A Dual-MMC Chain-Link Structure for Multifrequency Power Transfer

A dual-modular-multilevel-converter (MMC) structure is presented that allows multifrequency power transfer between its ports. By exploiting a three-winding transformer, where the converter-side windings can multitask by accommodating currents with multiple frequency components, the proposed MMC structure can be deployed for two different applications: 1) a dc–ac MMC with independent control of currents in the two dc poles to accommodate unbalanced conditions in bipolar dc systems, and 2) a three-port MMC that can achieve simultaneous dc–dc and dc–ac conversions between an ac grid and two dc systems. DC flux cancellation is imposed in the transformer core by using a center-tapped connection of the converter-side windings. A unified control scheme is developed that independently controls dc and ac terminal power transfers while keeping submodule capacitor voltages balanced. Converter operation and dynamic controls are validated by simulation and experiment.