Study on development of partial discharges at the defect caused by a needle damage to a cable joint

Insulation faults caused by the damages to the semi-conductive outer layer of silicon rubber prefabricated cable joints often occur. Studies on the partial discharges (PDs) development of such insulation defects, may help on the on-line assessment of their severity. In this paper, a needle damage was made in a 35 kV prefabricated cable joint. The voltage applied to the test object was raised step by step, which causes PDs and accelerates the deterioration of insulation. After the defect was seriously deteriorated, a constant voltage was applied until insulation breakdown. During the tests, PDs were all recorded and analyzed. The trend of PD energy per unit time was obtained. The development of PDs was classified into five stages according to the trend of PD energy. For each stage, phase characteristic spectrograms were determined, and three characteristic parameters were extracted. The investigation results showed that, during the development of PDs, there were differences in phase characteristic spectrograms at different stages. They increased monotonically, and then suddenly increased in the last period. Based on these characteristics, the severity of needle-damaged insulation defects could be more accurately assessed.