Design of flexible HVDC control system based on MMC

Modular multilevel converter (MMC) can be used as an important component of ultra-high voltage flexible DC transmission to achieve long-distance DC transmission, island power supply and other important areas of power supply. During the transition between active and passive networks in the DC transmission grid, issues such as active power imbalance and AC voltage oscillations can lead to instability. To address these challenges, a control strategy for active/passive network switching based on the Modular Multilevel Converter (MMC) is introduced. This strategy enhances external power and phase angle control, mitigates the instantaneous active power imbalance during the switch, and ensures a seamless transition in the DC power grid. A three-phase MMC simulation model is developed in PSCAD/EMTDC to test the proposed control strategy. The simulation outcomes demonstrate that the system operates stably during active, passive, and switching phases, thereby validating the effectiveness and accuracy of the proposed method.