Circulating Current Control in Arm Link Enhanced Modular Multilevel Converter for Low Voltage and Variable Frequency Applications

Modular multilevel converters have shown clear advantages over other multilevel topologies, thanks to its modularity and easy scalability to any number of levels, making them ideal for medium and high voltage applications. However, they require several control loops in order to achieve multi-variable control, where the main challenge is the voltage balancing of the Sub-Modules capacitors. The circulating current of the converter can be controlled to improve the capacitor voltage balancing by injecting specific harmonics or, alternatively, to improve the efficiency by minimizing this current. This work analyzes the circulating current suppression control in a modified MMC topology, where Dual Active Bridges are used to interlink capacitors of the arm Sub-Modules to reduce their power ripple. This allows to extend the MMC application to low voltage and variable frequency applications using a reduced capacitance and control complexity. The objective of the suppression is to reduce losses and improve the converter performance, including the function of the Dual Active Bridge. A study of the overall effects of suppressing the circulating current in the modified topology is presented. It greatly reduces the losses at medium and high frequencies, but also improves the Dual Active Bridge performance by reducing low frequency currents in the transformer. The paper shows whether it is convenient or not to suppress the circulating current depending on the output frequency and the output power of the converter.