Voltage source control strategy of three-port modular multilevel converter

Abstract

AC & DC hybrid power grid is an important trend of the future renewable energy based new-type power system. Power electronic transformer with multi AC and DC ports is the key device for renewable energy conversion and transmission. Global concerns are growing about the power electronic transformer and its control in the academic and industry communities. The existing topologies have disadvantages such as large volume, too many power conversion stages, difficult power balancing among submodule units, and complicated circulating control. On the other side, the lack of inertial of common power converter control strategies brings potential instability for renewable energies. TP-MMC has a compact structure and provides two independent AC ports and one DC port. It is suitable for the interconnection among AC & DC power grids. A voltage source control strategy is proposed in this paper for one of the AC ports in TP-MMC. The inertial link is added in the phase lock loop and the inner impedance is added in the dual loop control strategy. Thus, the proposed control strategy provides virtual inertial and brings self-synchronous ability. The AC port works as a voltage source with flexible virtual impedance. Active and reactive power can be controlled by frequency and voltage respectively. The proposed voltage source control strategy is validated by simulation results.