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

DEGRADATION OF POLYMERS DUE TO HIGH ELECTRIC STRESS

Suwamo Jurusan Teknik Elektro ITBJI. Ganesha 10Bandung 40132 e-mail:[email protected]

ABSTRACT

DEGRADATlONOFPOLYMii:RSDUii:TOHIGIIELii:CTRICSTRESS. Some polymers like low-density (LDPE),Cross-linked polyetJ1ylenes(XLPE) and Ethylene propylene rubber (EPR) are now widely used as high voltage cable insulation. During operation, the polymers are subjected to high electric field. Due to manufacturing process or operation 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 reports the experimentalresultsof the degradationof somepolymersdue to high electricfield.The effectsof applied voltage, temperature and gas absorbed on the degradation are explained.

ABSTRAK

DEGRADASI POLlMii:R AKIBAT MEDAN LlSTRIK TINGGI. BeberapabahanpolimersepertiPE,LDPE daDEPR banyak 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) daD peluahan sebagian (partia/ discharge). Makalah aniakan menjelaskanhasil eksperimen akan degradasi beberapa polimer akibat rnedantinggi. Pengaruh tegangan, temperaturpolimer daDabsorbsi gas akan dibabas dalam makalah ini.

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

INTRODUCTION put into service. In France, 90 kV LOPE cable was first The world demand for electric power has manufactured by Electric de France (EDF) network and increased steadily over the past few years. High voltage in 1969,225 kV LDPE cable was introduced to the field. and extra high voltage have been chosen for transmitting In 1985,400 kV LDPEcableswere also introduced. large amount of electric power. In rural areas, overhead- Long distance extra high voltage (EHV) XLPE high voltage transmission lines have been widely used. cable lines have been laid over of distance of 26.9 km However, in metropolitan areas, transmission of electric (South Route)from Chita Dai-ni ThermalPower Plant to power by means of underground cables has been found Minami Buhei sub station in Central Nagoya, Japan [2]. to be more practical. Theworldfirstpracticalapplicationof500kVXLPEcables Due to rapid advance in technology, polymeric took place in 1988 in Japan at Shimogo Power Station. cables have come into wider use and their performance Long distance transmission lines using 500 kV XLPE has improved as well [1]. There are many advantages of cables are being planed to be laid near Tokyo Bay by polymeric cables compared with oil-filled cables. They TEPCO in corporation with several Japanese cable makers. are excellent resistivity to thermal aging, free from failures associated with the migration of oil, free from fire risk due to cable oil, maintenance-free and low CRITICAL POINTS OF POLYMERS FOR losses. POWER CABLES There are several polymers being used for power cables from 5 kV up to 500 kV. They include low-density During operation, the polymeric insulation is polyethylene (LDPE), crosslinked polyethylene (XLPE), subjectedto high electricfield.Degradationat the critical tree-retardant crosslinked polyethylene (TRXLPE) and points may leadthe failure of the insulation. The critical ethylene-propylene rubber (EPR). points that govern the performance are defects. Defect In Japan, Central Research Institute of Electrical may arise in the form (1) voids and impurities in the Reset Institute (1971) organised a development of250 polymers(2)protrusionsin the interfaces.Ifthe insulation kVXLPEcables. In 1979,275 kV XLPE cables were than degradation 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 the alternating current voltage was applied to the needle insulation, insulation failure occurs. The investigation electrode silicone oil to avoid surface discharges. The on XLPE insulation showed that 74% of breakdowns area around the needle tip was observed by using a originated from impurities in the insulation and 26% ChargeCoupledDevice(CCO) camera.The inceptionof originated from protrusions. No breakdown was seen to tree determinedoptically. originate from voids [3]. Overthe lastfewyears,thequalityofXLPE cables has been improved by suppressing the level of defects. YII"UT~MII\II:tIIlI"1II J This has been achieved mainly by improving the -, . manufacturing technology of XLPE cables and improvement in materials. In the manufacturing II~WIl1rKUC:=.J technology,steamcuringhasbeenreplacedby drycuring and the separated tandem extrusion has been replaced by three-layer simultaneous extrusion. The application of dry curing has suppressed voids the insulation to a levelmuch smaller than 10f.1m. By using the three-layer simultaneous extrusion, the IN-Ir protrusion between semicon layer and XLPE insulation NUllDW IILIIC'I"IIUUU !'IAN!! 1ILIIC'IAClUII has decreased fromabout 100f.1mto smallerthen 10f.1m. Figure 1. Experimental set up Impurities also suppressed to less than 50 f.1m[1]. RESULTS AND DISCUSSION DEGRADATION DUE TO THE HIGH ELECTRIC FIELD The effectsof the applied voltage (applied electric field) The applied voltage greatly affected the tree Eletrical treeing degardation initiation. The initiation voltage of various polymers is Electrical tree consists of many fine channels. shown in Table I. Table 1shows that each polymer has Scanning electron microscopy has revealed that the its own treeing inception voltage, EPR represents diameter ofthe tree channel is 2-5 f.1mand less than 111m weakest polymer and EAA represents a strongest for minute channel just after tree initiation [4]. Electrical polymer. tree grows in regions of high electrical stress, such as metallic asperities, conducting contaminants and Table 1. Inception voltage of electrical treeing structural irregularities. Electrical tree may also develop for several polymers from voids. Material Inceptton voltage (kV, Rms) In the case of needle-plane electrode the maximum EPR 7 field (Em)can be expressed as [5] EVA 8 E = 2V LDPE 8 m 4d r In(-;-) EAA 10 whereV istheappliedvoltage,r istheradiusofcurvature of the needletip and d is the distancebetweenelectrodes. Effectoftemperature If the high electric field exceeds the strength of the polymers, a tree or partial dischargeoccur. The inception Temperature of -30 °Cwas obtained by using dry of tree is defined as time or voltage to produce tree icewhile high temperatureof60 °Cwasreachedby using observable in the measurement system. a controllable heater. It was surprised that in this temperature there was no tree started from the needle tip EXPERIMENT at 130 minutes. Compared with the results at room temperature, treeing at low temperature showed the Samples being used were low-density followingbehaviour: polyethylene (LOPE), ethylene-vinylacetatecopolymer . Trunk and branch number were very small (EVA, 3,4 .% wt), ethylene-acrylic acetate copolymer . Propagation was very siow (EAA), 13 %wt) and ethylene propylene rubber (EPR) Two reason were proposed as the causes ofphenomena. from Mitsubishi. A steel needle electrode (Ogura Firstly, at low temperature, the mechanical strength may Jewellery) with a tip radius of curvature of 3 f.1mwas be better than that of room temperature and the increase inserted into the sample. of the treeing resistance. Secondly, the activity of With a needle-plane electrode systems, an discharge was lower than at room temperature which will

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

reduced the degradation process. temperatur. in void will cause Treeing at high temperature of60 °C degradation to surrounding polymer. Experimental results showed that PD activity was At 60 °C, treeing appearedjust after the voltage strongly affected by applied voltage and therefore the application. The treeing grew very fast and the number effect on degradation also depend on the applied voltage. of channels was very much. PD number as well as PD magnitude increased linearly The effects of temperature on the treeing with the applied voltage. behaviour in other materialssuch as Ethylene Propylene Rubber (EPR), Ethylene Vinyl Acetate (AVA) and Ethylene Acrylic Acetate (EAA) were also investigated. The results showed that the behaviour of the treeing in i 5-1 the various materials were same especially at low and J. 4 high temperatures. Effects ofabsorbe4 gas 'I ,.,a '~ In order to know the effects of absorbed gas on I the electrical treeing degradation, several samples were 0 prepared. The are including D a y...', 4 . . - Virgin samples without treatment /lrruUD VOLT/lOI! CKYI - Soft vacuum samples, which were degassed by using Figure1. Dependenceof PO numberon applied rotary, pump soft vacuum voltage - High vacuum samples, which were degassed by using diffusion, pump vacuum 70 - Oxygen absorbed samples, which were doped using ~ 80 . II'OS""IV "" oxygen gas after vacuum treatment 0 IttIIOl\l1VE " - IIOSI11VI The experimental results showed that removal of gasses ~ 50 ,NEGA11VE . in samples by soft vacuum process considerably ~ ., increased the treeing resistance of polymers compared 51 " withthevirginsamples.Highvacuumtreatmentincreased ~ 2 ~Y more treeing resistance. These fact showed that gasses ~ I: inside virgin samples may promoted the initiation of 0 treeing during application of high electric field. It is a . . . speculated that oxygen played important role [6,7]. To /lI'I'UED VOLT/IOI CleVI prove this speculation oxygen treated samples were Figure 3. dependence of PO magnitude on applied investigated. The results showed that higher treeing voltage activities took place in these samples. These facts confirmedtheroleof oxygen.Themechanismisstillopen for discussion. CONCLUSIONS

Table 1. Time inception for gas treated samples Degradation of polymers under high electric can be observed mainly in the form of electrical treeing and Samples Inception tim e (rn in) partial discharge. The performance of the polymers as V irg in 5 insulation is affected by degradation. Applied voltage, Low vacuum 120 temperature and absorbed gasses inside the polymers greatly affected the degree of degradation speed. 0 x Yg e n -tre ate d 3 High vacuum 3 I 5 REFERENCES

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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 of Electrical Trees in XLPE Cable Insulation", IEEE Trans.,EI-14,No.3, 1979,PP...l48-158. [5]. J.H.MANSON, "Breakdown,of Solid Dielectric in Divergent 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,"RoleofOxygeninElectricaltreeing", Int. Workshopon El.Insul.,Nagoya, 1993, pp.ll- 16

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