Amplitude-Varying High-Frequency Voltage Injection Method for Reducing the Number of Submodules in DR-MMC-Based Hybrid Converters in Offshore Wind Farms

The hybrid topology with a modular multilevel converter-based (MMC-based) auxiliary converter and diode rectifiers connected in paralleled (DR-MMC) has emerged as a promising solution for offshore wind power transmission. To reduce the submodule (SM) number of this hybrid topology, an amplitude-varying high-frequency voltage injection method is proposed in this paper. The amplitude of the injected highfrequency voltage is changing with the instantaneous value of the fundamental output voltage. Thus, the high-frequency voltage can be injected without increasing the SM number of arms. By replacing the MMC with a three-phase seriesconnected MMC (SC-MMC), the AC side voltage is unaffected by injection. Meanwhile, the amplitude fluctuations of injected high-frequency voltage can be canceled out on the DC side. As a result, the high-frequency voltage with constant amplitude can be injected in the DC side of SC-MMC without increasing the SM number. Therefore, the SM number of the converter can be further reduced. The equation of injected high-frequency voltage is derived, and the SM number of the proposed and existing methods are compared. The feasibility of the proposed method is verified via 800-kV 110-MW/330-Mvar simulation.