# Investigation of Impulse Aging of Energy-Absorption Elements for Hybrid DC Circuit Breakers
## 4 Microstructure Testing and Analysis of MOV  
The main focus of the previous content on the aging characteristics of MOV is the changes in the macroscopic electrical properties of MOV before and after impulse aging. Since the nonlinear characteristics of MOV originate from its microstructure, the impulse aging characteristics from the internal microstructure of MOV valve sheets are studied in this section using the VE-9800S scanning electron microscope of the State key laboratory of electrical insulation and power equipment of Xi'an Jiaotong University. The MOV valve sheet is made by sintering different metal oxides mixed by the solid-state ball milling method, and its main component is zinc oxide (ZnO) [23]. The basic structure unit of the MOV valve sheet is the ZnO grain, which also includes the grain boundary wrapped around the ZnO grain, the spinel dispersed within the grain boundary, and the pores distributed within the ZnO grain and the grain boundary [24]. Figure 11: The pictures of typical failure modes of MOV: (**a**) no damage; (**b**) insulation breakdown; (**c**) body breakdown. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Test Groups** & **Sample 1** & **Sample 2** & **Sample 3** \\ \hline
300 A-1 & Insulation breakdown & Body breakdown & Insulation breakdown \\
150 A-10 & Body breakdown & Insulation breakdown & Insulation breakdown \\
175 A-10 & no damage & Insulation breakdown & Body breakdown \\
200 A-10 & Body breakdown & Insulation breakdown & Insulation breakdown \\
200 A-15 & Body breakdown & Insulation breakdown & Body breakdown \\
200 A-20 & Insulation breakdown & no damage & Insulation breakdown \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The states of the sample under different test conditions. The specific experimental steps for the morphological scanning of the MOV valve sheet using a scanning electron microscope are as follows:  
1. Take a small piece of relatively flat cross-section from the MOV sample without impulse aging, and smash a small piece of the section from the damaged surface of an MOV sample that has undergone impulse aging tests;
2. Smooth the other surfaces of the sample to facilitate smooth placement in the instrument for observation;
3. Observe the microstructure morphology of the sample at appropriate magnification. The average grain size of the MOV sample can be obtained by the line intercept method, so that the impulse aging characteristics of the MOV valve sheet can be analyzed from the microscopic perspective according to the variation in grain size. The average size of grain can be calculated using Equation (8):  
\[\overline{D}=1.56\frac{C}{MN} \tag{8}\]  
where \(D\) is the average size of the grains, \(C\) is the total length of the line segments for testing, \(M\) is the magnification of the image, and \(N\) is the number of lines for the test. The microstructure of the selected MOV sample without impulse aging is shown in Figure 12.  
As shown in Figure 12a, the surface morphology of the MOV valve sheet can be observed from the overall perspective. After magnifying it 50 times, it can be found that the cross-section is relatively flat, where the black dots represent the pores, and it can be seen that the distribution of pores in the new MOV valve sheet is more uniform and the pore size is small. The gray area in Figure 12a represents the sintered MOV valve sheet. The ZnO grain structure can be clearly observed after further magnifying its image 1000 times. As shown in Figure 12b, the black porous parts and the irregular polygons represent the pores and the individual ZnO grains, respectively, and the distributions of both are relatively uniform. The grains are intact and full, except for some fractures caused by stress during sampling. The bright line between two ZnO grains is the grain boundary, which is clearly visible. According to Equation (7), the grain size of the sample is calculated to be 14.69 \(\upmu\)m.  
The pictures of the microstructure of the samples under the three impulse aging tests mentioned above are as follows.