A Multi-terminal Flexible Frequency Support Method Based on Inertial-enhanced VSG Control

When large-scale offshore wind power is connected to the grid through VSC-MTDC, the inertia of the power system decreases, posing a threat to grid frequency and system stability. To address the above issues, we present a multi-terminal flexible frequency support method based on inertial-enhanced VSG control. First, the virtual inertia technology is employed to couple the AC frequency with the DC voltage, enabling offshore wind power to actively participate in frequency support. Secondly, to improve the inertia dynamic adjustment performance of traditional VSG, the frequency differential feedforward is utilized in the virtual synchronous control, while the equivalent DC capacitor active power output control is used to provide VSG inertia support. Subsequently, to enable VSG to have multi-terminal coordination capability, and considering the risk of DC voltage exceeding the limit during multi-terminal flexible frequency support, the frequency-power-voltage sag control link is cascaded before virtual synchronous control. In addition, the sag coefficient is adjusted adaptively according to voltage deviation to rationally allocate the power demand of each end, enabling the disturbed terminal to get effective frequency support. Consequently, the balanced control of power grid frequency and DC voltage can be realized. Finally, simulation results demonstrate that the proposed method is capable of improving the frequency change rate and frequency deviation of the AC system, while also maintaining the stability of the DC voltage.