Response and mechanisms of water trees in XLPE cable insulation under superposition voltage

When two voltages with different wave shapes or frequencies are applied to insulation which has been degraded by water trees in XLPE cable, it is found that the characteristic signals of deterioration in the insulation appear in the ground circuit of the cable screen. Detecting this signal, one can judge the water-treeing situation of the cable insulation. The superposition response of Ultra-low Frequency Square Wave Voltage (ULFSW) on Power Frequency Voltage (PFV) for off-line diagnosis of XLPE cable is explored and the optimum superposing voltage of power frequency is obtained. The superposing response of difference frequency modulated voltages for water treeing in XLPE cable insulation is also studied and the optimum superposing frequency of voltage is found by a simulation circuit in this paper. Finally, the mechanisms of superposing response are discussed