Time resolved measurements of electroluminescence in XLPE under impulse voltage conditions

Time resolved electroluminescence (EL) measurements were performed on high voltage grade XLPE cable insulation using negative impulse voltages of different shape and duration. To create divergent fields in the insulation, needle electrodes made of semiconductive polymer were used. It is shown that the EL emission probability is independent of the impulse rise time but increases with the impulse fall time and amplitude. Independently of the impulse shape and duration, EL pulses are always grouped into two distinct time intervals between which there is no EL activity. The first EL period begins at the rising portion of the impulse and the second at the falling portion when the impulse voltage drops below 50% of its peak value. EL emission during the first interval is believed to be caused by electron injection and the impact excitation processes, whereas the EL in the second interval is seen to be due to the local electric field reversal and the electron-hole recombination.