A four-leg three-level neutral-point-clamped inverter for offshore wind turbine with VSC-based HVDC transmission

A kind of multiconverter wind power conversion system with HVDC transmission based on VSCs is discussed for 10 MW offshore wind turbine. HVDC transmission is implemented by multiple modular VSCs that are serially connected by means of multiphase PMSG in the nacelle of wind turbine and multiwinding transformer in onshore power substation. As basic building blocks, power loss properties of different VSC topologies are compared. Then, a traditional three-level neutral-point-clamped voltage source inverter (3L-NPC VSI) mixed with an added FC leg is proposed for HVDC transmission. The added FC leg is used to deal with the intrinsic neutral-point imbalance issue of 3L-NPC VSI, including voltage fluctuation and DC drift. Four kinds of different balance control strategies are presented, and their performances are compared. Simulation and experiment results verify the effectiveness of the control strategies. Finally, it is proved that the power loss property of four-leg 3L-NPC VSI is better than the traditional 2L VSI regardless of additional power loss offered by the added FC leg.