# Surface Discharge Mechanism on Epoxy Resin in Electronegative Gases and Its Application
## 2 Experimental Setup and Method  
A parallel needle-plane electrode system enclosed in an experimental gas-insulated switchgear (GIS) filled with different gases was used in this study. The specifications of the electrodes used in this experiment are as follows: The needle electrode (\(N\)) had a diameter of 5 mm and 20\({}^{\circ}\) angle of a point (\(\theta\)). The diameter of the plane electrode (\(P\)) was 59 mm. Both electrodes were made of stainless steel and were arranged vertical to each other. The solid dielectric used was made of epoxy resin and was in the form of a disc with a diameter (\(d\)) of 100 mm and a thickness (\(t\)) of 2 mm. The sample of epoxy resin was inserted between the electrodes. The distance between the epoxy dielectric and the needle electrode was zero, and the needle electrode was carefully contacted, without damaging the epoxy surface, since damage to the epoxy surface affects the surface flashover voltage. Figure 1a illustrates a schematic diagram of the experimental electrodes and a solid dielectric arrangement.  
The experimental gas-insulated switchgear (GIS) chamber is shown in Figure 1b. This chamber is made of stainless steel of a 20 mm thickness and consists of two layers: An interior part with a diameter of 260 mm, height of 460 mm, and volume of 25 L, and an exterior part with a diameter of 460 mm, height of 500 mm, and volume of 83 L. Two windows were used to observe the inside of the experimental chamber. These windows were made of acryl with a diameter of 110 mm and thickness of 20 mm. The chamber was designed and manufactured to endure a fixed temperature range (\(-\)90-100 \({}^{\circ}\)C) and pressure up to 1.0 MPa. It was also capable of withstanding an AC voltage of 300 kV. A pressure gauge was installed to measure the pressure inside the chamber. The chamber could be preserved at a pressure of \(6.67\times 10^{-2}\) Pa by using a vacuum pump (SINKU KIKO Co., Ltd. Miyazaki, Japan, GUD-050A).  
The insulation gases used were SF\({}_{6}\), dry air, and N\({}_{2}\). Dry air was produced by a dry air production device. The device had three types of filters that could reduce impurities and lower the dew point of dry air. The final dew point of dry air produced using this device was less than \(-60\)\({}^{\circ}\)C. These gases were pressurized from 0.1 to 0.6 MPa, inside the chamber of the device.  
The experiments were performed under AC voltage supplied by a high-voltage generator (DY-106-Korea, AC 300 kV/120 mA) with a voltage rising speed of 3.15 kV/s. As an AC power source is actually used as the operating power of GIS, gas-insulated transmission lines (GIL), and gas circuit breaker (GCB), it is very important to analyze the surface discharge characteristics and mechanisms for the device and equipment working under AC power. After ventilation of the chamber to \(6.67\times 10^{-2}\) Pa, each type of gas was inserted and pressurized from 0.1 to 0.6 MPa, inside the chamber. AC voltage was applied to the electrodes at each pressure level that the gases were subject to. Then, the surface flashover voltages were sequentially measured five times. The average value of this measurement is referred to as the mean surface flashover voltage (\(V_{B}\)) in this study. The surface flashover voltages were measured using a high-voltage probe.