98_Bamji_elf_impulse - IEEE Transactions on Dielectrics and...

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204 IEEE Transactions on Dielectrics and Electrical Insulation Vol. 5 No. 2, April 1998 lectroluminescence Technique to Evaluate the Effect of Impulse Tests on High Voltage Cables S. S. Bamji, M. Kaufholdl and A. T. Bulinski National Research Council of Canada, Ottawa, Ontario, Canada ABSTRACT EL (electroluminescence) in XLPE (crosslinked polyethylene) subjected to impulses of various amplitudes, shapes and repetition rates has been investigated. Defects that could occur in the polymeric insulation of underground HV cables were simulated by embedding semicon pro- trusions in to create highly divergent electric fields. Time resolved measurement of the pulses emitted during positive, negative or alternating polarity impulses has shown that the pulses are grouped into two distinct time periods between which there is no activity, The emission probability increased with fall time of the impulse voltage but was independent of the impulse rise time. A study of electroluminescence characteristics from unaged and aged XLPE, subjected to a pair of positive and negative impulses, indicated that the charge trapped in the polymeric insulation during aging plays an important role during impulse application and could affect insulation failure. 1 INTRODUCTION HE electrical insulation of underground HV cables is not only sub- jected to ac stress but also to surges caused by lightning and switching impulses. To determine the operating conditions of power cables in service it is important to develop diagnostic tests that can be applied without causing any adverse effects to the cable insulation. Im- pulse voltage tests are commonly employed to check the integrity of in- sulation of power cables either immediately after installation or during service. In North America, the AEIC (Association of Edison Illumination Companies) impulse breakdown test [l] is widely recognized for PE (polyethylene) and insulated and shielded power cables. This test is performed at a nominal conductor temperature of 90°C for PE and 130°C for by applying 3 impulses of each polarity corresponding to the basic impulse level of the transformer supplying the cable. The voltage is then raised in steps of 25% of the basic impulse level with 3 negative pulses produced at each step. In Europe, the IEC (International Electrotechnical Commission) impulse test [2] is employed on cables and their accessories. The starting voltage for this test is also the basic impulse level with 10 impulses at each polarity applied to the cable. The voltage is then increased by 5% and 10 impulses at each polarity are re- peated. Breakdown often occurs at the first impulse after the polarity is reversed. This is probably due to a space charge effect, because although the cable is short circuited between impulses by the impulse generator circuitry, the internal electric stress of the dielectric is not completely re- lieved before the subsequent impulse is applied. If that impulse is of the reverse polarity, the stress within the dielectric is enhanced and could cause failure. Although AEIC and IEC follow different procedures, the
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98_Bamji_elf_impulse - IEEE Transactions on Dielectrics and...

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