A three-level flying-capacitor (four-leg) neutral-point-clamped (3L-FC-NPC) inverter is proposed for a large offshore wind power conversion system with light high voltage direct current (HVDC) transmission. The fourth flying-capacitor (FC) leg is added to eliminate mid-point voltage imbalance of the DC link including DC drift and low-frequency voltage fluctuation. The principle of the added FC leg is discussed first. Then three different voltage closed-loop control methods are proposed and compared. Finally

simulation and experiment show the correctness of modeling and analysis.