DC Voltage Control of MMC-based HVDC grid with Virtual Capacitor Control

Ensuring that the DC voltage remains within an acceptable range is a major concern for HVDC systems. In DC systems, the capacitance is the dominant factor that determines the volatility of the DC voltage against sudden disturbances in analogy to inertia of the AC system characterizing the rate of change of the frequency. As a solution to this problem, the authors of this paper previously proposed a novel control method for Modular Multilevel Converter (MMC), named Virtual Capacitor Control. This enables to make the MMC behave as if there were a physical capacitor whose size is adjustable. Thus, it is possible to attenuate the fluctuations of the DC voltage by virtually enhancing the equivalent capacitance of the system. This paper presents a method to determine the required size of this virtual capacitor to limit the peak of the DC voltage to a desired value in the case of the HVDC grid collecting the power generated by wind farms where a master/slave control may be applied. Its advantages are explicated with analysis on the DC voltage dynamics, and its feasibility is verified by EMT simulations of a three-terminal DC grid.