## 4 Simulation Results  
To verify the performance of the proposed LVRT control strategy and analyze the variations of the PCC voltage, a simulation that uses the grid-connected ESS as shown in Figure 4 was conducted using the PSIM software. The simulation parameters are listed in Table 1. DC-link voltage (\(V_{DC}\)) generated by the battery was 600 V, and the three-phase grid line-to-line voltage (\(V_{G}\)) was 60 Hz/380 Vrms. In addition, \(L_{f}\) was 3 mH and \(R_{G}\) and \(L_{G}\) were 0.05 \(\Omega\) and 0.04 mH, respectively. Figure 10 shows the simulation results of the voltage-level calculation process (given in Figure 6) that uses the three-phase grid voltages \(V_{R}\), \(V_{S}\), and \(V_{T}\). Under normal conditions, \(V_{R}\), \(V_{S}\), and \(V_{T}\) are 60 Hz/310 Vpeak. However, in Figure 10a, the grid voltage drops, and the magnitudes of \(V_{R}\), \(V_{S}\), and \(V_{T}\) decrease to 80%, 60%, and 40%, respectively, of the magnitudes of the phase voltages under normal \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters** & **Value** & **Unit** \\ \hline DC-link voltage (\(V_{DC}\)) & 600 & V \\ DC-link capacitor (\(C_{DC}\)) & 2200 & \(\mu\)F \\ Three-phase grid line-to-line voltage (\(V_{G}\)) & 380 & Vrms \\ Three-phase grid frequency (\(f_{G}\)) & 60 & Hz \\ Filter inductance (\(L_{f}\)) & 3 & mH \\ Resistive impedance of grid (\(R_{G}\)) & 0.05 & \(\Omega\) \\ Inductive impedance of grid (\(L_{G}\)) & 0.04 & mH \\ Rating power (\(P_{rating}\)) & 5 & kW \\ Rating current (\(I_{rating}\)) & 10.7 & Apeak \\ Switching frequency (\(f_{sw}\)) & 10 & kHz \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simulation parameters. conditions for intervals ranging from 0.1 to 0.5 s. After that, the magnitudes increase to 70% and 100% as compared with the magnitudes of the phase voltages under normal conditions. The magnitude (\(V_{mag}\)) of \(V_{R}\), \(V_{S}\), and \(V_{T}\) is precisely calculated using the voltage-level calculation process as shown in Figure 6. Moreover, \(V_{LEVEL}\) as the voltage-level of the three-phase grid is determined depending on \(V_{mag}\) (as shown in Figure 10b), and \(V_{LEVEL}\) is used for the detecting the LVRT requirement. Similar to the scenario in Figure 10, Figure 11 shows the simulation results of the proposed LVRT control strategy depending on the detecting the LVRT requirement based on the voltage-level. Figure 11a shows \(V_{LEVEL}\) of the three-phase grid when the voltage drops (as in Figure 10). Figure 11b,c show the \(d\)-axis and \(q\)-axis currents (\(I_{de}\) and \(I_{qe}\)) and the reference currents (\(I^{*}_{de}\) and \(I^{*}_{qe}\)) of the synchronous reference frame. \(I_{de}\) and \(I_{qe}\) stand for the reactive and the active currents in the synchronous reference frame, respectively. \(I^{*}_{de}\) and \(I^{*}_{qe}\) are determined by the proposed LVRT control strategy depending on the detecting the LVRT requirement using \(V_{LEVEL}\). In other words, the injection quantity of the active and reactive currents is determined from the voltage drop ratio of the three-phase grid using the method as shown in Figure 5.  
If \(V_{LEVEL}\) is 80% during the interval from 0.2 to 0.3 s in Figure 11a, \(I^{*}_{de}\) as the injection quantity of the reactive current is determined to 40% (approximately 4.28 A) of \(I_{Intating}\) in the grid-connected ESS as shown in Figure 11b. In addition, if \(V_{LEVEL}\) is lower than 50% during the interval from 0.4 to 0.5 s in Figure 11a, \(I^{*}_{de}\) is determined to 100% (approximately 10.7 A) of \(I_{Intating}\) in the grid-connected ESS as shown in Figure 11b. After determining \(I^{*}_{de}\) using the proposed LVRT control strategy, \(I^{*}_{qe}\) as the injection quantity of the active current is calculated using \(I^{*}_{de}\) and \(I_{Intating}\). As a result, although the grid voltage drops, \(I_{de}\) and \(I_{qe}\) are controlled by \(I^{*}_{de}\) and \(I^{*}_{qe}\), respectively, using the proposed LVRT control strategy.