## 6 Simulation Verification
### Verification of the Wind Turbine's Transient Current Characteristics  
Figure 9 presents simulation results of the wind turbine's output current with different ground-connection resistor when short-circuit fault occurs, where the virtual resistor control is not activated, the grid-side filter inductor, L\({}_{t}\), is 0.038 mH, the grid voltage phase, \(\varphi\), is 0, the output active power of the wind turbine before short-circuit fault is 2 MW, the output reactive power is 0, and the short-circuit ratio is 2.5, i.e., the line inductor, L\({}_{\text{line}}\), is 0.304 mH, and the line resistor, R\({}_{\text{line}}\), is 0.045 \(\Omega\). In Figure 9a, the three-phase short-circuit fault occurs at 2 s, and the ground-connection resistor, R, at the fault point is 1 \(\Omega\). After  
Figure 8: Simulation results of inertial response and primary frequency regulation when the grid frequency fluctuates.  
the short-circuit fault, the output current of the wind turbine increases for a short instant and then enters a steady state. In Figure 9b, the three-phase short-circuit fault occurs at 2 s, and the ground-connection resistor, R, at the fault point is 0.1 \(\Omega\). After the short-circuit fault, the output current of the wind turbine increases significantly and then enters a steady state. Comparing Figure 9a with Figure 9b, it is not difficult to find that the smaller ground-connected resistor, R, corresponds to the higher steady-state value of the wind turbine's output current after the short-circuit fault. In addition, after the short-circuit fault, the output current of the wind turbine quickly enters the steady state. The transient process during the fault is short and the short-circuit impulse current is very small. The simulation result of the wind turbine's output current in Figure 9 is consistent with the analytical calculation result based on Equation (8), which verifies the correctness of the theoretical analysis.  
Figure 10 presents simulation results of the wind turbine's output current with different grid voltage phase, \(\varphi\), when the short-circuit fault occurs, where the virtual resistor control is activated and the value of the virtual resistor, R\({}_{\rm v}\), is 0.3 \(\Omega\), the grid-side filter inductor, L\({}_{\rm f}\), is 0.038 mH, the ground-connection resistor, R, is 0.1 \(\Omega\), the output active power of the wind turbine before the short-circuit fault is 2 MW, the output reactive power is 0, and the short-circuit ratio is 2.5, i.e., the line inductor, L\({}_{\rm line}\), is 0.304 mH, and the line resistor, R\({}_{\rm line}\), is 0.045 \(\Omega\). In Figure 10a, the wind turbine's output current quickly enters a steady state after the short-circuit fault, and the transient process is short. After the virtual resistor is put in, the amplitude of the wind turbine's output current is smaller, indicating that the virtual resistor control can effectively suppress the wind turbine's overcurrent after the short-circuit fault. In Figure 10b,c, where the grid voltage phases, \(\varphi\), are 60\({}^{\circ}\) and 150\({}^{\circ}\), respectively, it can be seen that the virtual resistor control can effectively suppress the wind turbine's output current. In addition, the simulation result of the wind turbine's output current in Figure 10 is consistent with the analytical calculation result based on Equation (8), verifying the correctness of the theoretical analysis.  
Figure 9: Simulation results of the wind turbine’s output current with different ground-connection resistor when the short-circuit fault occurs.