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

Connecting large offshore wind farms using high-voltage direct current (HVDC) transmission systems based on the modular multilevel converter (MMC), the offshore alternating current (AC) voltage and frequency are regulated by the offshore MMC station and are important for the stable wind power generation and transmission. This study 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. A fault current injection control is also proposed, where the offshore MMC station actively provides fault currents during an offshore AC fault to enable overcurrent protection for the network. To ride-through onshore AC faults, a direct current (DC) voltage-dependent AC voltage controller is introduced to actively reduce the offshore AC voltage during an onshore AC fault to alleviate the DC overvoltage of the HVDC system. Simulation results in normal operation and during offshore and onshore faults confirm the feasibility of the proposed control scheme.