A Power Losses and Thermal Energy Optimization Method for Hybrid DC Chopper Based on Hysteresis Control Strategy

DC chopper used to absorb the surplus power to prevent the devices from been damaged by the dc overvoltage is one of the important equipment for the offshore wind farm grid connection via the modular multilevel converter-HVdc system. A hybrid dc chopper structure composed of cascaded submodules and a lumped resistor is proposed in which the distributed resistor is deployed inside each submodule. Due to the cascaded structure, a hysteresis control strategy with dc bus voltage coordinated is proposed to reduce the large charging power of the dc chopper. By doing so, the capacitor in each submodule is only charged during the inserting and bypassing stages giving an opportunity to greatly reduce the charging energy and capacitance of capacitor. As a result, the switching frequencies and power losses of insulated gate bipolar transistors (IGBTs) as well as the thermal energies of the distributed resistors are reduced by 95%, 99%, and 97.7%, respectively, compared to that of the original control strategy. Consequently, the press-packed IGBTs can be replaced with welded-type ones resulting in air-cooling device instead of water-cooling device. Finally, an experimental prototype is established for the validation of the proposed method.