AC Side Connected-In-Series Modular Quasi-Single-Stage Isolated AC-DC Converter

In order to solve the problems of low operation efficiency and short lifetime of medium-voltage (MV) ac grid-connected converter, a topology of ac side connected-in-series modular quasi-single-stage isolated ac-dc converter is proposed in this article. The electrolytic capacitor applied in the traditional two-stage topology is removed in the proposed topology to improve the life time and power density. An improved triple-phase-shifted (ITPS) modulation strategy to minimize the transformer current is adopted in each module to achieve the full soft-switching operation and improve the efficiency. A mathematical model is built to analyze the reasons for the unbalance of the capacitor voltage on the ac side of each module and the effect of grid harmonics on the grid current. A system level closed-loop control strategy is proposed, which includes the ac side voltage balance control of each module and the grid current closed-loop control, to achieve the voltage balance and high grid connection performance. The feasibility and practicability of the proposed topology and control strategy are verified by the experiments.