## 4 Simulation and Analysis
### Three-Phase-to-Ground Fault
Additionally, Figure 9f shows that the fault stator current of the DFIG cannot occur in the overcurrent during the fault period. Additionally, it can be concluded from Section 2.3 that during fault periods, the frequency of the fault current is affected by the voltage dips and the crowbar circuit. Therefore, during the fault period, when the voltage of the 35 kV line is compensated by DVR system, the stator voltage of the DFIG can be less affected by the fault. Theoretically, during fault periods, the DVR-based SMES compensating for the fault voltage not only achieves the fault ride through of the DFIG, but also mitigates the frequency offset of the fault current of the DFIG. Figure 8: The system’s responses with a DVR-based SMES during a three-phase-to-ground fault (depth of 25%). (**a**) Fault voltages on the 220 kV side; (**b**) fault voltages on the 35 kV side; (**c**) compensating voltage of the DVR; (**d**) compensated fault voltages on the 35 kV side; (**e**) stator voltage of the DFIG; (**f**) stator current of the DFIG. Therefore, Figure 10 shows a comparison of the frequency of the fault current with a DVR-based SMES and with a conventional crowbar circuit protection at the different voltage dip depths. The two faults occurred at around \(t\) = 0.22 s and their duration was 0.2 s. It can be observed that the frequency of the fault current with the crowbar circuit protection not only fluctuated during the fault period, but also continued to fluctuate for a short period of time after the fault was cleared. This is because during fault periods, the rotor overcurrent caused by the voltage dips can result in the crowbar circuit being activated. However, the stator voltage of the DFIG was still in a severe dip, so there was still a serious frequency offset in the fault current. Additionally, after the fault is cleared, due to the withdrawal of the crowbar circuit, the system voltage and current will fluctuate again. Thus, the frequency of the fault current still existed after the fault was cleared. Figure 11 shows a comparison of the DFIG output power when using an adopted DVR-based SMES at a voltage dip depth of 80%; an adopted DVR-based SMES at a voltage dip depth of 80%; crowbar circuit protection at a voltage dip depth of 80%; and crowbar circuit protection at a voltage dip depth of 25% when the three-phase-to-ground fault occurs. By comparing the crowbar circuit protection at voltage dip depths of 80% and 25%, it can be seen that when the DVR-based SMES is adopted to compensate for the fault voltage, the DFIG output power is mostly maintained in the normal output state. Figure 9: The system’s responses with an adopted DVR-based SMES during a three-phase-to-ground fault (depth of 80%). (**a**) Fault voltages on the 220 kV side; (**b**) fault voltages on the 35 kV side; (**c**) compensating voltage of the DVR-based SMES; (**d**) compensated fault voltages on the 35 kV side; (**e**) stator voltage of the DFIG; (**f**) stator current of the DFIG. To sum up, with DVR-based SMES to compensate the fault voltage of 35 kV line, the DVR-based SMES can isolate the DFIG from the 80% and 20% depth of three-phase-to-ground faults. Thus, it can prevent the voltage of DFIG from being affected by the faults. Moreover, the crowbar circuit cannot be activated during fault periods, so the frequency offset of the fault current is almost non-eistent and can be maintained at 50 Hz. Additionally, the DFIG can output the normal power during the faults.