**3. Simulation Results and Analysis**

Figure 4a schematically shows the simulated VSC-based transmission circuit to test the DCCB proposed in this paper. The simulation was performed assuming that a fault occurs in 230 kV, 50 Hz, and 200 MVA transmission between the DCCB and a 30 km cable [22,23]. Assuming the fault current flowing to the DCCB in the rectifier stage in the schematic, the DCCB in this paper can break the fault current in both directions, so in the example, the DCCB could be placed on the adjacent stage of the modular multilevel converter (MMC) on the right.

Figure 4b is a graph showing the magnitude of the fault current when a fault occurs within 3 s under a given simulation condition. Simulations performed under these conditions show that the current increases rapidly in 3 s and then gradually decreases [24].

The basic parameters used in the simulation are summarized in Tables 1–3. These parameters are default values. Optimized values are identified by analyzing the results as described later. Also, the current limiting reactors in Table 2, not shown in the schematic diagram of Figure 2, refer to the inductors used at both ends of the transmission line of a general DCCB.

**Figure 4.** *Cont.*

**Figure 4.** DCCB simulation condition proposed in this paper (VSC-Based HVDC transmission line) (**a**) Schematic diagram the DCCB simulation; (**b**) Magnitude of the fault current (Fault current occurrence time = 3.00 s).



**Table 2.** Basic characteristics of the elements used to simulate the proposed DCCB.


**Table 3.** Characteristics of the switching elements used to simulate the proposed DCCB.


#### *3.1. Effectiveness of the Reverse Charging Process*

Figure 5 is a graph comparing the reverse-charge method and the normal-charge method used in the proposed DCCB. The resonant current signals the switch's gate (in this case, the left thyristor), assuming that it is emitted within 0 s when turned on. In general, the larger the value of the resonant current emitted, the faster the same amount of fault current can be zero-crossed. However, as the amount of generated resonant current increases, more energy remains, making it difficult to eliminate residual current.

**Figure 5.** Comparison of the magnitude of resonant current between the reverse-charge method and the normal-charge method in the DCCB in the proposed DCCB.

When comparing the reverse-charge method and the normal-charge method used in the proposed DCCB, the reverse-charge method generates a peak resonant current that is 7.03 times larger than that of the normal-charge method. The slope of the current before peaking is also steeper when using the reverse-charge method than the normal-charge method [25]. This means that when using the reverse-charge method, zero-crossing can be performed faster on the mechanical switch of the main current branch. However, a corresponding problem arises, more energy must be dissipated. This issue will be addressed in Section 3.4.2, but in the case of the DCCB in this paper, energy can be dissipated efficiently due to the path of current to ground.
