Topology and Control Strategy of Offshore Wind Power via DR-MMC Parallel Hybrid DC Delivery System

With the rapid development of flexible direct current (DC) transmission technology and offshore wind power, the offshore converter station using modular multilevel converter (MMC) has become the primary choice for large-capacity long-distance offshore wind power transmission. However, because of the large number of sub-modules of MMC converter, the volume of offshore converter with MMC converter is too large and the construction cost increases. This paper proposes a low-cost hybrid converter consisting of an auxiliary MMC converter and a diode rectifier in parallel. The auxiliary MMC converter establishes the wind farm intranet voltage and returns reactive power during the wind farm start-up phase to achieve the black start of the wind farm; it compensates the reactive power of the wind farm and reduces the current harmonics of the diode rectifier during the wind farm generation phase. The diode rectifier undertakes active power delivery in the wind farm generation stage. With the diode rectifier, the number of sub-modules of the MMC converter can be greatly reduced to achieve the purpose of reducing the size and investment and improving the transmission efficiency of the converter station. In this paper, a 600MW offshore converter station is taken as a case study, and the proposed converter topology, control principle and the feasibility of cost reduction are verified through simulation analysis.