A second-order harmonic circulating current injection method for MMC capacitance reduction in offshore DC wind turbine

Abstract

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 PMSG, 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 to maintain the ripple within a reasonable limit, which decreases the SHCC amplitude. The proposed method not only mitigates the voltage ripple, but also reduces the MMC power loss over the entire wind speed range. Firstly, the voltage ripple is modeled with 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 and power loss without SHCC, with conventional and proposed injection methods, are analyzed and compared. The results demonstrate that, the maximum voltage ripple of conventional method can be more than 2 times that of the proposed method. Moreover, the SM capacitance can be reduced by 40% with the proposed method, which also features power loss reduction over the entire wind speed range.