Study on AC Asymmetric Fault Control Strategy of MMC-HVDC Based on CRC and PI-IVPI Bridge Arm Current Decoupling

To solve the problem of unified control of AC side current and circulating current of MMC-HVDC when asymmetric fault occurs in AC power grid, in accordance with three kinds of current control information of bridge arm current, including DC current, AC side output current of converter and circulating current, a control strategy for AC asymmetric fault of MMC-HVDC based on complex resonant controller (CRC) and the hybrid controller based on proportional integral (PI) and improved vector proportional integral (IVPI) resonant regulator is proposed. For the coupling problem between the current and the voltage bridge arm which is difficult to calculate, a decoupling network based on feedforward compensation method which can simplify the control structure is constructed. Under the two-phase static coordinate system, a calculation method of bridge arm current reference value based on CRC sequence component extraction is proposed. Finally, a PI-IVPI1-IVPI2 controller with bandwidth cut-off frequency is proposed is proposed to control three kinds of current effectively and reduce the use of additional controller. Simulation results of MMC-HVDC model by MATLAB/simulink show that the CRC component extraction method proposed in this paper can quickly extract positive and negative sequence components when unbalanced faults occur in power system; the PI-IVPI1-IVPI2 controller with bandwidth cut-off frequency can effectively suppress negative sequence and double frequency circulating current, and reduce the fluctuation of DC current.