A Control Method of DC Capacitor Voltage in MMC for HVDC System using Negative Sequence Current

The application of Modular Multilevel Converter (MMC) for High Voltage Direct Current (HVDC) system is expected to enhance power transmission capacity and improve power system stability. To control an MMC-HVDC system properly, the ac current, circulating current among arms and submodule (SM) capacitor voltages are considered. In the conventional balance control method of the dc capacitor voltage, the reference value of the circulating current control includes ac component as same frequency as ac grid. In this case, it is difficult to follow the reference value perfectly by proportional-integral (PI) controller. This article presents a new control method of dc capacitor voltage using negative sequence of ac grid current and circulating current. The validity of the proposed control method is verified by a numerical simulation.