Understanding Electrical Treeing Phenomena in XLPE Cable Insulation under Harmonic AC Voltages Adopting UHF Technique

In the present study, impact of high frequency ac voltage and harmonics in ac voltages on treeing were studied. When the applied voltage is high, a bush type tree forms irrespective of frequency of the supply voltage. Influence of harmonics on shape of tree formed indicates that a voltage wave with high total harmonic distortion have characteristics much similar to high frequency ac voltage. It is observed that the tree inception time is much lower with 11th harmonic ac voltages with higher THD. Presence of odd harmonics of equal magnitude, irrespective of amount of total harmonic distortion, the shape of the tree formed is almost same as the tree generated with 50 Hz ac voltage. The injected PD current pulses during treeing process have rise time of few nanoseconds. The UHF signals generated during tree growth (at the point of inception and propagation) have signal bandwidth in the range of 1-2 GHz. Measure of UHF signal magnitude during treeing process could help one to classify the shape of tree formed. Weibull distribution studies were carried out to estimate the life of XLPE cable insulation failure due to electrical treeing under different harmonic ac voltages. Independent higher order harmonic ac voltages with increase in total harmonic distortion reduce the life of cable insulation due to electrical treeing.