Electroluminescence properties and degradation of XLPE

Electroluminescence (EL), faint light observed at electric field concentrating points is well accepted as a precursor of the electrical tree initiation. EL is considered to be related to material degradation; the start of EL is the start of the degradation. When cross-linked polyethylene (XLPE) is subjected to long time voltage application of 5 kVrms for 25 hours, time-variation of EL intensity can be classified into five stages. In the fourth stage a small tree channel seems to initiate with a sudden increase of the light intensity. But the light intensity drops in few minutes. At that time, discharge light does not seem to occur. We have studied the relation between the light intensity and the material degradation in detail, aiming at estimation of the degree of material degradation by the time-variation of EL intensity.