## 7 Simulation Assessment
### Performance of the Separated Phase-Current Controls Using IBDGs
#### 7.2.1 Temporary SLG Fault  
The SLG fault was created by short-circuiting phase \(b\) to the ground with 0.01 \(\Omega\) of \(R_{f}\). As shown in Figure 9a, the total fault current amplitude was higher than the grid current amplitude after the fault occurred at 0.14 s. When the fault was detected at 0.155 s, the total fault current amplitude was equated to the grid current amplitude by conventional current controls. Soon after, in Figure 9b, \(V_{dc}\) reached the withstanding voltage of \(C_{dc}\) at 1.2 times the operating voltage (i.e., 1200 V) [39]. Consequently, the IBDGs were disconnected from the grid at 0.164 s, and the total fault current amplitude became greater than the grid current amplitude once again. As shown in Figure 9c, the fuse melted at 0.348 s before the tripping operation of the recloser and the recloser-fuse were no longer coordinated. After the fuse melted at 0.348 s, the grid current was substantially reduced. Figure 9: Conventional current control during temporary SLG fault: (**a**) total fault current and grid current; (**b**) DC-link voltage; and (**c**) operating time of recloser–fuse and SDG breaker. In Figure 10a, the separated phase-current control effectively reduced the total fault current amplitude to the grid current amplitude immediately after the fault was detected at 0.155 s. The estimated grid and SDG currents used for calculating the current reference are shown in Figure 10b. The estimated currents were equivalent to the real-time currents after the fault was detected. The current reference, thus, could be calculated correctly. The \(V_{dc}\) regulation within the withstanding voltage at 1200 V is illustrated in Figure 10c. The disconnection of IBDGs caused by excessive \(V_{dc}\) was avoided. As shown in Figure 10d, the tripping operation of the recloser occurred prior to the melting of the fuse, i.e., the recloser-fuse coordination was preserved. If the recloser trips, the IBDGs and SDG should be disconnected to prevent damage caused by out-of-phase reclosing. By applying the teleprotection, the IBDGs and SDG breaker received the tripping signal sent by the recloser. After the tripping event of the recloser and SDG breaker (at 0.406 s and 0.414 s), the total fault current was reduced significantly. In conventional current control, the active current utilized to transfer the energy in \(C_{dc}\) and the reactive current used to regulate the grid voltage are arbitrary. The three-phase current references are reliant on Equation (6), which is obtained by considering only the fault current. As shown in Figure 11a,b, after the fault was detected at 0.155 s, negative IBDG1 currents indicated that the active currents were drawn from phases \(a\), \(b\), and \(c\). At the same time, in IBDG2, the active currents were drawn only from phases \(a\) and \(c\). Although the IBDG2 injected a noticeable amount of the active current into phase \(b\), there was hardly any active power transmitted by IBDG2 to the grid due to the deep voltage sag of phase \(b\). In Figure 11c, both IBDGs absorbed the average active power (\(P\)) and reactive power (\(Q\)) from the grid, an agreement with the active and reactive currents. Consequently, the power was delivered to \(C_{dc}\) by IBDGs. The energy was accumulated in \(C_{dc}\) and eventually caused excessive \(V_{dc}\) during fault. Figure 10: Separated phase–current control during temporary SLG fault: (**a**) total fault current and grid current; (**b**) real-time currents and estimated currents; (**c**) DC-link voltage; (**d**) operating time of recloser–fuse and SDG breaker. For the separated phase-current controls, \(V_{dc}\) could be lower than the withstanding voltage due to the VC strategy applied in IBDGs. In the nonfaulty phase, the VC strategy was activated. The active and reactive currents were controlled to transfer energy from \(C_{dc}\) to the grid and regulate the grid voltage. The energy in \(C_{dc}\) was sent to the grid by injecting the active current.