## 1 Introduction
In some previous studies [16, 17], an RSC was used instead of a GSC to handle dc link regulation during grid faults, as the latter had to control its active as well as reactive power support during a grid fault. Thus, it may be more beneficial to exchange the roles of the GSC and RSC with respect to DC link regulation. In this case, rotor speed can be increased by storing the remaining mechanical power as WT inertia similar to that observed during grid fault. In Reference [17], a feedback linearization strategy for RSC control was designed. In Reference [18], a nonlinear robust controller was designed to mitigate the dc link voltage fluctuation after taking the nonlinear behavior of the dc link voltage into consideration. For LVRT operation, some authors considered the methods using additional device installations. In recent years, ESSs are being connected to WPSs for achieving many objectives by modulating the WP [6]. This can aid in the integration of WP into the grid and also makes the WPS more cost efficient. According to one strategy, an ESS can assist a WT as an LVRT solution [19]. This methodology resulted in a better LVRT response when compared to the response obtained using only the WT converter control; in addition, an ESS stores power instead of simply dissipating it. In Reference [20], the authors proposed an energy storage-based LVRT method with a direct-drive WPS having no power control capability. In References [21, 22], LVRT methods were proposed using an ESS to enhance the LVRT response; in these cases, an ESS was interconnected with wind farms to help voltage restoration at the PCC. For better reactive power support during grid faults, the optimal reactive power control method was proposed to minimize production cost in Reference [23]. Even though many LVRT control methods utilizing an ESS have been introduced, the coordinated control method for the LVRT method using an ESS has not been sufficiently studied. This coordinated LVRT method is important both for economic and reliable operation. However, to the best of the authors' knowledge, there are no methods to share the burden of LVRT power reduction between the WT and the ESS after considering operation constraints. In this study, we propose a coordinated LVRT control method using pitch, PMSG WT converters, and an ESS. Several previous studies on LVRT methods focus on improving its transient response. We focused on coordinating the WPS (pitch and inertia modulated) and ESS controls and on recognizing when the WT cannot handle the LVRT operation by itself. Before applying the coordinated control, we evaluate if the ESS LVRT response is required by analyzing WP equation. In the case of high wind speeds, an ESS LVRT response is required in addition to the WPS LVRT response, after taking into consideration the rotor speed and converter current limits. If an ESS LVRT response is not required, the WPS LVRT response alone can be used. When an ESS LVRT response is needed, the power reference considering the pitch dynamics and rotor speed limit is used for the LVRT. By using the proposed method, the capacity required for the ESS to support the LVRT could be evaluated. Therefore, the energy required for the ESS could be conserved according to the current operational status of the WPS, resulting in a cost-effective ESS state of charge (SoC) management. From the simulation results, we validated that the proposed method can effectively achieve the LVRT at high wind speeds that can result in significant rotor speed violations during WT LVRT control. We consider two case studies. One case study compares the LVRT response of the WT itself and that of the proposed method. The other study compares the response of the conventional LVRT method that uses the WT and  
Figure 1: Limit curves for the voltage to allow generator disconnection. the ESS with that of the proposed method. From these case studies, it can be concluded that the proposed method achieves the best performance in operating within the constraints of the WT rotor speed and ESS SoC limits.