Analysis and Enhanced Control of Hybrid-MMC-Based HVDC Systems During Asymmetrical DC Voltage Faults

This paper presents an enhanced control for HVDC systems employing hybrid modular multilevel converters (MMCs) during asymmetrical dc voltage faults. The characteristics of pole-to-ground (PTG) fault under different system grounding schemes are analyzed. Then, the principle of enhanced control strategy to deal with the PTG fault is presented for a hybrid-MMC-based HVDC system with symmetrical monopolar configuration. A new control variable is proposed on the arm voltages to distribute the power transmitted from the ac side between the upper and lower arms. By adopting the enhanced control, the overvoltage and the fault current can be eliminated when a PTG fault occurs. Furthermore, the hybrid-MMC-based HVDC system can operate continuously with adjustable active power and controllable reactive power by controlling the upper and lower arms of the converter asymmetrically during a fault period. Finally, the feasibility of the proposed control scheme is verified by simulation results in PSCAD/EMTDC.