00_montanari - 322 I EEE l?aiisartions on Dielectrics a nd...

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322 IEEE l?aiisartions on Dielectrics and Electrical hisulation Vol. 7 No. 3,Juiie 2000 Evaluation of dc Insulation Performance Based on Space-Charge Measurements and Accelerated Life Tests G. C. Montanari D. Fabiani 1)1E, Univcrsity nf liologna, Ilaly ABSTRACT Space charge observation is becoming thc most widely used techniquc to evaluate polymcric materials for dc insulation applications, particularly HV cablcs. However, quantities are still lacking that can help to summarizc and interpret thc huge amount of data resulting from space charge measurements, and that also are associated with the electrical performance of the in- sulation. The purpose of this payer is to propose test methods and parameters, based mi the pulsed elcctroacoustic tcchiiique, which can be used for the evaluation of the electrical per- formance of polymeric insulation under dc ficlds. These parameters are related to the space charge magnitudc and mobility, as well as to the internal elcctric field amplification, the rate of charge accumulation, and the threshold field above which charge is stored in the insulation. In ordcr to investigate the meaning of the parameters proposed for thc asscssment of iiisula- tion performance, the corrclation between these parainctcrs associated with space charge and insulation lifc is discussed. Examples arc reported with rcfcrence to five different caiididatc matcrials for de cable insulation. 1 INTRODUCTION largc potential markct would be open to pnlyincr-insulated dc A cablcs oiice tlie problcms associatcd with tlieir rcliability Ihavc bcen solved. It is generally accepted that thc accumulation of spacc charge can constitute an important causc of insulation degradation, es- pecially in the prcsence ol voltagc inversions aiid surges to which dc cables are subjccted during scrvicc operation. I’olymer-iiisulatcd ca- bles suffer iron1 this problem, sincc space charge is stored at quite low elcctric fields. hi the case ol crosslinked yolycthylciic (XI,IJI+, for cxa~ii- plc, crosslinking by-products can provide ions and/or dipoles giving rise lo net heterocharge closc to the electrodes even at fields ‘1s low as 10 IkV/mni [1-51. Injected carriers play a inore significant role when the ficld increases, so that heierochargc can turn cventually into ho- inocharge [6]. Both lioniocharge aiid hetc,rocharpc can, in principlc, affect cable reliability through local ficld am~lificalion, encrgy storage aiid hol clcctroii formalion [1, 7-11 1. New polymeric matcrials, mostly obtained irom polycthylenc (PE) or crossliiikcd polyethylene (XLI’E) with appropriate addiiivcs or fillers, are being investigated intensively as candidates for dc cablc insula- tion, with tlie inaiii aim of reducing the amount of charge stored at tlie desigti field [Z, 5,12-161. FIowcvcr, characterization of the electrical performancc of these new materials sulicrs from the lack of clcar and concise informatioii coming froin test procedures which try to achieve an evaluation of performance in a short time. Short tiincs are, in fact,
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00_montanari - 322 I EEE l?aiisartions on Dielectrics a nd...

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