DRU-based LFAC Transmission for Offshore Wind Farm Integration Using Grid-Forming Wind Turbines

The low frequency alternating current (LFAC) transmission is regarded as a competitive method for offshore wind power integration compared with conventional HVAC and HVDC transmission in the range of 70~200km due to its high reliability and low cost. To further reduce the cost and increase the reliability of the LFAC transmission system, a possible method is to replace the commonly used modular multilevel matrix converter (M3C) with a back-to-back converter, which consists of a diode rectifier unit (DRU) and a modular multilevel converter (MMC). However, the uncontrollable and unidirectional characteristics of DRUs require the offshore wind turbines to realize black start and establish the offshore AC system. To solve this problem, this paper proposes a novel grid-forming (GFM) control strategy for wind turbines, which only makes a few modifications in the grid-side converter compared with current grid-following control. Furthermore, the start-up process of the system is studied and a black start strategy for offshore grid based on the auxiliary converter is proposed. Finally, the effectiveness of the proposed GFM control strategy and black start strategy is verified by simulations conducted in PSCAD/EMTDC.