## 3 Simulation Results and Experimental Validation
### Experiment Setup  
The experiments are conducted according to IEC60099-4 [32]. The experiments are the current distribution experiment and temperature distribution experiment. The current distribution experiment measures the current's distribution inside the \(\pm 66\) kV DC GISA at 8/20 us lightning impulse current. The current distribution experiment aims to obtain the arrester prototype's current distribution and the current amplitude's influence on the uneven coefficient of current distribution. The temperature distribution experiment measures the arrester's temperature distribution in a power frequency voltage of 57 kV (RMS). The temperature distribution experiment can obtain the temperature difference of different column ZnO varistors and spacers inside the arrester prototype and then analyze the energy absorption difference. IEC60099-4 defines the uneven coefficient \(\beta\) of the current distribution of lightning arresters [32]:  
\[\beta=n\times I_{\max}/I_{arr} \tag{16}\]  
where \(I_{arr}\) is the peak value of the total arrester current; \(I_{max}\) is the maximum peak current through any column of ZnO varistors or arrester elements; and \(n\) is the number of parallel columns. The current distribution experimental program includes the following:  
1. The current distribution factor \(\beta\) of the three-column parallel core is measured under the single-column 1000-3000 A amplitude lightning impulse current, and it should be ensured that \(\beta\) is not greater than 1.10;
2. The effect of current magnitude on the homogenization characteristics is determined. The current distribution non-uniformity coefficients under 8/20 us lightning strikes in the range of 1000-3000 A currents in a single-column core are measured separately. The experimental equipment includes an 8/20 us lightning surge current generator, a \(\pm 66\) kV DC GISA prototype, a current measuring device, and an oscilloscope. An electrical equipment's insulation systems are more susceptible to failure under negative polarity voltages. The reference voltage of the surge arrester is of negative polarity. This is significant as lightning overvoltage in power systems is mostly negative polarity, and \begin{table}
\begin{tabular}{c c} \hline \hline Mass density \(\rho\) (kg/m\({}^{3}\)) & \\ \hline Zinc oxide & \(5672.5-1.4\times 10^{-3}T-1.4\times 10^{-4}T^{2}-7.6\times 10^{-8}T^{3}\) \\ Aluminum & \(2736.9-2.8\times 10^{-2}T-1.0\times 10^{-3}T^{2}-71.7\times 10^{-5}T^{3}\) \\ Epoxy Resin & 1673.0 \\ \hline Heat capacity Cp (J/(kg-K)) & \\ \hline Zinc oxide & \(41.6+3T-6.8\times 10^{-3}T^{2}+7.3\times 10^{-6}T^{3}\) \\ Aluminum & \(596.7+1.5T-2.1\times 10^{-3}T^{2}+1.3\times 10^{-6}T^{3}\) \\ Epoxy Resin & 550.0 \\ \hline Thermal conductivity k (W/(m-K)) & \\ \hline Zinc oxide & \(255.5-1.7T+6.5\times 10^{-3}\,T^{2}-1.4\times 10^{-5}T^{3}\) \\ Aluminum & \(39.6+1.7T-5.4\times 10^{-3}T^{2}+8.4\times 10^{-6}T^{3}\) \\ Epoxy Resin & \(-0.03+0.002\times T\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Characteristics of the materials. applying negative polarity voltage can help detect moisture defects more easily. The circuit diagram of the current distribution experiment can be simplified in Figure 7. The current waveform is related to R, L, and C, and the output current amplitude is adjusted by changing the capacitor charge voltage [33]. The impulse current generator is connected to the \(\pm\)66 kV DC GISA via a connecting cable. The prototype has three columns connected in parallel. In order to easily measure the three-column current, the prototype does not have an internal earth wire. Three cables are connected to the bottom of the three columns separately and make a connection to the three-column current lead terminal of the tank. In the current distribution experiment of the prototype, the three-column current lead terminal of the tank is grounded separately. Three Hall current sensors are used to detect the current of the three columns, and the current data and waveforms are obtained through an oscilloscope.