Modulation Coordinated Voltage Balance Strategy for a Dual-T-Type Modular Multilevel Converter

This article presents a dual-T-type modular multilevel converter (TMMC) composed of stackable ac–dc–ac power cells. Each cell is constructed by two high-frequency T-type legs and one line-frequency switched common leg. Hybrid legs make the system feature less switching semiconductors and flexible leg reusing. Series configurations on both input–output ac sides obtain high voltage grid accessing interfaces. Nevertheless, the fact that copies of capacitors are employed makes the dc voltages imbalance inner cell a tough issue. Furthermore, resulted from cell-mismatch arising with nonideal elements like parameter variations, the voltage imbalance inter different cells need to be settled. Seeing that TMMC has considerable switching combinations, a modulation coordinated method is proposed to realize capacitors’ energy-flexible routing. A voltage vector modulation is deduced to facilitate the planning of proper primary switching patterns, to achieve inner-cell voltage balancing. Moreover, the intercell voltage imbalance is foreknown by using the arranged patterns and a predicted grid current. Taking this feed-forward imbalance voltage value into account, a secondary alternated pulse distribution LS-pulsewidth modulation is proposed to achieve intercell voltage balancing. The simulation and experiment results are supplied to verify the validity of the proposed topology and its modulation coordinated voltage balancing strategy.