## 4 Simulation Result
### Comparison with WT LVRT  
In this study, we first considered LVRT performance without additional devices. In this case, the WT must take up the entire LVRT operation burden by controlling the pitch angle and rotor speed. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Value** & **Unit** \\ \hline Rated power & 1.63 & MW \\ Rated wind speed & 12 & m/s \\ Max. power coeff. & 0.5 & \\ Optimal tip speed ratio & 9.9495 & \\ Blade radius & 33.05 & m \\ Air density & 1.12 & kg/m\({}^{3}\) \\ Max. rotor speed & 1.2 & pu \\ DC link voltage & 1150 & V \\ Turbine inertia & 6500 & kgm\({}^{2}\) \\ ESS capacity & 1 & kWh \\ \hline \hline \end{tabular}
\end{table} 
Table 1: System parameters used in simulation. Figure 5: Flow chart of GSC switching operations during normal and fault operation. At low wind speeds, this control method can handle LVRT operation, by itself. However, at high wind speeds, it is impossible to satisfy the LVRT grid code while meeting its rotor speed limit constraint. The performance of the proposed LVRT method is illustrated from Figures 6-12. At 80 % voltage sag, the WT should limit the active power to zero and supply full reactive power to the grid. As shown in Figure 6, the WT produced zero active power from the GSC (can be considered as active power to grid) and a reactive power of 0.2 pu during the fault period as shown in Figure 7. A different grid active power profile was observed immediately after the duration of the grid fault. This is because of the GSC control law where the GSC has different power references owing to the difference in the dc link voltage violation. However, we focused more on the behavior during the grid fault and it acts similarly in the two methods during the duration of the fault. Because the grid voltage is 0.2 pu, reactive power current from the GSC is 1 pu, which is the maximum achievable current. Thus, the GSC successfully achieved LVRT control during the fault; that is, full reactive power production was obtained. The results of the two different methods have similar behaviors during the grid fault. As shown in Figure 8, the RSC (can be considered generator power) produces a predefined power considering the pitch angle and the maximum rotor speed limit of the proposed method. Hence, it can be stated that the proposed method realized a suitable power reduction because of the WT during the fault period. However, in the case of WT LVRT control, it tried to produce zero active power to regulate the dc link voltage. The tracking performance was not good in WT LVRT control, and resulted in a high deviation of the dc link voltage as shown in Figure 9. In the proposed method, the dc link voltage regulation was quite improved. The dc link voltage regulation could be further improved in WT LVRT control by modulating the controller or by using nonlinear controller. However, this is not the only problem. The rotor speed was also within the limit as shown in Figure 10 in the proposed method, but, in the case of WT LVRT control, the rotor speed violated its limit. Thus, even though the WT LVRT control is improved, the rotor speed violation will not be eliminated. Because the proposed method calculates the amount of power reduction during the grid voltage sag, the required ESS capacity can be properly conserved. Thus, the SoC requirement for grid faults can be efficiently satisfied using the WT and ESS scheduling operation. The ESS scheduling operation can be implemented for several objectives, such as the smoothing of WP fluctuations and peak load reduction. These operations, however, are out of the scope of this study. However, we defined the required ESS capacity for the LVRT and it can be used as a constraint in that optimization problem. Thus, defining the required ESS capacity for LVRT is meaningful for the efficient operation of the WT during normal and LVRT operation. In Figure 11, the ESS SoC was within its limit of 1 pu in the proposed method. In terms of the power coefficient, the proposed method had a higher value during the grid fault.