AC-AC Type Isolated Modular Multilevel Cascade Converter Based on Common LVDC Bus

This paper introduced a novel single-stage isolated modular multilevel cascaded converter (I-MMCC), which has triple ports of three-phase medium-voltage AC (MVAC(T-P)), single-phase medium-voltage AC (MVAC(S-P)) and low-voltage DC (LVDC). The converter can realize single-stage power conversion from LVDC to MVAC, and MVAC(T-P) and MVAC(S-P) port can realize flexible AC/AC power conversion for same or changeable frequencies. Its unipolar modulation strategy can avoid the voltage spike during the transient commutation process of bidirectional switches in isolated AC-AC matrix converter. The topology and modulation strategy of I-MMCC sub-module were proposed at first. The average equivalent model of sub-module and single-phase I-MMCC was established, respectively. In addition, the working principles towards frequency and voltage conversion of MVAC(T-P) and MVAC(S-P) port were introduced. The steady-state power and port characteristics of the two ports were analyzed as well. Based on the concept of orthogonal virtual circuit, the control model of single-phase AC port was established, and the power constraints of MVAC(S-P) and MVAC(T-P) ports were deduced. Finally, a set of laboratory prototype was built to verify the effectiveness and superiority of the proposed converter. © 2021 Chin. Soc. for Elec. Eng.