# Application of Surge Arrester in Limiting Voltage Stress at Direct Current Breaker
## 2 The Proposed Mechanism
As SW1 is still closed, the fault current flows through SW1 and D1 forming and RL circuit. At the end of this stage, SW1 starts to separate its contacts and the Stage 4 operation starts. The upstream current flows in the surge arrester in this stage (See Figure 3b). Stage 4: In this stage, switch SW1 opens, and parallel capacitor C1 starts to re-route the fault current. As the contacts separate, the voltage across capacitor C1 increases. The current flows through C1 and D1, forming an RLC circuit. The current continues to flow and decay until the voltage across capacitor C1 exceeds the surge-arrester voltage, turning diode D2 on. If \(I_{downstream}\) significantly drops in Stage 3, the C1 voltage may not reach the surge-arrester voltage and a current zero-crossing may occur here, meaning Stage 5 is skipped (See Figure 3c). Stage 5: Here, the fault current (\(I_{downstream}\)) flows through diodes D2 and D1 back to the surge arrester, returning part of the fault energy to the source. The reverse DC voltage applied across the downstream inductance helps with its faster current decay. Theoretically, we have a zero-crossing of the fault current at the end of this stage and thus a low-cost AC breaker (Figure 1) can isolate the faulty line at this point (see Figure 3d). Stage 6: A low-cost conductor can be used to isolate the breaker from source (\(I_{downstream}=0\)) at the end of Stage 5. Otherwise, the Stage 6 operation takes place. Here, the direction of the current reverses after crossing zero, and D1 and D2 turn off. Because of the stored energy in the capacitors, a small oscillating current flows through C1, the RL load/fault, C2, and the surge arrester, partially discharging the capacitors. Stage 7: Due to the termination of the upstream fault current, low capacitor voltages, and downstream fault current at this stage, the surge arrester switches back to the blocking  
Figure 3: Current path during **(a)** Stage 2, **(b)** Stage 3, **(c)** Stage 4, **(d)** Stage 5.  
mode, transferring the oscillating current to the DC source and causing the fault current to disappear. Current breaking takes place in Stages 2 through 5 (shown next). Due to the capacitor charging in these stages (Stages 2 and 4), the voltages across switches can reach very high values. In order to overcome the voltage overshoots, the redirecting diodes are utilized to limit the voltage rise to that of the breaker terminal voltage, which is the surge arrester (SPD)'s voltage. In the absence of the surge arrester, the capacitor (switch) voltages can be excessively high due to the upstream-inductor fault energy. The protection voltage level of the surge arrester is set according to system needs. In the steady-state condition, the surge arrester is disconnected. In the case of a short-circuit fault, as the surge arrester and breaker contacts start to separate, both the upstream inductor and the breaker capacitors' rising voltages impose a high voltage on the surge-arrester terminals, forcing it to enter the conductive mode, with a small resistance, as explained. Thus, unlike [11], for faults away from the source, the switching overvoltage is controlled using both the diodes and surge arrester, instead of only diodes.