A Compact MMDTC-MMC Hybrid HVDC System for Offshore Wind Power Integration: Topology and Control

The modular multilevel converter (MMC) is currently the mainstream solution for connecting offshore wind power to high-voltage direct-current (HVDC) grids. However, its large number of submodules and bulky energy-storage capacitors have become key bottlenecks that limit the compactness and cost reduction of offshore converter platforms. To achieve a compact offshore converter station, this paper proposes a novel MMDTC-MMC hybrid DC transmission system for offshore wind power. A modular multilevel DC-link T-type converter (MMDTC) is used at the sending end. While effectively overcoming the volume disadvantage of conventional MMCs, the proposed converter retains the inherent advantages of a fully controlled topology, including bidirectional power transmission and black start capability. Considering the physical characteristic that the AC- and DC-side voltages of this topology are directly coupled, a steady-state analytical model of the system is derived. A coordinated control strategy based on an adaptive closed loop for submodule capacitor voltage is then proposed to solve the transient energy-balancing problem caused by strong active- and reactive-power coupling. Quantitative analysis shows that, under the same DC-voltage level and operating conditions, the total number of submodules required by the sending-end MMDTC is only 35.6% of that required by a conventional MMC, while the required capacitance is reduced to 33.41%. With a reasonable sacrifice in device current stress, the hardware cost and physical space required by the offshore converter platform are significantly reduced. Finally, MATLAB/Matlab of R2024a simulation results verify the effectiveness of the proposed compact topology and adaptive control strategy under high-quality steady-state bidirectional power transmission and transient power-step conditions.