Optimal Control for AC and DC Power Quality of VSC-HVDC

In Voltage Source Converter-based High Voltage DC transmission (VSC-HVDC), the unbalance of AC grid is caused by the power quality deterioration, such as AC grid current disturbance and DC voltage periodic ripple. Firstly, the forming reason of system power ripple is introduced under the unbalance of AC grid. An optimal control strategy for AC and DC is proposed. Taking the instantaneous active power without ripple, the voltage outer-loop based on sliding mode control is designed. Secondly, a grid current control under single d-q frame is proposed. With the VSC mathematical model of single d-q frame under the unbalance AC grid conditions, and the reference expressions of positive and negative sequence current components under single positive sequence d-q frame are obtained according the reverse-model principle. Furthermore, the proportional integral plus repetitive control strategy is proposed to implement the control of the current under the single d-q frame, as a result, the effect of the AC grid unbalance on the AC and DC power quality of VSC can be reduced. The corresponding simulation results verify the accuracy and feasibility of the proposed strategy.