# Investigation of Impulse Aging of Energy-Absorption Elements for Hybrid DC Circuit Breakers
## 4 Microstructure Testing and Analysis of MOV
The picture of the non-visibly damaged area of MOV valve sheets under SEM is displayed in Figure 16; it  
Figure 13: The pictures of the impulse damage area of the 300 A-10 group of MOV valve sheets under SEM: (**a**) magnified 200 times; (**b**) magnified 200 times; (**c**) magnified 2000 times. can be seen that there was a significant increase in internal pores compared to before the impulse, and a decrease in grain size and a higher integrity of grain. After ten rounds of impulse aging tests, there was no visible damage on the group of 175 A-10 test samples. Compared with the valve sheet that did not undergo impulse aging, a small piece of the sample was taken and observed under the microscope. The pictures of the impulse damage area of the 175 A-10 group of MOV valve sheets under SEM are displayed in Figure 17. It was found that there was no significant change in the number of internal pores compared to before the impulse, the grain size was slightly reduced with lower integrity, and the grains were damaged under impulse. Figure 16: The picture of the non-visibly damaged area of the 150 A-10 group of MOV valve sheets under SEM. Figure 14: The picture of the non-damaged area of the 300 A-10 group of MOV valve sheets under SEM. \[\overline{D}=1.56\frac{C}{MN} \tag{8}\]0 The pictures of the impulse damage area of the 150 A-10 group of MOV valve sheets under SEM: (**a**) magnified 50 times; (**b**) magnified 200 times; (**c**) magnified 2000 times. After ten rounds of impulse aging tests, the pictures of the impulse damage area of the 200 A-10 group of MOV valve sheets under SEM are displayed in \[\overline{D}=1.56\frac{C}{MN} \tag{8}\]1 When noticeable damage area was magnified 50 times, the clear breakdown pores and burn marks caused by heat release were revealed. It can be found that the impulse damage occurred between the MOV sintered body and the insulation layer, and the number of pores on the breakdown aperture was much larger than the number of pores on the sintered body of the MOV. When magnified 200 times, multiple pores were found to be connected to form larger ones. When magnified 2000 times, it was found that the ZnO grains lost their original shape due to melting after being heated. The picture of the non-visibly damaged area of MOV valve sheets under SEM is displayed in Figure 19; it was observed that the internal pores were significantly enlarged compared to before the impulse. It can also be found that the grain size was reduced, and the grain was damaged with low integrity after the impulse. \[\overline{D}=1.56\frac{C}{MN} \tag{8}\]2 The pictures of the non-visibly damaged area of the 200 A-10 group of MOV valve sheets under SEM. \[\overline{D}=1.56\frac{C}{MN} \tag{8}\]3 The pictures of the impulse damage area of the 175 A-10 group of MOV valve sheets under SEM: (**a**) magnified 50 times; (**b**) magnified 1000 times. \[\overline{D}=1.56\frac{C}{MN} \tag{8}\]4 The pictures of the impulse damage area of the 200 A-10 group of MOV valve sheets under SEM: (**a**) magnified 50 times; (**b**) magnified 200 times; (**c**) magnified 2000 times. Test 3: Impulse current amplitude of 200 A and the number of impulses per round is 10, 15 and 20. The state of the 200 A-10 test samples was analyzed in test 2. After fifteen rounds of impulse aging tests, the pictures of impulse damage area of the 200 A-15 group of MOV valve sheets under SEM are displayed in \[\overline{D}=1.56\frac{C}{MN} \tag{8}\]5 When test samples were magnified 50 times, clear breakdown pores and ablation marks caused by heat release on the entire surface were revealed. The areas magnified 200 times revealed pores distributed densely on the aperture, with some pores connected to form larger ones, and cracks running through the pores. Compared to valve sheets that did not experience impulse aging, the number of pores in the areas receiving an impulse increased significantly. Magnifying the areas to 2000 times showed that ZnO grains melted and expanded under thermal stress, resulting in a radial pattern after exploding.