Frequency-division and Hierarchical Control of Modular Multilevel Matrix Converter for Flexible Low-frequency Transmission

This paper proposes a frequency-division and hierarchical control strategy for three-phase flexible low-frequency transmission system based on the full-bridge modular multilevel matrix converter (M3C). The operation principle of M3C is analyzed from the mechanism of dual-frequency power coupling. The control of fundamental-frequency and low-frequency side of M3C is realized in the rotating coordinate system. On the premise of maintaining the total active power balance of the system, the balance of capacitor voltage of power modules among three phases is realized by injecting the negative sequence current into the fundamental-frequency side. And the balance of capacitor voltage of power modules inside each phase is realized by the fundamental-frequency circulating current control inside the bridge arm in the static coordinate system. The proposed hierarchical control can realize the stable operation of the flexible low-frequency transmission system based on M3C under symmetrical and asymmetrical conditions. According to the analysis of the instantaneous power of each phase unit, it is proposed to use only two kinds of sliding filters with different frequencies to ensure the transient stability in the capacitor voltage deviation control of power modules. Finally, a simulation model of the dual-terminal flexible low-frequency transmission system is built to verify the effectiveness and the feasibility of the proposed control strategy. © 2021 Automation of Electric Power Systems Press.