OF CORONA ANDPARTIAL DISCHARGE.IEEE Trans . Elec t r . I n s u l . , Vol. EI-17(1982) , p . 143-150.

[Bo 84) Boone, W. and E.F. Steennis , P.A.C. Bentvelsen , A.M.F.J.van de LaarD E V E ~ O P M E N T AND TRIAL OF EHV XLPE CABLES IN THE NETHERLANDS.In : r roc . 30th Session I n t . Conf. on Large High VoltageElec t r i c Systems (CIGRE), Par i s , 29 Aug.-6 Sept . 1984.Par is : CIGRE, 1984. Paper 21-02.

[C h 83) Cheo, P.K. and R. Luther , J.W. Por t e rDETECTION OF VOIDS AND CONTAMINANTS IN POLYETHYLENE INSULATEDCABLE UTILIZING A FIR LASER BEAM.IEEE Trans . Power Appar. & Sys t . , Vol. PAS-I02(1983) ,p. 521-526.

[Er 84J Eriksson, A.J. an d H. KroningerACCELERATED LIFE PERFORMANCE STUDIES ON A SAMPLE 13 2 kVXLPE INDUSTRIAL CABLE INSTALLATION.In : Proc. 30th Session In t . Conf. on Large High VoltageElec t r i c Systems (CIGRE), Par i s , 29 Aug.-6 Sept . 1984.Par i s : CIGRE, 1984. Paper 15-05.

[Ge 84) Geur ts , W.S.M.VEROUDERING VAN KABELS MET ISOLATIE VAN VERNET POLYETHYLEEN:Chemische begele iding va n onderzoek par t i e l e ontladingen.[Aging of cables with XLPE insu la t ion : Chemical aspec ts ) .Arnhem: KEMA, 1984.In t e rna l r epo r t I I I 9610/84.


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DISCHARGE DETECTION IN HIGH VOLTAGE EQUIPMENT.London: Heywood, 1964.

[Le 79] Lemke, E.A NEW METHOD FOR PD MEASUREMENT ON POLYETHYLENE INSULATEDPOWER CABLES.In: Proc. 3rd Int . Symp. on High Voltage Engineering, Milan,28-31 Aug. 1979.Milano: Associazione Elettrotecnica ed Elettronica I ta l iana ,1979. Paper 43.13.

[Ly 81] Lyle, R. and J.W. KirklandAN ACCELERATED LIFE TEST FOR EVALUATING POWER CABLE INSULATION.IEEE Trans. Power Appar. & Syst . , Vol. PAS-100(1981) ,p. 3764-3774.

[Ok 77] Okamoto, H. and M. Kanazashi, T. TanakaDETERIORATION OF INSULATING MATERIALS BY INTERNAL DISCHARGE.IEEE Trans. Power Appar. & Syst . , Vol. PAS-96 (1977) ,p. 166-177.[Ra 84] Rageb, M.H.S.A. an d A.J. Pearmain

AN APPROACH TO THE PREDICITON OF THE LIFETIME OF ELECTRICALINSULATIONS.IEEE Trans. Elect r . Insul . , Vol. EI-19(1984) , p. 107-113.

[Re 81] Reeves, J.H.THE LOCATION OF PARTIAL DISCHARGE SITES IN H.V. SWITCHGEARAND SHORT CABLES BY A TRAVELLING WAVE METHOD.In: Proe. 6th Int . Conf. on Elec tr i c i ty Distr ibut ion (eIRED),Brighton, 1-5 June 1981.London: lEE, 1981. lEE Conf. Publ. , No. 197. P. 246-250.

[Ru 59] Ruthroff, C.L.SOME BROAD-BAND TRANSFORMERS.Proc. IRE, Vol. 47(1959), p. 1337-1342.

CSt 81] Steennis , E.F.METINGEN VAN PARTlfLE ONTLADINGEN IN HOOGSPANNINGSKABELS.[Measurements of par t ia l discharges in high-voltage cables] .M.Sc. Thesis. High Voltage Laboratory, Department ofElec t r i ca l Engineering, Eindhoven Universi ty o f Technology,1981.Internal report EH.81.A.65.

CSt 82] Stone, G.C. and S.A. ~PROPAGATION OF PARTIAL DISCHARGE PULSES IN SHIELDED POWERCABLE.In: Annual Report 51st Conf. On Elec t r i ca l Insu la t ion andDie l ec t r i c Phenomena, Amherst, Mass. , 17-21 Oct. 1982.Piscataway, N.J . : IEEE Service Center , 1982. P. 275-280.

[Te 60] Tempelaar., H.G.HET AANTONEN VAN ONTLADINGEN IN HET ISOLATIEMATERlAAL VANHOOGSPANNINGSKABELS. [The detection of discharges in theinsulat ion of high-voltage cables] .Electrotechniek, Vol. 38(1960), p. 403-410.


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[We 82] Weeks, W.L. an d J .P. SteinerINSTRUMENTATION FOR THE DETECTION AND LOCATION OF INCIPIENTFAULTS ON POWER CABLES.IEEE Trans. Power Appar. & Syst. , Vol. PAS-IOl(1982),p. 2328-2335.

[Wi 85] Wieringa, L.LOCATION OF SMALL DISCHARGES IN PLASTIC INSULATED HIGHVOLTAGE CABLES.IEEE Trans. Power Appar. & Syst . , Vol. PAS-l04(1985) , p. 2-8.

CWo 83] Wolzak, G.G.THE DEVELOPMENT OF HIGH-VOLTAGE MEASURING TECHNIQUES.Ph.D. Thesis. Eindhoven University of Technology, 1983.


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Eindhoven University of Technology Research ReportsDepartment of Electrical Engineering(138) Nicola , V.F.

ISSN 0167-9708Coden: TEUEDEA SINGLE SERVER QUEUE WITH MIXED TYPES OF INTERRUPTIONS, Applicat ion to th e modell ing ofcheckpoint ing an d r e c o v ~ r y in a t r an s ac t i ona l system.EUT Report 83-E-138. 1983. ISBN 90-6144-138-2

(139) Arts , J .G.A. an d W.F.H. MerckTWO-DIMENSIONAL MHO 8 0 U N ~ L A Y E R S IN ARGON-CESIUM PLASMAS.EUT Report 83--139. 1983. ISBN 90-6144-139-0

(140) Willems, F.M.J.COMPUTATION OF THE WYNER-ZIV RATE-DISTORTION FUNCTION.EUT Repor t 83--140. 1983. ISBN 90-6144-140-4

(141) Heuvel , W.M.C. van den and J .E . Daalder , M.J.M. Boone, L.A.H. WilmesINTERRUPTION OF A DRY-TYPE TRANSFORMER IN N O - L O A D ~ VACUUM CIRCUIT-BREAKER.EUT Repor t 83-E-141. 1983. ISBN 90-6144-141-2

(142) Fronczak, J .OATA COMMUNICATIONS IN THE MOBILE RADIO CHANNEL.EUT Report 83-E-142. 1983. ISBN 90-6144-142-0

(143) Stevens , M.P.J . en M.P.H. va n LoonEENlMULTlFUNCTIONELE I/O-BOUWSTEEN.EUT Report 84-E-143. 1984. ISBN 90-6144-143-9

(144) Dijk , J . an d A.P. ver1ijsdonk, J .C. ArnbakDIGITAL TRANSMISSION EXPERIMENTS WITH THE ORBITAL TEST SATELLITE.EUT Report 84-E-144. 1984. ISBN 90-6144-144-7

(145) Weert, M.J.M. va n~ H A L I S A T I E VAN PROGRAMMABLE LOGIC ARRAYS.EUT Report 84-E-145. 1984. ISBN 90-6144-145-5

(146) Jochems, J .C. en P.M.C.M. va n de n EijndenT O E S T A N D - T O ~ W I J Z I N G IN S E Q U E N T I ~ L E CIRCUITS.EUT Report 85-E-146. 1985. ISBN 90-6144-146-3

(147) Rozendaa1, L.T. en M.P.J. ~ , P.M.C.M. va n de n EijndenDE R[ALISATIE VAN EEN MULTIFUNCTIONELE I/O-CONTROLLER MET BEHULP VAN EEN GATE-ARRAy.EUT Report 95-E-147. 1985. ISBN 90-6144-147-1

(148) Ei jnden , P.M.C.M. va n de nA COURSE ON FIELD PROGRAMMABLE LOGIC.EUT Report 85-E-148. 1985. ISBN 90-6144-148-X

(149) Beeckman, P.A.MILLIMETER-WAVE ANTENNA MEASUREMENTS WITH THE HP8510 NETWORK ANALYZER.EUT Report 85-E-149. 1985. ISBN 90-6144-149-8

(150) Meer, A.C.P. va nEXAMENRESULTATEN IN CONTEXT MBA.EUT Repor t 85-E-150. 1985. ISBN 90-6144-150-1

(l51) Ramakrishnan, S. an d W.M.C. va n de n H,'uve1SHORT CIRCUIT CURRENT INTERRUPTION IN A LOW-VOLTAGE FUSE WITH ABLATING WALLS.EUT Repor t 95-E-151. 1985. ISBN 90-6144-151-X

(152) Ste fanov , B. an d L. Zarkova , A. VeefkindDEVIATION FROM L O C A L ~ D Y N A M I C EQUILIBRIUM IN A CESIUM-SEEDED ARGON PLASMA.EUT Report 85-E-152. 1985. ISBN 90-6144-152-8

(153) Hof, P.M.J . Van den and P.H.M. JanssenSOME ASYMPTOTIC PROPERTIES OF MULTIVARIABLE MODELS IDENTIFIED BY EQUATION ERROR TECHNIQUES.EUT Report 85-E-153. 1985. ISBN 90-6144-153-6

(154) Geer1ings . J .H .T .LIMIT CYCLES IN DIGITAL FILTERS: A bib l iography 1975-1984.EUT Report 85-E-154. 1985. ISBN 90-6144-154-4

(ISS) Groot, J .F .G . deTHE INFLUENCE OF A HIGH-INDEX MICRO-LENS IN A LASER-TAPER COUPLING.EUT Report 85-[-155. 1985. ISBN 90-6144-155-2

(156) Amelsfort , A.M.J. va n an d Th . SchartenA THEORETICAL STUDY OF THE ELECTROMAGNETIC FIELD IN A LIMB, EXCITED BY ARTIFICIAL SOURCES.EUT Report 86-E-156. 1986. ISBN 90-6144-156-0

(157) Ladder, A. an d M.T. va n St iphout , J .T . J . va n EijndhovenESCHER: Eindhoven SCHematic EditoR r eference manual.EUT Report 86-E-157. 1986. ISBN 90-6144-157-9

(158) Arnbak, J .C .DEVELOPMENT OF TRANSMISSION FACILITIES FOR ELECTRONIC MEDIA IN THE NETHERLANDS.EUT Report 86--158. 1986. ISBN 90-6144-158-7

partial discharges and xlpe cable insulation

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