08_Muramoto_tree_ini_ELF -...

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Unformatted text preview: 2008AnnualReportConferenceonElectricalInsulationDielectricPhenomena Tree InitiationatElectroluminescence Starting Voltage in X L P E N. Shimizu,T.Mito,S.Mizuno andY.Muramoto MeijoUniversity Nagoya,468-8502,Japan Abstract- The degraded region isformed even at low voltage almost equal to electroluminescence (EL) startingvoltage, and then tree channel grows from the degraded region.Thispaper reportstherelationbetweentheprogressofpolymerdegradation andEL intensity,anddiscussestheprocessattheveryearlystage oftreeinitiationincludingtheformationoftreechannelwithout gasdischarge. I.INTRODUCTION Extrudedpolymericelectricalinsulationofpolyethylene(PE) orcross-linkedpolyethylene(XLPE)iswidelyusedinpower transmission systems,because oflow cost,lightweight,and excellentworkability.Electricaltreeingismaindrawbackof polymeric materials. When electrical tree channel bridges electrodes, the final breakdown may occur; itdecides the practicallifetimeofinsulationsystems.Intheinitiationstage ofelectricaltree,adegradedregionisformedatelectricalfield enhancement points such as contaminant particles or protrusionofasemiconductinglayer.Thetreechannelgrows fromthedegradedregion[1]. At the field enhancement point in polymeric materials, electroluminescence (EL), very faint light emission, is observed under ac stress [2,3].The lightofEL isclearly distinguishedfromlightduetopartialdischarge(PD)inagas voidinrespectsofintensityandspectrum.Thelightintensity ofEL ismuch lowerthanthatofPD. EL lighthasabroad spectruminthevisiblerange,whilePD lighthaspeakinthe UV range. Inbothprocessesofthedegradationinelectricaltreeinitiation phase and EL, electrons injected from electrode and their collisionimpactplayessentialroles[1,4,5].Electronsinjected fromelectrodeareacceleratedinmicrocavityorpolymerfree volume by high electric field, and collide with polymer molecules or chromophores which are excited or ionized. Whiledegradationofmaterialtakesplacewhenpolymerbond scissioniscausedbyelectroncollision,lightisemittedwhen some excited chromophores return to their ground states (impactexcitation)or upon chargerecombinationinvolving luminescencecenters(impactionization). However, therelationbetweenEL intensityanddegradation progress is stillunclear. In this paper we will report the relationbetweenEL intensityandmaterialdegradationatlow voltage of 5kVrms, which is almost equal to EL starting voltage. The discussion will be done on the process of formation oftreechannel from the degraded region atEL startingvoltage. II.EXPERIMENTALRESULTS A.Experimental Experimental isthesame asthepreviousreports [6]. Tree testingspecimen consisting ofa XLPE block (HFDJ-4201, 0.910 g/cm3,25x25x4mm) andasteelneedlewithatipradius of 3pm andatipangle30°shown inFig. 1was used.The distancefromtheneedleelectrodetiptothebottom ofthe blockis3mm. Priortothevoltageapplication,thespecimen was subjectedtovacuum treatmentformore than10daysin order to remove oxygen in polymer free volume. The specimenwasplacedonagroundedplaneelectrodeinalight...
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This note was uploaded on 06/11/2011 for the course ELECTRICAL 124 taught by Professor Ghjk during the Spring '11 term at Institute of Technology.

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08_Muramoto_tree_ini_ELF -...

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