Analysis and mitigation of subsynchronous resonance in VSC-HVDC-based offshore wind farms

Offshore wind farms typically employ either doubly fed induction generators (DFIG) or permanent magnet synchronous generators (PMSG) due to their enhanced flexibility. They usually have high-rated capacity and high voltage DC transmission (HVDC) technologies have been used for the bulk evacuation of power from offshore wind farms. Among several challenges that have been faced in the integration of offshore wind energy to grid, this work aims to analyze and mitigate subsynchronous oscillations associated with DFIG-based offshore wind farms that are integrated to the grid by HVDC transmission systems employing voltage source converters (VSC). Integration of the DFIG-based offshore wind farm with the power system network via VSC-based HVDC transmission test system has been simulated and analyzed through electromagnetic transient type simulation (EMTP) in PSCAD/EMTDC. The system has been found susceptible to the two major oscillation modes, namely, subsynchronous controller interaction (SSCI), subsynchronous shaft torsional oscillation (SSTO), and the highly correlated variables to these two major oscillation modes have been identified. Further, excitation damping and subsynchronous resonance damping controllers have been proposed and placed optimally to comprehensively mitigate SSTO and SSCI respectively.