Exploiting Redundant Energy of MMC–HVDC to Enhance Frequency Response of Low Inertia AC Grid

This paper presents a grid frequency responsive inertial control for the modular multilevel converter-high voltage DC (MMC-HVDC) using the MMC design attribute, i.e. submodule redundancy for high reliability. This control does not rely on the external energy sources in providing the inertial response. With an increase in the number of levels of MMCs via the proposed nlevel control, the submodule capacitor voltage decreases, and the electrostatic energy then flows from the submodule capacitors and supports extra power to the grid in need. Decoupled control of the AC, DC and submodule dynamics of the MMC maintains the desired control performance during and after the inertial response, emulating the inherent behavior of the synchronous machine. This study further quantifies the inertia constant of the MMC, equivalent to that of the synchronous machine. The proposed control performance is demonstrated in the context of a Jeju Island power grid with MMC-HVDC systems for the offshore wind interconnection and interconnection between mainland and Jeju. Simulation results present that the proposed method improves the frequency response in harmony with existing synchronous generators and line-commuted converter based HVDC system.