*5.3. Simulation Results and Analysis*

This section simulates the controller designed in the previous Section using the programming tool and analyzes the result. When a permanent fault occurs in a feeder with DG, the voltage waveforms at the PCC are shown in Figure 10. Therefore, DG perceives a permanent fault and disconnects.

**Figure 10.** Phase Voltages at the PCC of DGs in the case of a permanent fault, (**a**) phase voltages at the PCC of DG2, (**b**) phase voltages at the PCC of DG3.

Firstly, the simulation waveforms for voltage and current when DG exceeds load are shown in Figure 10. Figure 10a shows the voltage of the PCC stage of DG2 and DG3. In Figure 11a, voltages are unbalanced between 3.1 s and 3.2 s. Conversely, as the is fault eliminated after 3.2 s, voltage is balanced. In this case, over-voltage occurred because the aggregated output of DGs is larger than the aggregated loads. However, when the proposed TFRT control method is applied, the voltage at the PCC was maintained at 0.5 p.u. In addition, over-voltage during reclosing of the recloser did not occur. Figure 11b shows the output current of DG in islanding operation. After 3.2 s, the output of DG1 upstream of the RC1 recloser does not change. The output currents in DG2 and DG3 remain constant after the fault is eliminated. However, inrush current occurred temporarily at 3.6 s when the RC1 recloser is reclosed. To solve this problem, when the proposed inrush suppressing control method was applied, the output currents of DGs increased and reduced the inrush current flowing from upstream. As a result, it is confirmed that the method can suppress the inrush current occurring during the reclosing period.

**Figure 11.** Comparison of the voltage and current with and without Temporary Fault Ride-Through (TFRT) control methods when the aggregated output of DGs is greater than the aggregated loads.

Secondly, voltage and current waveforms are shown in Figure 12 when the aggregated loads are greater than the aggregated output of DGs. As above, after the fault is eliminated, control was implemented when the voltage is balanced. If control is not implemented, the voltage is less than 0.5 p.u. In this case, a fault is eliminated, but DG is disconnected as it is outside the permissible operating range of the VRT. However, when the voltage is controlled at 0.5 p.u. using the proposed voltage control algorithm, after the fault is eliminated, the voltage kept a constant within the permissible range so that DG2 and DG3 can continue to operate. In the case of current, the inrush current is reduced as shown in Figure 12b using the proposed control method. Since the voltage is raised to maintain a voltage within the permissible range, the current was also higher than before the control.

**Figure 12.** *Cont*.

**Figure 12.** Comparison of the voltage and current with and without TFRT control methods when the aggregated loads are greater than the aggregated output of DGs.

As seen from the above simulation results, if a fault is permanent or temporary can be decided according to whether the output voltage of DG is balanced or not. In the case of a permanent fault where the output of voltage of DG is not balanced, DG is disconnected. In the event of a temporary fault where the output voltage is balanced, it was proofed that DG can maintain the voltage within the VRT permissible voltage range by maintaining a constant voltage regardless of the amount generation and load. It was also proofed that the inrush current during the reclosing of the CB or recloser is lower than without TFRT. Simulation results show that the proposed TFRT method can work e ffectively for temporary faults in distribution systems.
