Virtual capacitance based control for MMC-HVDC systems connected with weak AC grids

Abstract

The modular multilevel converter (MMC) topology is recognized as the most viable technology option applied in high-voltage and high-power transmission systems. In this paper, the virtual capacitance based control for MMC-HVDC (MMC-High Voltage Direct Current) systems is proposed to increase the frequency support ability and enhance the system stability when the receiving-end ac grid is weak. The auxiliary control derived from the dynamic energy balance equation with the introduced virtual capacitance for the arm current control of MMC is used as the additional control degree of freedom to enhance the response of ac system frequency. With the proposed control, the MMC can simulate a virtual capacitor with adjustable and larger capacitance capacity value to help increase the dynamic response of ac frequency. Finally, a benchmark two-terminal MMC-HVDC simulation model is established on PSCAD/EMTDC to verify the validity of the proposed control.

Keywords

• MMC-HVDC
• Virtual capacitance
• Inertia response
• Weak ac grids