Structure optimization based on phase‐locked loop and controller parameters optimization of Y‐connected modular multi‐level converter for fractional frequency offshore wind power system under weak grid

Based on a practical long-distance large-capacity transmission engineering case, the small-signal stability of fractional frequency system (FFTS) with Y-connected modular multi-level converter (Y-MMC) is mainly researched in this paper. Different from previous control strategies Y-MMC, paper establishes decoupling mathematical model and proposes strategies. Then, Y-MMC obtained. Considering existence phase locked loop (PLL) will deteriorate under weak grid, applies short-circuit ratio (SCR) for analysis proposed new structure PLL, called phase-shifted PLL (PS-PLL), to improve system. In addition, better dynamic response performance closed-loop system, parameter optimization objective function parameters adjustment. Controller conducted by using particle swarm (PSO) algorithm evaluation based eigenvalue distribution. Finally, feasibility superiority are verified MATLAB/ Simulink.