Black-Start and Frequency Response Control Strategies for Offshore Wind Power Transmitted by DR-MMC Series-Connected HVDC System

In recent years, the modular multilevel converter (MMC) based high-voltage direct current (HVDC) system have become the mainstream technological approach for remote offshore wind farms. In order to reduce the the volume and weight of offshore converter platforms, many scholars have studied the offshore hybrid converter composed of large-capacity diode rectifier and small-capacity MMC (DR-MMC) in series connected on the DC side. The most crucial research point is the difficulty of black-start caused by the unidirectional conduction characteristic of diode rectifier. In addition, the requirement to provide frequency response from HVDC is proposed by many countries' grid guidelines. Therefore, addressing the black-start and frequency response issues of the DR-MMC HVDC system, this paper proposes to transform the existing HVDC chopper into HVDC direct-mounted energy storage device (DM-ESD) based on modular cascaded topology. The DM-ESD retaining the AC fault ride-through capability, while the stored energy can provide seconds of frequency support for the grid. Furthermore, it can also transmit the power needed for black start to offshore converter station. This paper presents detailed control strategies of black-start and frequency response. Finally, a system model is established based on PSCAD/EMTDC, and the effectiveness of the control strategies is verified.