# Investigation of Impulse Aging of Energy-Absorption Elements for Hybrid DC Circuit Breakers
## 3 Research on Impulse Aging Characteristics of MOVs
### Result and Discussion
The V-I characteristics in the large current region are determined by the grain characteristics, and \(U_{\rm res}\) is basically unchanged during the aging process, so that the grain resistance value is also basically unchanged. This indicates that the change in the residual voltage ratio \(K\) is mainly caused by the change in the grain boundary barrier. According to the double Schottky barrier theory [21], it is generally believed that the initial aging of the MOV is caused by the double Schottky barrier distortion in the grain boundary layer. At the beginning of the impulse, the trap in the grain boundary captures the injected electrons (electron trapping process), so that they cannot go to the anti-bias side, which leads to an increase in the anti-bias-side Schottky barrier holes and reduces the height of the Fermi energy level. This causes an increase in the height of the anti-bias-side grain boundary barrier, which leads to the increase in the energy required by the electron leap barrier. From the macro-perspective, there is a small increase in \(U_{\rm 1 mA}\), while \(U_{\rm res}\) remained basically unchanged, resulting in a decrease in the residual voltage ratio \(K\). As shown in Figure 8d, the nonlinear coefficient \(\alpha\) shows a trend of linearly decreasing with the increase in the number of impulses, and decreased from 37.5 to 27.0. Generally, the nonlinear coefficient \(\alpha\) of the MOV ranges from 30 to 100 [22]. At about the 23 rd current  
Figure 8: The relationship between various parameters and the number of impulses n: (**a**) impulses n and various voltage change rates; (**b**) the number of impulses n and leakage current \(I_{\rm L}\); (**c**) the number of impulses n and residual voltage ratio \(K_{i}\) (**d**) the number of impulses n and nonlinear coefficient \(\alpha\). impulse, the value of \(\alpha\) started to be less than 30. This suggested that the MOV samples entered the early stage of aging. This indicates that the nonlinear coefficient \(\alpha\) can more accurately characterize the deterioration and aging degree of the MOV compared with other parameters. Test 2: The amplitude of the impulse current is 150 A, 175 A, and 200 A, ten impulses per round. Nine MOV samples with similar initial parameters were selected and divided into three groups (group 150 A-10, group 175 A-10, and group 200 A-10), with three samples in each group. Multiple rounds of impulse current with an amplitude of 150 A, 175 A, and 200 A generated by an impulse current generator were applied to each MOV sample in the three groups, with ten impulses per round. The DC static parameters of the MOV were measured after each round of impulses, and the main reference index values were recorded. The voltage change rate in the small current region, the residual voltage ratio \(K\), and the nonlinear coefficient \(\alpha\) were calculated. Since the recorded data trends for each parameter of the three groups of samples were similar, one sample was selected from each group to plot the relationship curves between the change rate of voltage in the small current region, leakage current \(I_{\mathrm{L}}\), residual voltage ratio \(K\), nonlinear coefficient \(\alpha\), and the number of impulses n under different impulse current amplitudes, as shown in Figures 3-8 and 9a-d. Regarding the 150 A-10 group of samples, Figures 3-8 and 9a show that test samples were damaged after 430 impulses, and the trends in \(U_{\mathrm{1 mA}}\), \(U_{\mathrm{0\cdot 1 mA}}\), and \(U_{\mathrm{0.01 mA}}\) were the same as those in the 300 A-1 group. After the 430 th impulse, \(U_{\mathrm{1 mA}}\) increased by 3.83% compared to its initial value, and \(U_{\mathrm{0.01 mA}}\) decreased by 7.56% compared to its initial value.