Electroluminescence Properties under Long Time Voltage Application in XLPE

Electroluminescence (EL), faint light observed at electric field concentrating points is well accepted as a precursory phenomenon of the electrical tree initiation. We consider that EL is caused by electron impact. EL is considered to be closely related to material degradation; the start of EL is the start of the degradation. But the relation between EL intensity and degradation is still unclear. In this paper we'll report time-variation of EL intensity in XLPE over long-time of voltage application. When XLPE is subjected to long time voltage application of 25 hours at 5 kVrms, which is as low as EL starting voltage, the light intensity showed unexpected change. That is, even after formation of tree channel, which is very small but certain, no discharge light was observed. Instead of discharge light we observed unstable EL. This result leads us to better understanding of the early stage of degradation