Hybrid Topology of a Diode-Rectifier-Based HVDC System for Offshore Wind Farms

This paper proposes a hybrid converter for the high-voltage direct-current (HVDC) transmission of offshore wind farms. The proposed converter consists of a diode rectifier and a paralleled auxiliary modular multilevel converter (MMC). The auxiliary converter is a low-power bidirectional ac/dc converter with a high conversion ratio. During the startup procedure of a wind farm, the auxiliary converter actively sets up the ac voltage of the collection grid and supplies the necessary startup power to the wind turbines. In addition, the auxiliary converter provides the reactive power and partly compensates harmonic currents to the wind farm collection system, so that the hybrid topology can overcome the disadvantages of a diode-rectifier-based HVDC system. After the startup sequence, power delivery is shifted from the auxiliary converter to the diode rectifier by controlling the ac voltages. Compared with HVDC systems based on full-power MMCs, the proposed system significantly decreases the number of submodules, resulting in lower volume, weight, and cost. The effectiveness of the proposed system is verified by the simulation results of a 160-MW offshore wind power system and the experiment results of a 300 W system.