Power factor correction and harmonic suppression of bipolar DRU-based LFAC transmission system for offshore wind farm integration

Using the back-to-back AC-AC converter consisting of a Diode Rectifier Unit (DRU) and a Modular Multilevel Converter (MMC) to replace the Modular Multilevel Matrix Converter (M3C) in Low Frequency Alternating Current (LFAC) transmission systems can realize low-cost grid-integration of offshore wind farms. However, the DRU may introduce high-order harmonics into the LFAC-side current and deteriorate the power quality of the offshore AC system. To solve this problem, a novel AC-side harmonic current suppression strategy is proposed. In this strategy, a specific harmonic current is injected into the DC-link current of both poles to suppress the LFAC-side current harmonics, and the bipolar structure is adopted to provide harmonic current path. The proposed strategy has minimal interference with the normal operation of the system and can effectively reduce the total harmonic distortion (THD) of the offshore AC system as well as increase the power factor of the DRU. Finally, the effectiveness of the proposed harmonic suppression strategy is verified by PSCAD/EMTDC simulations.