Modulation and control of MMC based multiterminal HVDC

Multi-terminal High Voltage dc Transmission (MTDC) is used to reduce the power losses over two distant regions. It is difficult for typical three level inverters to reach the desired dc bus voltage levels, e.g. > 500 kV, due to the large voltage stress on each switch and switching transients. For this reason, Modular Multilevel Converters (MMC) have gained popularity in High Voltage dc Transmission (HVDC) applications and are currently a promising solution. This paper first re-visits the dynamic equations of the circulating current of an MMC to incorporate a disturbance term. Based on the modified equation, a disturbance rejection controller is proposed to eliminate the ac part of this current. In a larger scale, different strategies are presented to regulate the active power, reactive power, and dc bus voltage in an MTDC system. Based on these, a case study on the Lower Churchill MTDC project is illustrated to show the feasibility of an actual MMC implementation.