# Investigation of Impulse Aging of Energy-Absorption Elements for Hybrid DC Circuit Breakers
## 4 Microstructure Testing and Analysis of MOV
Test 1: The amplitude of the impulse current is 300 A, one impulse per round. The pictures of the impulse damage area of the 300 A-10 group of MOV valve sheets under SEM are displayed in Figure 13.  
Figure 13a shows the image of the microstructure of the impulse break cross-section with a clear damaged area after magnifying it 50 times. Figure 13b shows the microstructure of the yellow area in Figure 13a at 200 times magnification. Figure 13c shows the microstructure of the yellow area in Figure 13b at 2000 times magnific  
Figure 12: Microstructure picture of MOV valve sheet without impulse aging under SEM: (a) magnified 50 times; (b) magnified 1000 times. the breakdown pores caused by impulse damage can be clearly seen in the blue line box, and the ablation marks caused by heat release can be seen on the whole section. Comparing Figure 13a with Figure 13b, it can be found that the pores were densely distributed on the impulse aperture, and some of the pores on the aperture were connected to form a larger pore. Comparing Figure 12a with Figure 13a, it can be found that the number of pores was greatly increased after impulse. From Figure 13c, it can be seen that the crack penetrated the pores, which may be one of the reasons for the cracking of the sample after breakdown. It can be found that the longer the length of the crack, the more pores it will penetrate, and the easier it can crack. It can also be observed from Figure 13 that the denser the pores on the breakdown aperture, the easier it can crack along the aperture. The cause of the above phenomenon is that under the impulse of high current, the current easily flows through the area with a thin grain boundary of the MOV valve sheet, such as the breakdown aperture that has low resistance, resulting in uneven temperature distribution in different regions. The grain boundary melts when the temperature reaches a certain value, leading to the perforation and damage of the MOV valve sheet. The uneven temperature also causes thermal expansion stress, which generates pores and cracks during stress release, and the MOV valve sheet breaks if its tensile stress exceeds its resistance. As shown in Figure 13c, ZnO grains melted and expanded under heat stress, resulting in a radial pattern after exploding, with white noise representing electrostatic accumulation that cannot be completely eliminated. Figure 14 shows the microstructure of the damaged surface of the test sample at a magnification of 1000 times, and no visibly damaged area was observed. Regarding the non-impulse damage area, the grain size of the sample was calculated to be 8.79 \(\upmu\)m according to Equations (3)-(5). Compared with Figure 12b, the grain size of the impulse area was significantly reduced, and the integrity of the grains was low, with most of them being damaged, which was caused by heat stress. As shown in Figure 14, the internal area of the selected section contains elastic substances, which may be caused during sampling, by the products of ion migration during impulse aging, or by debris from grain damage, and further analysis of its composition is needed. Test 2: The amplitude of the impulse current is 150 A, 175 A, and 200 A respectively, ten impulses per round. After ten rounds of impulse aging tests, the picture of the impulse damage area of the 150 A-10 group of MOV valve sheets under SEM is displayed in Figure 15. When the damaged area was magnified 50 times, the visible breakdown holes and ablation marks caused by heat release were revealed, with cracks running through the entire aperture. When magnified 200 times, the same phenomenon as that of the 300 A-1 group damage area magnified 200 times was observed. When magnified 2000 times, it was found that ZnO grains lost their original shape after being heated and melted.