Suppression of Subsynchronous Resonance in a Hybrid Steam-Turbine Generator/Offshore Wind Farm System Using a Static Synchronous Compensator with a Vanadium Redox Flow Battery

Due to the intermittent nature of renewable energy sources (RESs) such as wind farms or solar farms, integrating these RESs into power systems can result in instability, affecting the power system's quality and stability. To address this issue, various energy storage systems (ESSs) used to compensate for power fluctuations or different flexible AC transmission systems (FACTS) used to control voltage magnitude and active/reactive power injections were proposed to improve the stability of the connected power systems. Some effective supplementary damping controllers (SDCs) need to be designed by using suitable control theory for the ESSs or the FACTS devices to achieve the goal of stability improvement. A static synchronous compensator (STATCOM) joined with a vanadium redox flow battery (VRFB)-based ESS is proposed to suppress subsynchronous resonance (SSR) occurring in a hybrid steam-turbine generator (STG)/offshore wind farm (OWF) system fed to an infinite bus through a series-capacitor compensated line. The OWF is based on a doubly-fed induction generator, and the d-q axis equivalent-circuit model of the studied system under three-phase balanced loading conditions is derived to establish the complete system model. An SDC of the STATCOM is designed using modal control theory to improve the damping of the dominant modes of the studied system. Small-signal stability and dynamic simulation results of the studied system are systematically performed to demonstrate that the STATCOM joined with the proposed VRFB-ESS with the designed SDC effectively suppresses the studied system's SSR.