**2.1 Lifetime and partial discharge characteristics of polyimide under repetitive impulse voltages**

**Figure 1** shows the system for testing lifetime and partial discharge characteristics of polyimide under repetitive impulse voltages. Bipolar continuous square impulse voltage was conducted. The amplitude, risetime, duty cycle, and frequency of applied voltage were 1 kV, 40 ns, 50%, and 10 kHz, respectively. The electrodes used in the test were designed according to the standard of ASTM 2275 01, including a rod electrode, a plate electrode, and an insulating board [5]. By this configuration, we can obtain the voltage endurance of solid electrical insulating materials subjected to surface partial discharges. The PD signal was measured by an UHF antenna.

**Figure 2** shows the lifetime of PI under repetitive impulse voltages, which decreases sharply with increasing temperature. The lifetime values are much shorter than that under AC voltages [6].

**Figure 1.** *Schematic diagram of the PD aging test system.*

**Figure 2.** *Lifetime of PI under repetitive impulse voltages [6].*

**Figure 3.** *PD characteristics of PI films with increasing temperature. (a) PDIV, (b) PD amplitude, (c) PD number.*

## *Polyimide for Electronic and Electrical Engineering Applications*

**Figure 4.** *Sample discharge area and eroded area of sample [6].*

**Figure 5.** *Microtopography of PI films after 3-h aging and breakdown. (a) 3-h aging and (b) breakdown.*

**Figure 3** shows the PD characteristics, including the PDIV, PD amplitude, and PD number, of PI films under repetitive impulse voltages as a function of temperature. PDIV decreases with increasing temperature; on the contrary, PD amplitude and PD number increase with increasing temperature. All these parameters are higher than that under AC voltages [6].
