The changes in electrical properties of XL4421 insulation under repetitive lightning impulse voltages

XLPE is commonly used as the bulk dielectric for HV power cables. During a long service life, the insulation material will age and may ultimately degrade and fail. The service life of the cable will be affected by multifactor ageing processes. If the cable suffers a lot of lightning or switching impulse over-voltages during its working period, the ageing process may be accelerated. By analyzing the changes in electrical property of a material before and after experiencing impulse over-voltages, mechanisms for repetitive lightning and switching impulse ageing of XLPE can be distinguished. This research project involves sample preparation, impulse ageing of samples using laboratory equipment controlled by real time monitoring software, breakdown strength measurements and analysis using Weibull statistics, dielectric relaxation measurements in the frequency domain and space charge measurements using the pulsed-electroacoustic method. Obtained results are presented and their analysis highlights the mechanisms behind lightning impulse ageing.