Nonlinear Subsynchronous Oscillation Damping Controller for Direct-Drive Wind Farms With VSC-HVDC Systems

Due to the subsynchronous interaction between the grid-side converter (GSC) of wind farms and the rectifier (REC) of voltage source converter-based high-voltage direct current (VSC-HVDC) transmission system, subsynchronous oscillations (SSOs) may occur in direct-drive wind farms with VSC-HVDC systems. Considering the nonlinearities and uncertainties of the system, a nonlinear SSO mitigation strategy is proposed in this article based on the feedback linearization theory and sliding mode control (SMC). The feedback linearization theory is used to eliminate the nonlinearities, and the SMC is adopted to improve the robustness against uncertainties and disturbances. The proposed feedback linearization SMC (FLSMC) takes the advantages of feedback linearization control (FLC) and SMC. The FLC transforms the nonlinear forms of the GSC and REC into the linear forms through the coordinate transformation and feedback. Considering that the FLC is sensitive to parameter uncertainties and external disturbances, the SMC is combined with the FLC to improve the system robustness. An eigenvalue analysis and time-domain simulations are carried out, which demonstrates that the FLC outperforms over the traditional proportional-integral control for the SSO mitigation and decoupling. Meanwhile, the FLSMC shows better robustness against parameter uncertainties and external disturbances over the FLC and traditional damping control.