**2.2 Partial discharge-induced degradation under repetitive impulse voltages**

Due to the electrode structure as shown in **Figure 2**, surface discharge would occur in the air gap between the sample and the upper rod electrode during lifetime testing. Certain region on the film surface, as shown in **Figure 4**, would be corroded by surface discharge generating high-energy electrons, ultraviolet rays, and highactivity chemical groups [7, 8]. The microtopography of PI films after 3-h aging and breakdown (lifetime: 6 h and 10 min) under repetitive impulse voltages is shown in **Figure 5**. An obvious circle can be seen on the surface of aged PI film, which is eroded as the consequence of surface discharge, resulting in surface roughness and morphology changing. From the microscopic view of the breakdown point as shown in **Figure 5(b)**, the mesh structure and voids can be observed on the surface microtopography around the breakdown point obviously, indicating degradation of PI occurred during discharge.
