**2. Circuit Operation Process**

Figure 1 shows the current flow in the DCCB proposed in this paper is in a steady-state. If the DC steady-state persists, the blocking inductor has no effect. It does not cause any additional power loss. The steady-state current charges the reverse charge capacitor, so no further action is required when operating later. Even in this case, there is no power loss when the reverse charge thyristor is fully charged. The proposed DCCB works well even if there is a fault current anywhere in the transmission line.

**Figure 1.** The overall circuit diagram of the DCCB proposed in this paper and the steady-state current flow.

Figure 2 shows the flow of the fault current from the rectifier stage to the DCCB and the emitted the inductor and the capacitor (LC) resonant current. When a fault current is detected, it sends a signal to the gate terminal of the reverse charge thyristor and proceeds with the reverse charging process. At this time, some of the fault currents are used in the reverse charging process to create a larger resonant current. Immediately after that, the LC resonant current is emitted by turning of the left or right thyristors according to the direction of the fault current [21]. When the fault current and LC resonant current are zero-crossed, the mechanical switch on the main transmission line turns off to eliminate the effect of fault current.

**Figure 2.** Current flow of LC resonant emission current and fault current from the rectifier stage to the DCCB.

The DCCB in this paper can break in both directions, as described above. Unlike the example in Figure 2 where there is a fault current in the rectifier stage, when the fault current flows from the inverter stage to the DCCB, the reverse charging process is the same, and when the right thyristor is turned on, the LC resonant current is emitted to generate a zero crossing point and the right mechanical switch is turned off.

Figure 3 shows the flow of residual current after zero-crossing of the DCCB main switch. To minimize the effect of residual current on the rectifier and inverter stages, the current is returned to the DCCB through parallel diodes and bypass diodes. At this time, the reverse charge thyristor is turned on continuously, dissipating the energy in the DCCB to the ground. When the residual current is removed to the ground, it resonates through the ground inductor passing through, leaving a small amount of energy in the DCCB but with little effect.

Figure 3a shows the current flow after breaking the fault current passing from the rectifier stage to the DCCB, and Figure 3b shows the current flow after breaking the fault current passing from the inverter stage to the DCCB. The mechanism for directing current flow through a diode connected in parallel to the mechanical switch, which is usually the main switch, is the same. Depending on the direction of the fault current according to the placement of the DCCB, the flow of residual current is symmetrical and the process of dissipating energy through the ground is the same.

**Figure 3.** Flow of residual current after zero-crossing of the main switch of the proposed DCCB: (**a**) Current flow after breaking the fault current passing from the rectifier stage to the DCCB; (**b**) Current flow after breaking the fault current passing from the inverter stage to the DCCB.
