抄録
Partial discharge (PD) characteristics and degradation mechanism of an oil-impregnated insulation system with an oil gap at the electric field adopted in the design of OF cables at 275kV class were investigated. First, we investigated its time transition of PD characteristics under a continuous alternating current (AC) voltage application. As a result, PDs continued with the AC voltage that was lower than that of the PD inception voltage and the test sample broke down after approximately 16 days. These results suggest that a certain amount of lead time is expected even when oil gaps are overlapped on the inner conductor or under the outer shield due to core movement of OF cables after lengthy operation and PDs continuously occur. Also, phase angle characteristics of PD charge magnitude (ϕ-q pattern) showed typical patterns of PDs in a void until before breakdown that changed just before that point. Therefore, ϕ-q patterns are useful for removing outer noises and for determining the occurrences of PDs in OF cable systems under operation. Finally, we observed insulation papers subjected PDs for certain time. As a result, carbonization due to PDs was found especially around the edge of an oil gap and a pin hole was formed in the carbonized area of the oil gap edge after more than 8 days. Based on this result, degradation progress and mechanism was discussed.
