Energy-Utilizing Frequency Support for MMC-MTDC System Integrating Asynchronous Grids

This paper proposes an energy-utilizing frequency support method for modular multilevel converter-based multi-terminal high voltage direct current (MMC-MTDC) system to participate in the frequency regulation of connected AC grids. By coordinating the energy reserves in asynchronous AC grids as well as MMC capacitors, the proposed method achieves comprehensive frequency support performance adaptive to different load disturbance conditions. Firstly, a disturbance identification algorithm is developed to precisely recognize the load disturbance degree and location. Then, an inertia control is developed for minor load disturbances to offer inertia support by only using the MMC energy. For major load disturbances, the energy in both asynchronous AC grids and MMC capacitors is utilized to avoid severe frequency deviation. Furthermore, a supplementary DC voltage-energy control is designed to increase the MMC energy utilization, thereby improving the frequency support performance. Finally, the simulations in an MTDC system integrated with four asynchronous grids are carried out to verify the effectiveness and robustness of the proposed method.