Synthetic-Inertia-Based Modular Multilevel Converter Frequency Control for Improved Micro-Grid Frequency Regulation

Abstract

High penetration of renewable energies through fast-response power converters results in a considerable displacement of conventional synchronous generators and losing of system inertia for frequency control. In a micro-grid with high shares of renewables, Modular Multilevel Converters (MMCs) can be employed serving as an interface between the renewable generation and the micro-grid. The MMC submodule (SM) capacitors can be used as energy storages to provide an considerable amount of synthetic inertia for system frequency support. This paper proposes an MMC synthetic inertia concept that is mainly affected by renewable penetration ratio, SM capacitance and the modulation index of the MMC, with the system constraints determined in the design phase. A substantial portion of the system inertia can be provided by the MMC with properly selected system parameters. By simultaneously controlling the MMC capacitor voltage and output active power based on the system frequency variation, an amount of energy can be released or absorbed by SM capacitors to improve the system frequency response during the transients of frequency event, while the renewable power is scarcely influenced. The proposed concept is verified by simulations and experiments. The experimental results show that, by proper system parameters design, the frequency deviation and the rate of change of frequency (RoCoF) can be reduced by 13.9% and 18.5%, respectively.