Space charge induced in stressed polyethylene

Measurements of space charges induced by poling have been made on XLPE (cross-linked polyethylene) samples cut from AC cables aged under different conditions. The approximately 220- mu m-thick samples were obtained by peeling. Gold electrodes were evaporated on the samples, the samples were poled at approximately 0.12 MV/cm (2.75 kV per sample) for three hours at 70 degrees C, and the space charge was determined by the thermal pulse technique. The amount of space charge induced in different samples by identical poling conditions was larger in the samples with the longer exposure to AC stress. The total amount of space charge which could be induced by poling was also found to correlate with other parameters, such as AC breakdown strength. In addition, the space charge dissipation rate was monitored for several weeks after poling. Samples aged at room temperature with both electrodes grounded retained more than half their original space charge after one week. It is noted that it is highly probable that the determination of space charges will be a powerful method for estimating the degree of degradation of aged cables.<>