Cumulative degradation of electrical properties in polypropylene insulation for AC cables under repeated impulse voltage stress

In modern power transmission systems, AC cables are increasingly integrated with overhead lines, forming hybrid networks. These cables are frequently exposed to repeated impulse voltages from the overhead lines. While surge arresters offer partial protection, the long-term effects of these impulses on polypropylene (PP) insulation remain unclear. This study systematically investigates the cumulative degradation of the electrical breakdown properties of PP insulation under repeated impulse voltage stress. The 50% impulse breakdown voltage (U₅₀) was first determined, and a series of impulse tests were conducted at varying voltage levels to assess the number of impulses required for electrical breakdown, leading to the construction of an amplitude of impulse voltage (U) and the number of times required for breakdown (N), which is U–N curve. To evaluate the cumulative degradation, impulse voltage at 0.8 U₅₀ was applied for 50, 100, and 200 cycles, with the electrical conductivity current measured before and after each series of impulses. The results indicate significant degradation in the insulating properties of PP under repeated impulse stress. Mechanisms of cumulative degradation under impulse stress were further explored using isothermal relaxation current and space charge measurements. These findings provide critical insights into the performance of PP in hybrid transmission systems and offer valuable data to inform improved insulation design and protection strategies.