Balancing energy and low frequency operation of the Modular Multilevel Converter in Back to Back configuration

The modular multilevel topologies are the next generation of power converters for applications where high power-voltage is necessary, such as traction systems, marine propulsion and Wind Energy Conversion Systems. In this paper, a new model and its respective control scheme for the Modular Multilevel Converter are proposed. The model is able to represent separately the dynamic of each port and to show the degrees of freedom in the converter, which are two circulating currents and the common mode voltage. These three degrees of freedom were used to perform energy balancing and to mitigate the voltage fluctuations at low AC frequency operation. Additionally, the proposed control strategy is extended to control two Modular Multilevel Converters in Back to Back configuration. Extensive theoretical analysis and computer simulation validate the effectiveness and viability of the presented control algorithm.