Distributed PLL-Based Control of Offshore Wind Turbines Connected With Diode-Rectifier-Based HVDC Systems

Distributed phase-locked loop-based frequency control is proposed in this paper for offshore wind turbine converters connected with diode-rectifier-based high-voltage-direct-current systems. The proposed control enables a large number of wind turbines to work autonomously to contribute to the offshore ac frequency and voltage regulation. The proposed control also provides automatic synchronization of the offline wind turbines to the offshore ac grid. Stability of the proposed frequency control is analyzed using the root locus method. Moreover, active dc voltage control of the onshore modular multilevel converter (MMC) is proposed to ride-through an onshore ac fault, where the onshore MMC converter quickly increases the dc voltage by adding additional submodules in each phase, in order to rapidly reduce wind farm active power generation so as to achieve quick active power rebalance between the offshore and onshore sides. Thus, the overvoltage of the submodule capacitor is alleviated during the onshore fault, reducing the possibility of system disconnection. Simulation results in PSCAD verify the proposed control strategy during startup, synchronization, and under onshore and offshore fault conditions.