Enhanced AC voltage and frequency control on offshore MMC station for wind farm

For HVDC transmission systems based on modular multilevel converter (MMC) for connecting large offshore wind farms, the offshore AC voltage is regulated by the offshore MMC station and stable offshore AC voltage and frequency are important for the stable wind power generation and transmission. This paper proposes an enhanced AC voltage and frequency control strategy of the offshore MMC for wind farm integration, where an additional frequency loop is used to improve its AC voltage and frequency controllability. The proposed scheme considers the working principle of the phase locked loop (PLL) where the measured q-axis voltage drives the frequency output for the generated AC voltage. Thus, the output of the proposed frequency loop sets the q-axis voltage reference and feeds to the AC voltage loop to regulate the offshore AC frequency. Compared to conventional approach where no frequency loop is used and the converter simply produces the output as per the offline phase information, the proposed strategy can tightly control the AC voltage of the offshore network, which contributes to a stable transmission of the offshore wind energy. Simulation results in normal operation and during symmetrical fault confirm the feasibility of the proposed control scheme.