Prosiding Perlemu"" /Imiah /Imu Pengetahuan don Teknolog; Ballo" '99Serpong, 19 -20 Oktober 1999 ISSN 1411-2213

DEGRADATION OF POLYMERS DUE TO HIGH ELECTRIC STRESS

SuwamoJurusan Teknik Elektro ITBJI. Ganesha 10Bandung 40132

e-mail:suwamo@melsa.netid

ABSTRACT

DEGRADATlONOFPOLYMii:RSDUii:TOHIGIIELii:CTRICSTRESS. Some polymers like low-densitypolyethylene (LDPE),Cross-linked polyetJ1ylenes(XLPE) and Ethylene propylene rubber (EPR) are now widely used as highvoltage cable insulation. During operation, the polymers are subjected to high electric field. Due to manufacturing process oroperation the polymers may contain electric field enhancement sites sli~has conductive contaminants, protrusions and voids.The presence ofthe sites causes a very high electric field. During long term the degradation may take place. This paper reportsthe experimentalresultsof the degradationof some polymersdue to high electricfield.The effectsof applied voltage, temperatureand gas absorbed on the degradation are explained.

ABSTRAK

DEGRADASI POLlMii:R AKIBAT MEDAN LlSTRIK TINGGI. BeberapabahanpolimersepertiPE, LDPE daDEPRbanyak dipakai untuk isolasi listrik tegangan tinggi. Akibat pembebanan medan tinggi polimer dapat mengalami degradasi.Degradasi yang paling pentil1gdalam polimer akibat medan listrik tinggi adalah pennohonan listrik (e/ectrlca/treeing) daDpeluahan sebagian (partia/ discharge). Makalah aniakan menjelaskanhasil eksperimen akan degradasi beberapa polimer akibatrnedantinggi. Pengaruh tegangan, temperaturpolimer daDabsorbsi gas akan dibabas dalam makalah ini.

Kala tunc/: Polimer, Medan listrik tinggi, Pemohonan Ii~trik, Peluahan sebagian

INTRODUCTION

The world demand for electric power hasincreased steadily over the past few years. High voltageand extra high voltage have been chosen for transmittinglarge amount of electric power. In rural areas, overhead-high voltage transmission lines have been widely used.However, in metropolitan areas, transmission of electricpower by means of underground cables has been foundto be more practical.

Due to rapid advance in technology, polymericcables have come into wider use and their performancehas improved as well [1]. There are many advantages ofpolymeric cables compared with oil-filled cables. Theyare excellent resistivity to thermal aging, free from failuresassociated with the migration of oil, free from fire riskdue to cable oil, maintenance-free and low dielectriclosses.

There are several polymers being used for powercables from 5 kV up to 500 kV. They include low-densitypolyethylene (LDPE), crosslinked polyethylene (XLPE),tree-retardant crosslinked polyethylene (TRXLPE) andethylene-propylene rubber (EPR).

In Japan, Central Research Institute of ElectricalReset Institute (1971) organised a development of250kVXLPEcables. In 1979,275 kV XLPE cables were than

put into service. In France, 90 kV LOPE cable was firstmanufactured by Electric de France (EDF) network andin 1969,225 kV LDPE cable was introduced to the field.In 1985,400 kV LDPEcableswere also introduced.

Long distance extra high voltage (EHV) XLPEcable lines have been laid over of distance of 26.9 km(South Route)from Chita Dai-ni ThermalPower Plant toMinami Buhei sub station in Central Nagoya, Japan [2].Theworldfirstpracticalapplicationof 500kVXLPEcablestook place in 1988 in Japan at Shimogo Power Station.Long distance transmission lines using 500 kV XLPEcables are being planed to be laid near Tokyo Bay byTEPCO in corporation with several Japanese cablemakers.

CRITICAL POINTS OF POLYMERS FORPOWER CABLES

During operation, the polymeric insulation issubjectedto high electricfield.Degradationat the criticalpoints may leadthe failure of the insulation. The criticalpoints that govern the performance are defects. Defectmay arise in the form (1) voids and impurities in thepolymers(2)protrusionsin the interfaces.If the insulationdegradation occurs at these defects an electrical arises

48

Degradation of Polymers Due to High Electric Stress (Suwarno)

and propagates inside the insulation. If it bridges theinsulation, insulation failure occurs. The investigationon XLPE insulation showed that 74% of breakdownsoriginated from impurities in the insulation and 26%originated from protrusions. No breakdown was seen tooriginate from voids [3].

Over the lastfewyears,thequalityofXLPE cableshas been improved by suppressing the level of defects.This has been achieved mainly by improving themanufacturing technology of XLPE cables andimprovement in materials. In the manufacturingtechnology,steamcuringhasbeen replacedby drycuringand the separated tandem extrusion has been replacedby three-layer simultaneous extrusion.

The application of dry curing has suppressedvoids the insulation to a levelmuch smaller than 10f.1m.By using the three-layer simultaneous extrusion, theprotrusion between semicon layer and XLPE insulationhas decreased fromabout 100f.1mto smallerthen 10f.1m.Impurities also suppressed to less than 50 f.1m[1].

DEGRADATION DUE TO THE HIGHELECTRIC FIELD

Eletrical treeing degardation

Electrical tree consists of many fine channels.Scanning electron microscopy has revealed that thediameter ofthe tree channel is 2-5 f.1mand less than 111mfor minute channel just after tree initiation [4]. Electricaltree grows in regions of high electrical stress, such asmetallic asperities, conducting contaminants andstructural irregularities. Electrical tree may also developfrom voids.

In the case of needle-plane electrode the maximum

field (Em)can be expressed as [5]

E = 2Vm

(4d )r In -;-whereV istheappliedvoltage,r is the radiusof curvatureof the needle tip and d is the distancebetweenelectrodes.If the high electric field exceeds the strength of thepolymers, a tree or partial dischargeoccur. The inceptionof tree is defined as time or voltage to produce treeobservable in the measurement system.

EXPERIMENT

Samples being used were low-densitypolyethylene (LOPE), ethylene-vinyl acetatecopolymer(EVA, 3,4 .% wt), ethylene-acrylic acetate copolymer(EAA), 13 %wt) and ethylene propylene rubber (EPR)from Mitsubishi. A steel needle electrode (OguraJewellery) with a tip radius of curvature of 3 f.1mwasinserted into the sample.

With a needle-plane electrode systems, an

alternating current voltage was applied to the needleelectrode silicone oil to avoid surface discharges. Thearea around the needle tip was observed by using aChargeCoupledDevice(CCO) camera. The inceptionoftree determinedoptically.

YII"UT~MII\II:tIIlI"1II J-, .

II~WIl1rKUC:=.J

IN-IrNUllDW IILIIC'I"IIUUU !'IAN!! 1ILIIC'IAClUII

Figure 1. Experimental set up

RESULTS AND DISCUSSION

The effectsof the applied voltage (applied electric field)

The applied voltage greatly affected the treeinitiation. The initiation voltage of various polymers isshown in Table I. Table 1shows that each polymer hasits own treeing inception voltage, EPR representsweakest polymer and EAA represents a strongestpolymer.

Table 1. Inception voltage of electrical treeingfor several polymers

Effectof temperature

Temperature of -30 °Cwas obtained by using dryicewhile high temperatureof 60 °Cwas reachedby usinga controllable heater. It was surprised that in thistemperature there was no tree started from the needle tipat 130 minutes. Compared with the results at roomtemperature, treeing at low temperature showed thefollowingbehaviour:

. Trunk and branch number were very small

. Propagation was very siow

Two reason were proposed as the causes ofphenomena.Firstly, at low temperature, the mechanical strength maybe better than that of room temperature and the increaseof the treeing resistance. Secondly, the activity ofdischarge was lower than at room temperature which will

.on

Material Inceptton voltage (kV, Rms)EPR 7

EVA 8

LDPE 8

EAA 10

Pros;d;ng Pertemulln IlmJllh limu Pengelllhuan dan Teklfolog; Bahlln'99Serpong, 19 -20 OklobeT ]1)99 ISSN 1411-2213

reduced the degradation process.

Treeing at high temperature of60 °C

At 60 °C, treeing appearedjust after the voltageapplication. The treeing grew very fast and the numberof channels was very much.

The effects of temperature on the treeingbehaviour in other materials such as Ethylene PropyleneRubber (EPR), Ethylene Vinyl Acetate (AVA) andEthylene Acrylic Acetate (EAA) were also investigated.The results showed that the behaviour of the treeing inthe various materials were same especially at low andhigh temperatures.

Effects of absorbe4 gas

In order to know the effects of absorbed gas onthe electrical treeing degradation, several samples wereprepared. The are including- Virgin samples without treatment- Soft vacuum samples, which were degassed by using

rotary, pump soft vacuum- High vacuum samples, which were degassed by using

diffusion, pump vacuum- Oxygen absorbed samples, which were doped using

oxygen gas after vacuum treatmentThe experimental results showed that removal of gassesin samples by soft vacuum process considerablyincreased the treeing resistance of polymers comparedwiththevirginsamples.Highvacuumtreatmentincreasedmore treeing resistance. These fact showed that gassesinside virgin samples may promoted the initiation oftreeing during application of high electric field. It isspeculated that oxygen played important role [6,7]. Toprove this speculation oxygen treated samples wereinvestigated. The results showed that higher treeingactivities took place in these samples. These factsconfirmedthe roleof oxygen.Themechanismisstillopenfor discussion.

Table 1. Time inception for gas treated samples

Partial discharge (PD)

When AC voltage is applied, under a givencondition,there isa voltageBELOWWHICHbreakdownwill not occur across a gas-filled void. This voltage iscalled the inception voltage (V) and has been observedfor a wide range polymers. The inception voltage for agas-filled void depends on the nature of the insulation,the gas inside, shape and size of void, pressure and

temperatur. Partial discharge in void will causedegradation to surrounding polymer.

Experimental results showed that PD activity wasstrongly affected by applied voltage and therefore the

effect on degradation also depend on the applied voltage.PD number as well as PD magnitude increased linearlywith the applied voltage.

i 5-1

J. 4

'I ,.,a'~

I

0D a y...', 4 .

/lrruUD VOLT/lOI! CKYI

Figure1. Dependenceof PO numberon appliedvoltage

.

70

~ 80

~ 50

~ .,

. II'OS""IV

~Y""

0 IttIIOl\l1VE "- IIOSI11VI

,NEGA11VE .

51 "~ 2

~ I:0

a . ./lI'I'UED VOLT/IOI CleVI

.

Figure 3. dependence of PO magnitude on appliedvoltage

CONCLUSIONS

Degradation of polymers under high electric canbe observed mainly in the form of electrical treeing andpartial discharge. The performance of the polymers asinsulation is affected by degradation. Applied voltage,temperature and absorbed gasses inside the polymersgreatly affected the degree of degradation speed.

REFERENCES

[I]. T. KUBOTA, Y. TAKAHASHI, S.SAKUMA, M.WATANABE, M. KANA OK A and H.YAMANOUCHI,"Development of500 kV XLPECables al1d Accessories for Long DistanceUndergroundTransmissionLines",IEEETrans..PO-9,NoA, 1994,pp.1741-1749.

[2]. H.OTA, M.lCHIHARA, N.MIYAMOTO,"Applicationof AdvancedAfter-LyingTest to LongDistance275kV Cables",IEEETrans.,PD-I0,NO.2,

Samples Inception tim e (rn in)

V irg in 5

Low vacuum 120

0 x Yg e n -tre ate d 3

High vacuum 3 I 5

Degradation of Polymers Due to High Electric Stress (Suwarno)

1995,pp.567-577.[3]. H. SUZUKI, T.ENDOH, "Experimental Study on The

Cause ofEle.ctricalBreakdownofXLPR", CEIDP,1990,pp255-260.

[4]. R.J DENSLEY, "An Investigation Into Growth ofElectrical Trees in XLPE Cable Insulation", IEEETrans.,EI-14,No.3, 1979,PP...l48-158.

[5]. J.H.MANSON, "Breakdown,of Solid Dielectric inDivergent Fesulting from Ibtemal Discharges".

Proc.IEEVoI.98,Partl, 1951,p~59.[6]. RASlKAWAN, "Suppression of Treeing

Degradation in Polmeric Materials",Ph.D. Dissertation, Nagoya University, 1993.

[7]. N.SHIMIZU,"Roleof OxygeninElectricaltreeing",Int. Workshopon El.Insul.,Nagoya, 1993, pp.ll-16

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