Location of the prebreakdown region in a cross-linked polyethylene insulated cable

A method of locating discharges during breakdown tests on medium-voltage cables is described. The resistive screen of the cable attenuates the discharge pulses and the location is made by using the magnitude of the pulses recorded at each end of the cable, allowance must be made for pulse reflection. Tests on an 11 kV polyethylene insulated cable that contained large vented water trees are used to illustrate the method. Total discharge energy and location were recorded for a single discharging site. The test was stopped prior to breakdown and a volume of insulation containing a single electrical tree was cut from the cable. The computed discharge energy was compared with the eroded volume within the electrical tree.