Improved dual-mode control strategy of NPC based MMC under wide-load range for HVDC system of offshore wind farms

DC offshore wind farms require DC/DC converter with high step-up ratio and high power and voltage withstand capability to connect medium voltage (MV) DC and HVDC side, multi-level modular converter (MMC) technique is widely used in HVDC grid. In several MMC topologies, resonant three level neutral-point-clamped (3L-NPC) converter could reduce the stress of power device and adapt to high voltage level. However, in high-frequency operating condition, the zero-voltage-switching (ZVS) condition of resonant converter is related to load condition, which make the ZVS failure occur in light-load condition, efficiency degradation under light-load condition with traditional ON/OFF control limit the load range of resonant converter for HVDC application in wind energy system. In this paper, optimal control strategy with a novel dual-mode ON/OFF control strategy is proposed, reason of ZVS failure and efficiency degradation under light-load condition are analyzed and derived, the parameter derivation procedure is discussed in detail, duty ratio D and frequency parameters of two modes is determined with equivalent gain principle, then a downscaled power converter is built, simulation and experimental result verify the feasibility of this improved control strategy under wide-load condition.