Transmission level MMC DC/DC Converter for large scale integration of renewable energy into HVDC grid

This paper presents the detailed description of DC/DC operation for reliable grid integration of renewable energy into HVDC grid, under normal operation and different fault conditions. The DC/DC uses the isolated dual-active-bridge DC/DC converter based on MMC (Modular Multilevel Converter) technology. Within the MMC DC/DC converter, an optimal efficiency control strategy to achieve minimal inner current magnitude and fast control for the full power range is proposed. At grid level, the 3-level cascaded current controller design achieves reliable power transfer and grid voltage control. Under normal grid operating conditions, the tie line DC/DC converter enables flexible power exchange between interconnected HVDC links. During fault conditions, the proposed DC/DC cascaded current controller limits the converter currents. The DC/DC converter will therefore function differently, either in power control mode or voltage control mode, for a proper post-fault grid management without grid-wide communication. The validity of the proposed DC/DC control is verified using a 4-terminal DC system with a large wind farm on a detailed PSCAD model for both normal and fault conditions.