A Second-Order Harmonic Circulating Current Injection Method for Capacitance Reduction of MMCs in Offshore DC Wind Turbines

The offshore DC wind turbine, consisting of direct-drive permanent magnet synchronous generator (PMSG) and modular multilevel converter (MMC), has become a promising candidate for the large-capacity wind energy conversion system. However, due to the low-frequency output current of PSMG, submodule (SM) capacitors suffer from large voltage ripple especially at rated wind speed. Second-order harmonic circulating current (SHCC) injection is effective for ripple reduction, but conventional techniques only suppress the fundamental component, leading to large SHCC at low wind speed, where the ripple is naturally small. The main contribution of this paper involves that a SHCC injection method, considering all voltage ripple components, is proposed, which maintains the ripple within a reasonable limit. Firstly, the voltage ripple is modeled with variable SHCC. Then, the injected SHCC for defined ripple limit is derived, which facilitates capacitance selection. Finally, a case study of MMC connected with 10-MW and 10-kV PMSG is conducted, based on which the voltage ripple with conventional and proposed injection methods, are analyzed and compared. The results demonstrate that SM capacitance can be reduced by 40% with the proposed method.