Circulating Current Suppression in DAB Assisted Low-Voltage Variable Frequency MMC

Modularity, scalability, and efficiency of modular multilevel converters (MMCs) have shown clear advantages among the multilevel topologies, being ideal for medium and high voltage applications. However, they require several control loops in order to achieve multivariable control, where the main challenge is the voltage balancing of the submodules capacitors, usually requiring large capacitances. To extend the MMC application to the low-voltage range, the arm link enhanced MMC solves this issues by adding dual active bridges (DABs) between the upper and lower submodules (SMs) in order to transfer the power ripple between each other. This allows the use of small capacitances in the SMs and greatly reduces the control and measurement complexity, while maintaining a low voltage ripple in an extended output frequency range. In this scenario, this work studies how the circulating current suppression control (CCSC) affects the DABs power transfer, as well as the overall effects in the converter and its performance. Considering the variable frequency operation of the converter, the effects at low and high frequencies are analyzed, as well as the consequences in the DABs high-frequency transformers. The main objective is to determine whether it is convenient or not to enable the circulating current suppression depending on the operation conditions of the converter.