Control Strategy of MMC Receiving Converter Based on Virtual Synchronous Generator

The power electronic topological structure of Modular Multi-level Converter (MMC) determines its feature of zero inertia, so it cannot support the system inertia, thus both fluctuation of DC transmitted power and short-time power unbalance in AC system could leads to an evident frequency deviation in AC power grid connected to thermal power generating units. To remedy this defect, a virtual synchronous generator (VSG) based control strategy for receiving converter of MMC was proposed. In this control strategy a first-order damp link was added into the outer-loop control of MMC to provide inertial parameter and damping parameter for the system to make the converter similar to the synchronous generator in external operating characteristic. A mathematical model of VSG was established to analyze the dynamic response of both inertial parameter and damping parameter. Based on Opal-RT real-time simulation, a five-terminal MMC-MTDC system was constructed to verify the correctness and effectiveness of the proposed control strategy. Simulation results show that when the power in AC/DC system fluctuates, the proposed control strategy can effectively suppress the frequency fluctuation in AC power grid without the aid of complex communication system, so it is proved that this control strategy can effectively provide inertial and damping to the system and enhance the frequency stability of AC power grid.