Enhanced AC Fault Ride-through Control for MMC-integrated System Based on Active PCC Voltage Drop

When a renewable energy station(RES) connects to the rectifier station(RS) of a modular multilevel converterbased high-voltage direct current(MMC-HVDC) system, the voltage at the point of common coupling(PCC) is determined by RS control methods. For example, RS control may become saturated under fault, and causes the RS to change from an equivalent voltage source to an equivalent current source, making fault analysis more complicated. In addition, the grid code of the fault ride-through(FRT) requires the RES to output current according to its terminal voltage. This changes the fault point voltage and leads to RES voltage regulation and current redistribution, resulting in fault response interactions. To address these issues, this study describes how an MMC-integrated system has five operation modes and three common characteristics under the duration of the fault. The study also reveals several instances of RS performance degradation such as AC voltage loop saturation, and shows that RS power reversal can be significantly improved. An enhanced AC FRT control method is proposed to achieve controllable PCC voltage and continuous power transmission by actively reducing the PCC voltage amplitude. The robustness of the method is theoretically proven under parameter variation and operation mode switching. Finally, the feasibility of the proposed method is verified through MATLAB/Simulink results.