HB and FB MMC Based Onshore Converter in Series-Connected Offshore Wind Farm

In contrast to the traditional high-voltage direct current (HVdc) based offshore wind farms, series-connected offshore wind farms do not require transformers, voltage source converters (VSCs), and platforms in offshore, thereby significantly decreasing costs. In this study, the onshore converter, which is based on the half-bridge (HB) modular multilevel converter (MMC), and full-bridge (FB) MMC, is proposed to reduce the costs and power loss of the series-connected offshore wind farm. In this type of onshore converter, the dc voltage of the HB-MMC is controlled to be the rated dc voltage, while the dc voltage of FB-MMC can vary from negative rated dc voltage to positive rated dc voltage. With the variable dc voltage of the FB-MMC, the dc-link current of the series-connected offshore wind farm can be controlled at will. Moreover, the dc-link current control strategies are proposed to reduce the power loss of the offshore wind farm. The capacitor voltage balance control, which is suitable for FB-MMC with variable dc voltage, is proposed to ensure the normal operation of the onshore converter. Finally, simulation and experimental results verify the feasibility of the proposed onshore converter and the corresponding control algorithm.