Studies concerning the fast ageing by thermal cycling of power cables

In the present paper the results concerning the structural modifications and dielectric properties of polyethylene insulations taken from a medium voltage cable (20 kV) subjected to 120 thermal cycles between -35 °C and + 100 °C are presented. The structural modifications were investigated by thermal analysis methods (TG, DTG and DTA), showing that during the thermal cycling the degree of cross-linking of polyethylene increases and the degree of crystallinity decreases. Dielectric modifications of polyethylene due to thermal cycling were determined by using the dielectric spectroscopy method in range 10 ^-2 ÷ 10⁶ Hz, for 30 °C but also for 100 °C. After 120 thermal cycles the results obtained show that tgδ for 30 °C increases slowly for frequencies higher than 100 Hz and substantial for frequencies lower than 100 Hz. This increase is more pronounced for 100 °C (up to 100 %) especially for frequencies between 10^-2 ÷ 2·10³ Hz.