# Investigation of Impulse Aging of Energy-Absorption Elements for Hybrid DC Circuit Breakers
## 3 Research on Impulse Aging Characteristics of MOVs
### Result and Discussion  
By analyzing the relationship curves between the change rate of static parameters, leakage current value \(I_{\rm L}\), residual voltage ratio \(K\), nonlinear coefficient \(\alpha\), and the number of impulses n, the impact of impulse current on the aging degree of the MOV valve sheets is explored. The value of \(\alpha\) is usually calculated from the measured voltages \(U_{1}\) and \(U_{2}\) at two determined currents \(I_{1}\) and \(I_{2}\) using Equation (5). \[\alpha=\frac{\lg I_{1}-\lg I_{2}}{\lg U_{1}-\lg U_{2}}=\frac{\lg(I_{1}/I_{2} )}{\lg(U_{1}/U_{2})} \tag{5}\]  
When \(I_{1}\) and \(I_{2}\) are set to 1.0 mA and 0.1 mA at room temperature, Equation (5) can be reduced to Equation (6):  
\[\alpha=1/[\lg(U_{1 mA}/U_{0.1 mA})] \tag{6}\]  
The residual voltage ratio \(K\) is calculated by Equation (7):  
\[K=U_{res}/U_{1 mA} \tag{7}\]  
where \(U_{res}\) is the residual voltage. Three tests were carried out as follows:  
Test 1: The amplitude of the impulse current is 300 A, one impulse per round (300 A-1 group, impulse current amplitude, the number of impulses per round). Three MOV samples with similar initial parameters were selected. Multiple rounds of pulsed current with an amplitude of 300 A were applied to the MOV samples, one impulse per round. The DC static parameters and leakage current \(I_{\rm L}\) of the MOV in the low-current region were measured and recorded after each round of impulse. The change rate of DC static parameters, residual voltage ratio \(K\), and nonlinearity coefficient \(\alpha\) were calculated. It can be seen that the statistics of the three MOV samples showed similar trends in each parameter; thus, the statistics of one of them were selected to plot the relationship curves between the change rate of DC static parameters, leakage current \(I_{\rm L}\), residual voltage ratio \(K\), nonlinear coefficient \(\alpha\), and the number of impulses n, as shown in Figure 8a-d. It can be seen that test samples were damaged after 44 impulses; therefore, the statistics during the first 44 impulses were recorded for analysis. From Figure 8a, as the number of impulses increases, \(U_{\rm 1 mA}\) shows a trend of small increase, \(U_{\rm 0\mbox{-}\rm 1 mA}\) has a trend of small increase followed by a decrease, and \(U_{\rm 0.01 mA}\) shows a trend of decrease. The initial values of each parameter are as follows: \(U_{\rm 1 mA}=6.103\) kV, \(U_{\rm 0\mbox{-}\rm 1 mA}=5.740\) kV, \(U_{\rm 0\mbox{-}\rm 0\mbox{-}\rm 1 mA}=5.230\) kV, \(I_{\rm L}=1.67\)\(\mu\)A, \(K=1.34\), and \(\alpha=37.5\). After the 44 th impulse, it can be found that \(U_{\rm 1 mA}\) increased by 2.95%, and \(U_{\rm 0\mbox{-}\rm 1 mA}\) fluctuated by 0.52%, and \(U_{\rm 0\mbox{-}\rm 0\mbox{-}\rm 1 mA}\) decreased by 4.94% from the initial value. Figure 8b shows that before the damage occurs, the leakage current \(I_{\rm L}\) exhibits an increasing trend, reaching a value of 6.90 \(\upmu\)A. The change rate of voltage \(U_{\rm 1 mA}\) remains within \(\pm\)10% and the leakage current value \(I_{\rm L}\) does not exceed 20 \(\upmu\)A prior to the damage of the MOV valve sheet; this indicates that \(U_{\rm 1 mA}\) and \(I_{\rm L}\) have a delay in judging the aging degree of the MOV, so that the aging cannot be determined in the initial period. After the MOV is damaged, the voltage value in the small current area rapidly decreases to nearly 0 A, and the MOV is no longer insulated. The leakage current \(I_{\rm L}\) rapidly increases to 249.8 \(\upmu\)A. Figure 8c indicates that the residual voltage ratio \(K\) shows a trend of decreasing before damage occurs. Since the residual pressure ratio \(K\) is jointly determined by \(U_{\rm res}\) and \(U_{\rm 1 mA}\), the two parameters need to be studied simultaneously. Firstly, the change in residual voltage is analyzed. The initial value of \(U_{\rm res}\) of the test sample is 8.16 kV. The \(U_{\rm res}\) changed 40 V after the 44 th impulse with the change rate of 0.49%, which can be regarded as basically unchanged.