**1. Introduction**

Polyimide (PI) is used as insulating material for resist surface discharge because of its excellent electrical, thermal, and mechanical characteristics [1, 2]. PI films are usually expected to withstand surface discharge caused by repetitive impulse voltages. For example, surface partial discharge occurs during lifetime of turn-to-turn insulation of inverter-fed traction motors due to the driven pulse width modulation (PWM) converters, which has become one of the most important factors for surface degradation and final premature failures of the insulation of inverter-fed traction motors. Therefore, the PD-dependent lifetime of PI films under repetitive impulse voltages needs to be improved to meet the requirement for rapid development of industry applications [3, 4].

To provide a comprehensive understanding to lifetime of PI under repetitive impulse voltages and to clarify the emerging problems, we review the recent progress in partial discharge characteristics, degradation, and charge storage effect under repetitive impulse voltages. Particular attention is paid on lifetime improvement approaches, such as using nanocomposites and surface modification.
