## 3 Proposed LVRT Control Strategy for Grid-Connected ESSs and Analysis of PCC Voltage Variation
### Proposed LVRT Control Strategy for a Grid-Connected ESS  
The LVRT control strategy needs to be applied in the grid-connected ESS having a large power scale. This strategy generally considers the characteristic of the source type or the application of the system. However, regardless of the characteristics or application, the main purpose of the LVRT control strategy is to comply with the LVRT requirement and inject reactive power through the grid-side inverter on the basis of the gird-code regulations. The characteristics of the grid-connected ESS are different from those of the WPG and SEG systems; the grid-connected ESS with the battery in the DC-link has bidirectional power flow depending on the charging and discharging conditions, unlike the WPG and SEG systems, which have unidirectional power flow. Therefore, the LVRT control strategy for the grid-connected ESS needs to consider the charging and discharging conditions. When the grid voltage drops during ESS operation under the discharging and charging conditions, the proposed LVRT control strategy for the grid-connected ESS becomes similar to the strategy for the WPG and SEG systems. The proposed LVRT control strategy determines the injection quantity of the reactive current for injecting reactive power into the three-phase grid depending on the grid-code regulation. In addition, the active current for transferring active power is determined within the range of the rating current of the grid-connected ESS. The method for determining the injection quantity of the active and reactive currents depends on the voltage drop ratio of the three-phase grid as shown in Figure 5. The voltage-level (\(V_{LEVEL}\)) of the three-phase grid is calculated by the voltage-level calculation process using the three-phase grid voltages \(V_{R}\), \(V_{S}\) and \(V_{T}\). \(V_{LEVEL}\) is classified into three parts depending on the LVRT requirement of the grid-code regulations, and the method for determining the injection quantity of the active and reactive currents is selected by each part. If \(V_{LEVEL}\) is greater than 90% of the three-phase grid voltage under normal conditions, the reactive current injected into the three-phase grid is zero, and the active current becomes the reference current. If \(V_{LEVEL}\) is greater than 50% but less than 90% of the three-phase grid voltage, then reactive current to be injected into the three-phase grid is determined based on the voltage drop ratio of the three-phase grid. In addition, the active current is calculated using the reactive current and the rating current of the grid-connected ESS. If \(V_{LEVEL}\) is less than 50% of the three-phase grid voltage, the reactive current injected into the three-phase grid is the rating current of the grid-connected ESS, and the active current is zero. Figure 6 shows the voltage-level calculation process using \(V_{R}\), \(V_{S}\) and \(V_{T}\) as the three-phase grid voltages. The voltage magnitudes (\(V_{R(mag)}\), \(V_{S(mag)}\) and \(V_{T(mag)}\)) of each phase in the three-phase grid is calculated by \(V_{R}\), \(V_{S}\) and, \(V_{T}\) and the three-phase grid voltages (\(V_{R(shift)}\), \(V_{S(shift)}\) and \(V_{T(shift)}\)), which are transformed by the orthogonal signal generator such as the all-pass filter. In addition, \(V_{LEVEL}\) is determined by \(V_{R(mag)}\), \(V_{S(mag)}\), and \(V_{T(mag)}\) using the maximum value estimation. The \(V_{LEVEL}\) obtained by the voltage-level calculation process that uses the three-phase grid voltage magnitude is important because it is used for detecting the LVRT requirement. Figure 4: Circuit configurations of the grid-connected energy storage system (ESS) using a voltage source inverter (VSI). As a result, the proposed LVRT control strategy for a grid-connected ESS determines the injection quantity of the active and reactive currents depending on the voltage drop ratio of the three-phase grid, which is based on the grid-code regulation. Figure 7 shows the control block diagram of the grid-connected ESS with the proposed LVRT control strategy.