## 3 Experiment and Method  
The configuration of the experimental setup is shown in Figure 1. The applied voltage was supplied by a dry-type transformer with a maximum output of 50 kV and 30 mA, and was measured using a high-voltage divider with a ratio of 10,000:1. The transformer was controlled by an alternative current (AC) regulator. Two types of electrode systems were fabricated to simulate typical insulation defects in a gas-insulated structure, including a protrusion on conductor (POC) and a protrusion on enclosure (POE). A high voltage was applied to the upper electrode and the lower electrode was grounded through a 50 \(\Omega\) non-inductive resistor. The electrode systems were filled with SF\({}_{6}\) gas or g3 gas with 4% NOVECTM 4710/96% CO\({}_{2}\). Before each experiment, the electrode systems were vacuumed for 30 minutes by a vacuum pump with a pumping speed of 120 L/min, and were then filled with CO\({}_{2}\) with purity over 99.9% and vacuumed again. This procedure was repeated three times in order to prevent any pollution of gas [20]. PD pulses were measured using the detection resistor, and were  
\begin{table}
\begin{tabular}{c c c c} \hline
**Gas** & **Sulfur Hexafluoride** & **NOVECTM 4710** & **Carbon Dioxide** \\ \hline Chemical formula & SF\({}_{6}\) & (CF\({}_{3}\))\({}_{2}\)CFCN & CO\({}_{2}\) \\ Number of electrons & 77 & 48 & 16 \\ Relative dielectric strength to SF\({}_{6}\) & 1 & \textgreater{}2 & \textless{}0.4 \\ Liquefaction pressure at \(-30^{\circ}\)C (MPa) & 0.52 & 0.0311 & 1.43 \\ Freezing point (\({}^{\circ}\)C) & \(-51\) & \(-118\) & \(-78.5\) \\ Boiling point at 0.1 MPa (\({}^{\circ}\)C) & \(-63\) & \(-4.7\) & \(-79\) \\ Ozone depletion potential & 0 & 0 & 0 \\ Global warming potential (GWP) & 23,500 & 2100 & 1 \\ Atmospheric lifetime (year) & 3200 & 30 & 5–200 \\ Toxicity (LC 50 (rat)) & \textgreater{}500,000 & 10,000–15,000 & \textgreater{}300,000 \\ Flammability & No & No & No \\ Corrosion & No & No & No \\ \hline \end{tabular}
\end{table}
Table 1: Physical and chemical properties of gases.  
acquired using a digital oscilloscope (DSO, YOKOGAWA, 5 GS/s, Tokyo, Japan) and a peripheral component interconnect eXtensions for instrumentation (PXI, National Instrument, 250 MS/s, Austin, TX, USA). PD detection was carried out in a shielding box to reduce external interference [26].  
Figure 2 illustrates the electrode systems. The POC mainly consisted of a needle electrode with a curvature radius of 10 \(\upmu\)m and a plane electrode that was made of a tungsten-copper alloy with a diameter of 80 mm and a thickness of 20 mm. In the POE arrangement, the same needle electrode was attached to the plane electrode and a rod electrode with a curvature radius of 10 mm was used. The distance between electrodes was 5 mm and the permissible maximum gas pressure was 0.55 MPa. Before each test, the electrodes were conditioned with metal polishing paste and were cleaned carefully with isopropyl alcohol.  
In this paper, the background noise level was measured as 2.37 picocoulomb (pC). The PDIV was defined as the voltage at which discharges with a magnitude over 10 pC occurred clearly and repeatedly. The PDIV under the same condition was measured 10 times for calculating its mean value. Since discharge is a complicated process, PRPD patterns were acquired for 2 s for analyzing the average discharge magnitude, pulse count, and discharge phase.