# Investigation of Impulse Aging of Energy-Absorption Elements for Hybrid DC Circuit Breakers
## 2 Simulation Analysis of the Working Characteristics of MOVs during Short Circuit  
In order to study the impulse of short-circuit current on the MOV of the energy-absorption branch during the short-circuit fault breaking process, a dynamic simulation model was designed to investigate the operation process of hybrid DCCBs. MATLAB/Simulink was used to build this model [19]. The schematic diagram of the dynamic simulation model for the hybrid DCCB is shown in Figure 1, the construction of which is based on the topology of a diode full-bridge circuit [1, 13]. The simulation model mainly consists of three branches connected in parallel, namely, a main branch with a fast mechanical switch (FMS) and an auxiliary sub-module (ASM), a transfer branch mainly composed of a sub-module (SM), and an energy-absorption branch mainly consisting of a connected metal oxide varistor (MOV). Both the SM and ASM consist of an insulated-gate bipolar transistor (IGBT), an RC buffer circuit, and diode full-bridge circuit. Additionally, a DC voltage source, E, is used in the model to simulate the DC system voltage. An inductor, L, and a resistance, R\({}_{\rm L}\), are used in the model to represent all inductive components and all loads of the DC system, respectively. When closing S at \(t_{0}\), the short-circuit fault in the simulation model starts. The circuit parameters of the simulation model are shown in Table 1.  
The MOV in the simulation model is a non-linear resistor, and its nonlinear volt-ampere characteristics can be expressed as follows:  
\[\frac{U}{U_{ref}}=k_{i}\left(\frac{I}{I_{ref}}\right)^{1/\alpha_{i}} \tag{1}\]  
where \(U_{ref}\) is the reference voltage of the MOV, \(I_{ref}\) is the reference voltage of the MOV, \(k_{i}\) and \(\alpha_{i}\) are the coefficient of the \(i\)th section of volt-ampere characteristics curve. \(i=1\), \(2\), \(3\) represent a small current region, nonlinear region, and saturation region, respectively. The parameters of the MOV set in the model are shown in Table 2.  
Based on the design principle of per-unit value equality [19; 20], the dynamic simulation model with rated value 5 kV/20 A is used to simulate a practical 500 kV/2 kA DC transmission system. The ratio of voltage and current is set to 1/100. The system parameters of the simulation model and the corresponding real system are shown in Table 3. The short-circuit fault happened to the simulation system at t = 0.02 s, and the threshold of the action current is set to 30 A. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Value** & **Parameter** & **Value** \\ \hline Load Resistance (\(R_{L}\)) & 250 \(\Omega\) & Buffer capacitor of SM/ASM (\(C_{s}\)) & 15 \(\mu\)F \\ DC system inductor (\(L\)) & 100 mH & Buffer resistor of SM/ASM (\(R_{s}\)) & 200 \(\Omega\) \\ Start time of short circuit fault (\(t_{0}\)) & 0.02 s & Forward voltage of IGBT (\(V\)) & 2.7 V \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The circuit parameters of the simulation model. Figure 1: Schematic diagram of the dynamic simulation model for the hybrid DCCB. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \(U_{ref}\) (V) & \(I_{ref}\) (A) & (\(k_{1}\) and \(\alpha_{1}\)) & (\(k_{2}\) and \(\alpha_{2}\)) & (\(k_{3}\) and \(\alpha_{3}\)) \\ \hline
8000 & 500 & (0.995, 50) & (1.0, 25) & (0.9915, 16.5) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The parameters of the MOV. The simulation waveform of the whole process of breaking the short-circuit fault current of the hybrid DCCB is shown in Figure 2.  
At t\({}_{1}\) = 20 ms, a ground short-circuit fault occurs on the load side of the DC system and the fault current rises rapidly. When the fault current reaches the pre-set action threshold of \(I_{\text{th}}\) = 30 A, the hybrid DCCB starts to operate, triggering the SM and shutting down the ASM, and the fault current is instantaneously switched from the main branch to the transfer branch.