Research on Harmonic Oscillation Suppression Strategies for Offshore Wind Power Flexible DC Systems

With the advancement of the "dual carbon" goal, offshore wind power has achieved large-scale development due to its advantages of abundant resources and high power generation efficiency. Flexible direct current transmission (VSC-HVDC) technology has become the core technical path for offshore wind power grid connection, as it is suitable for long-distance and large-capacity wind power transmission needs. However, in the offshore wind power flexible DC system, the interaction between wind turbines, converters and transmission lines is prone to cause wide-frequency harmonic oscillation problems, which seriously threaten the system operation stability and power quality. Aiming at the generation mechanism and suppression difficulties of harmonic oscillation in offshore wind power flexible DC systems, this paper carries out systematic research. Firstly, a full-system mathematical model including wind turbines, MMC converters and DC transmission lines is constructed to analyze the excitation mechanism and key influencing factors of harmonic oscillation; secondly, the coupling influence law of converter control parameters, wind power output fluctuation and line parameter change on harmonic oscillation is revealed; then, a composite suppression strategy integrating adaptive virtual impedance and improved model predictive control (MPC) is designed. By reshaping the system impedance characteristics through virtual impedance and optimizing the converter output characteristics through improved MPC, the accurate suppression of wide-frequency harmonic oscillation is realized; finally, a 1000MW offshore wind power flexible DC system simulation platform is built based on MATLAB/Simulink, and simulation verification is carried out under different wind power output and fault disturbance conditions. The research results show that the proposed suppression strategy can effectively suppress the wide-frequency harmonic oscillation of 20~2000Hz, reduce the total harmonic distortion (THD), and improve the operation stability of the system under fluctuating conditions. The research results of this paper provide theoretical support and engineering technical reference for the harmonic oscillation suppression of offshore wind power flexible DC systems, and have important practical significance for promoting the large-scale safe grid connection of offshore wind power.