## 3 Field Overvoltage Waveform Acquisition and Statistics  
### Field Overvoltage Waveform Acquisition  
In order to make the calculation results of the simulation model more accurate, the overvoltage waveforms used in the model were the typical overvoltage waveforms suffered by the EMU in the actual operation. In order to acquire the typical overvoltage waveforms, an online monitoring system of roof overvoltage was established. The system consists of a pantograph, arrester, capacitive voltage divider, data acquisition card and computer. The model of the arrester is YH10 WT-42/105. The low-voltage side signal of the capacitive voltage divider was introduced into the HS4 data acquisition card inside the EMU through coaxial cable, and its sampling rate was set to 100 kHz. The HS4 data acquisition card has four sampling channels, which can realize high-speed continuous synchronous acquisition. In this paper, the flow disk mode is used to achieve continuous signal acquisition and avoid data leakage, and real-time signal acquisition is stored in the computer.  
The cumulative test distance was about 100,000 km, and 1274 groups of overvoltage waveforms were extracted from the original data. The overvoltage was classified into passing section overvoltage, high-harmonic overvoltage and steep impulse overvoltage according to their characteristics. The three typical overvoltage waveforms are shown in Figure 8.  
The passing section overvoltage is mainly caused by the action of the circuit breaker when the EMU passes the neutral section; its waveform is shown in Figure 8a. From Figure 8a, it can be seen that the voltage waveform of the arrester changed from a sinusoidal signal to a non-sinusoidal periodic signal and superimposed periodic impulse voltage. The maximum amplitude of impulse voltage is 94.5 kV, and the duration of a single impulse is about 0.2 ms.  
Due to the existence of power electronic equipment such as the rectifier and inverter in the traction power supply system of the EMU, the voltage at both ends of the arrester contains high-harmonic components. When the harmonic content is high, the voltage peak may increase, as shown in Figure 8b. The voltage waveform was highly distorted, reaching a peak of 52.8 kV compared to 38.9 kV under normal conditions.  
Steep impulse overvoltage has a higher amplitude and may occur continuously, which is likely to cause the arrester to heat up. The maximum amplitude of the steep impulse overvoltage detected during the EMU operation is \(-138.9\) kV, as shown in Figure 8c. The duration of impulse voltage is about 0.14 ms. During the overvoltage occurrence period, there is no abnormal harmonic voltage or process of passing the neutral section.  
In Figure 8, the passing section overvoltage and steep impulse overvoltage have higher amplitudes, reaching 94.5 kV and 138.9 kV, respectively, while the lightning shock residual voltage of the arrester under the nominal discharge current (10 kA) should be  
Figure 8: Three typical overvoltage waveforms. (**a**) Passing section overvoltage. (**b**) High-harmonic overvoltage. (**c**) Steep impulse overvoltage.  
less than 105 kV. If the arrester is attacked by overvoltage with such a high amplitude in a short period of time, the temperature of the valve plate will rise sharply, which may lead to thermal aging or the damage of the arrester. Therefore, it is necessary to study the temperature variation characteristics of the arrester under the action of overvoltage.