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The converter valve is the core equipment of the DC distribution systems. This paper proposes an autonomous control strategy for grid-connected and islanded operation of hybrid topology modular multilevel converter (MMC). The overall control structure is designed, including the outer loop for autonomous control and the inner current loop. Firstly, for the outer loop, it is proposed to use the equivalent capacitance storage energy of MMC power module to simulate the inertia of synchronous generator rotor to achieve the purpose of buffer power fluctuation. So, the outer loop provides system inertia. Second, the inner loop is designed for current tracking control and suppresses transient fault current effectively. It is proposed that the current inner loop reference value can be directly calculated through the grid-connected impedance matrix and voltage across it. And the small signal model of autonomous control for a hybrid MMC is established. Then the influence of the proposed control structure and control parameters on the impedance characteristics of the system is analyzed in detail. Finally, a hybrid MMC simulation model for DC distribution system is built based on PSCAD, simulating island operation, grid-connected operation mode and transient faults respectively for verifying the proposed strategies.

Autonomous Control Based on Capacitor Energy Storage of Converter for DC Distribution System