Internal Energy Based Grid-Forming Control for MMC-HVDC Systems With Wind Farm Integration

The virtual synchronous generator (VSG) control is regarded as an effective solution for operating converters in weak grid conditions due to its excellent grid support functions. However, successful implementation of the VSG control of an AC/DC converter relies on the existence of a steady DC voltage at the DC side, while this is not easy to achieve for the cascaded converter system of this work, i.e., the Modular Multilevel Converter-based High-Voltage Direct-Current (MMC-HVDC) transmission systems with offshore wind farm integration. To address this issue, a novel grid-forming control strategy with real-time inertia support and direct DC-link voltage regulation is proposed for the Receiving End Converter (REC) of the MMC-HVDC. The intrinsic power balancing regime of the internal energy stored in MMC's submodule (SM) capacitors is utilized for grid synchronization rather than emulating the swing equation as requested by the VSG control. For being more robust to sudden power variations, proper insertion strategies of the REC are developed to decouple the DC-link voltage of MMC-HVDC from the voltage of SM capacitors. Furthermore, in terms of the grid fault-ride-through issue of the REC, an associated FRT control strategy is proposed aiming for power angle stability and the stability of SM capacitor voltage. Finally, simulation results in PSCAD/EMTDC show that the proposed control can provide fast inertia support with satisfactory control of DC-link voltage and FRT capability, etc.