**Abstract**

Polyimide (PI) is a commonly used insulating material to resist surface discharge, for instance, as turn-to-turn insulating material in inverter-fed motors driven by pulse width modulation (PWM) converters. Under the effect of repetitive impulse voltages, PI is expected to withstand surface partial discharge (PD) during service. However, lifetime under repetitive impulse voltages is much shorter than that under AC voltages due to storage effect of charges. Many approaches have been proposed to improve lifetime of polyimide under repetitive impulse voltages, such as using nanocomposites, surface modification, and structure design. In this chapter, we will discuss the lifetime of polyimide under repetitive impulse voltages and corresponding theoretical mechanism, together with modification approaches and their effects on lifetime improvement.

**Keywords:** lifetime, repetitive impulse voltages, surface discharge, nanocomposites, surface modification
