Electrical Treeing in Xlpe Cable Insulation under Harmonic Ac Voltages

The shape of electrical trees gets altered due to applied voltage profile. Bush type electrical trees forms under harmonic voltages with lower THDs. With increase in harmonic level with increased THDs fibrillar type electrical trees were formed. Weibull distribution studies indicate that increase in harmonic level (odd/even) with higher THDs accelerates the tree growth in cable insulation causing early failure of cable insulation. The frequency content of the Ultra High Frequency (UHF) signals radiated due to tree formation lies in the frequency band of 0.5-2 GHz. Adoption of spectral power ratios to UHF signal can classify the shape of electrical trees that has incepted and propagating in the insulating material under different voltages. Phase Resolved Partial Discharge (PRPD) analysis using UHF signal indicates that discharges occurs under harmonic voltages where the rate of rise of voltage is high thereby accelerating the tree growth. During tree growth discharges occur in the entire phase of the triangular voltage. Partial discharges occur at the peak with the low frequency AC voltage and at the raising portion when the supply voltage is increased.