A Coordinated Control Strategy of MT-HVDC Grids to Reduce Real-Time Frequency Deviations by Considering Malfunction of Converter

Voltage source converter (VSC)-based high-voltage direct-current (HVDC) systems are being increasingly considered to support grid frequency regulation (FR) by using fast responses to reference signals. This paper proposes a control strategy for the optimal real-time FR of multi-terminal (MT) HVDC grids to reduce grid frequency variations. The novelty of the proposed strategy is that it realizes coordinated power control (CPC) of MT-HVDC system to consider the malfunctions of the HVDC converters and communication time delays between the malfunctioning and compensated HVDC converters. Accordingly, a state-space model of MT-HVDC grids is derived considering the DC-link voltage and active power controls at the rectifier and inverter, respectively, which are combined with inertia emulation (IE) and droop controllers. Based on the developed model for MT-HVDC grids, optimal secondary frequency control (SFC) is achieved using a linear quadratic Gaussian (LQG) controller. Moreover, CPC is integrated into the LQG to compensate for the inaccurate output power owing to malfunctions of the HVDC converter. Simulation case studies are performed to verify that the proposed strategy can alleviate real-time frequency fluctuations of inverter-side grids more effectively than conventional strategies.