Variable Voltage Variable Frequency Modular Multilevel AC/AC Converter With High-Frequency Harmonics Filtering Capability

This article proposes a variable voltage variable frequency modular multilevel ac/ac converter (VVVF-MMC) with a high-frequency isolation link, which can realize voltage and frequency conversion simultaneously with insulated-gate bipolar transistors (IGBTs) of the same rated voltage and current level. The parallel connection of multiple modules is realized by coupling reactors, which can realize large-scale voltage and current transformation and save volume. It combines parallel multiplexing and serial multiplexing technology of submodules (SMs) to increase the equivalent switching frequency, thereby canceling nonzero sequence high-frequency voltage and current harmonics, so it has good power quality in the steady-state. Under the improved carrier phase-shifted pulsewidth modulation (CPS-PWM) strategy, the pulse flip circuit can realize soft switching operation, thereby improving energy conversion efficiency. Furthermore, dc capacitor voltage balancing and parallel SM current balancing control strategies are designed. With the strong filtering ability, the proposed ac/ac converter can be used as the onshore converter station of fractional frequency offshore wind power system. Due to the advantages of small size and high efficiency, it also has broad application prospects in distribution networks and electric drive. Simulation and experimental results validate the effectiveness of the proposed converter topology.