Understanding treeing phenomena in XLPE cable insulation under composite voltages

Electrical trees were experimentally generated in XLPE cable insulation under AC and composite voltage formed with AC and DC voltages. Bush type and tree like tree structures were generated. Analysis of time to failure due to electrical trees was made by adopting Weibull distribution studies. The life of cable insulation due to electrical tree failure is enhanced in presence of offset voltage. The rate of tree propagation is high under AC voltage with positive offset. Applied voltage and failure time of insulation shows inverse relationship. It is realized that, the frequency content of the UHF signal generated during tree propagation lies in the range 1-3 GHz. The Phase resolved partial discharge (PRPD) analysis indicates that discharges occur in the rising portion of the applied AC voltage. With composite voltages, the discharges occurs near the zero crossing and is predominant with positive DC voltage superposed with AC voltage.