# Investigation of Impulse Aging of Energy-Absorption Elements for Hybrid DC Circuit Breakers
## 3 Research on Impulse Aging Characteristics of MOVs
### Result and Discussion
As shown in Figures 3-9 and 10b, the leakage current \(I_{\rm L}\) showed a rising trend and its value increased to 18.67 \(\mu\)A before the test samples were damaged. It can be seen from Figures 3-9 and 10a that test samples of the group of 200-15 were damaged after 405 impulses. During the 405 impulses before the samples were damaged, \(U_{\rm ImA}\) first slightly increased and then showed a decreasing trend, with its value increasing by a maximum of 4.61% compared to its initial value. \(U_{\rm 0.1 mA}\) had a decreasing trend, with its value decreasing by 2.34% compared to its initial value. \(U_{\rm 0.01 mA}\) changed with a decreasing trend, with its value decreasing by 9.92% compared to its initial value. As shown in Figures 3-9 and 10b, the leakage current \(I_{\rm L}\) showed a rising trend and its value increased to 15.93 \(\mu\)A before the test samples were damaged. It can be seen from Figures 3-9 and 10a that test samples of the group of 200 A-20 were damaged after 460 impulses. During the 405 impulses before the samples were damaged, \(U_{\rm ImA}\) first slightly increased and then showed a decreasing trend, with its value increasing by a maximum of 5.60% compared to its initial value. \(U_{\rm 0.01 mA}\) had a decreasing trend, with its value decreasing by 2.99% compared to its initial value. \(U_{\rm 0.01 mA}\) changed with a decreasing trend, with its value decreasing by 14.37% compared to its initial value. As shown in Figures 3-9 and 10b, the leakage current \(I_{\rm L}\) showed a rising trend and its value increased to 20.80 \(\mu\)A before the test samples were damaged. For the 200 A-10 and 200 A-15 groups, the change rate of \(U_{\rm 1 mA}\) did not exceed \(\pm\)10%, and the \(I_{\rm L}\) did not exceed 20 \(\mu\)A. After the damage of the test sample, the voltage in the low-current region rapidly decreased to nearly 0 V, indicating that the MOV valve no longer had insulation performance. The leakage current \(I_{\rm L}\) increased rapidly to 249.8 \(\mu\)A. The \(I_{\rm L}\) of the 200 A-20 group exceeded 20 \(\mu\)A in the first 460 impulses, and can be judged as aging. From Figures 3-9 and 10c, it can be seen that the residual voltage ratio \(K\) of the three groups of samples showed a trend of firstly decreasing and then increasing as the number of impulses increased. From Figures 3-9 and 10d, it can be seen that the nonlinear coefficient \(\alpha\) of the three groups of samples exhibited a rapid decrease followed by a steady decline with the increase in the number of impulses. During the whole impulse process, for the 200 A-10 group samples, the nonlinear coefficient \(\alpha\) decreased rapidly from 42.9 to 29.8 in the first 30 impulses, and then decreased gradually to 18.0 from the 30 th to 620 th impulses. For the 200 A-15 group samples, the nonlinear coefficient \(\alpha\) decreased rapidly from 37.7 to 27.3 in the first 30 impulses, and then decreased gradually to 18.6 from the 30 th to 405 th impulses. For the 200 A-20 group samples, the nonlinear coefficient \(\alpha\) decreased rapidly from 41.6 to 28.3 in the first 40 impulses, and then decreased gradually to 17.7. Therefore, the 200 A-10 group, 200 A-15 group, and 200 A-20 group can all be considered to begin aging after the 30 th impulse. By comparing the changes in the nonlinear coefficient \(\alpha\) under the three different impulse conditions, it can be observed that as the number of impulse increases, the required number of impulses for \(\alpha\) to be less than 30 did not change significantly. This is because impulse aging is mainly thermal aging, and if the amplitude of the impulse current did not change (the same amount of energy is injected into each impulse), as the number of impulse increases, the temperature of the MOV valve plate increased to a certain extent and no longer increased, which meant that the thermal aging degree of the MOV valve plate for each impulse cycle was not significantly different, and was more affected by the quality of the MOV valve plate itself.