Composition and Control of a New Type of Hybrid Voltage-Source Converter Based on DRUs and FB-MMC for Large-Scale Offshore Wind Power Integration and Transmission

The composition and control of a new type of hybrid voltage-source converters (H-VSCs) are proposed based on diode rectifier units (DRUs) and full-bridge modular multilevel converter (FB-MMC) to reduce the topside volume and weight of offshore wind farm (OWF) converter stations. The relationship between FB-MMC dc voltage ratio and its modulation index is presented to obtain the FB-MMC active power capacity. The equilibrium point is analyzed using numerical calculations. Then, a controller derived from proportional-integral controller is adopted to control FB-MMC dc voltage and a reactive power controller is added to balance the reactive power of OWF, FB-MMC and DRU, by which the PCC voltage is established for wind turbine converters grid access. In addition, the dc short-circuit fault and offshore short-circuit fault recovery strategy is also presented. Finally, two specific case parameters are calculated and a simulation model in RSCAD is developed to validate the effectiveness and feasibility of the topology as well as the proposed control strategy of the H-VSCs.