**2.3 Storage effect of charges under repetitive impulse voltages**

Under repetitive impulse voltages, partial discharge-induced degradation would be intensified because of charge storage effect. We discuss the electrical field distribution on discharge region of the sample surface, as shown in **Figure 6**, where Ei is the electric field in the air gap between the sample and the edge of the upper electrode. When external electric field reverses, accumulated charges on the film surface will enhance the electric field in the air gap between the rod and the sample, as shown in **Figure 6**. Assuming the external electric field is negative, the charge-induced electric field in the air gap Eq is proportional to the difference between the charges on the rod electrode Qup and that on the film surface Qdown, namely, Eq (Qup − Qdown). Thus, the effective electric field in the air gap can be obtained, Ei = Et + Eq, where Et is the external electric field. Since the charges on the film surface are unable to dissipate in such short rising time, the Qdown can be considered as a constant. When the external electric field reverses and becomes

### **Figure 6.**

*The electric field change in air gap when external electric field inverses from negative to positive side [9].*

positive, the Eq is proportional to the sum of the charges on the rod electrode Qup and that on the film surface Qdown, namely, Eq (Qup + Qdown). As the charge continues accumulating, the value of Ei can increase to a critical value, and then PD would take place.
