Coordinated Frequency Control Strategy for VSC-HVDC-Connected Wind Farm and Battery Energy Storage System

With the increasing penetration of renewable energy, power system inertia is reduced; thus, frequency stability faces tremendous challenges. Offshore wind farms (WFs) are often integrated to the grid through a voltage-source-converter-based high-voltage direct current (VSC-HVDC) transmission. However, traditional WFs cannot provide frequency support owing to the decoupling characteristics of VSC-HVDC. Modern WFs may support frequency regulation, but the recovery of rotor speeds of wind turbines (WTs) would cause a considerable second frequency drop (SFD). To resolve these problems, this paper presents a coordinated control strategy for a VSC-HVDC-connected WF with a battery energy storage system (BESS) for providing frequency support. The proposed strategy enhances the synthetic inertia by allowing WFs and BESS to participate in frequency regulation, in which the VSC-HVDC transmission supports frequency regulation by regulating its DC-link voltage, and BESS provides the required power during the rotor-speed recovery of WTs. Thus, SFD can be prevented and frequency deviation is minimized. In this study, the case study that considers the outage of synchronous generator and variable wind-speed scenario was conducted in PSCAD/EMTDC. The simulation results verify the effectiveness and the robustness of the proposed control strategy and demonstrate the superiority of the proposed strategy over other existing strategies.