A direct AC/AC Modular Multilevel Cascade Converter Based on Bridge Cells with Distributed Energy Resources

This paper presents the internal energy balance of a direct AC/AC modular multilevel cascade converter (MMCC) with photovoltaic (PV) arrays as distributed energy resources. This converter is characterized by a modular arm structure composed of bidirectional cascaded bridge cells with floating DC capacitors connected to the photovoltaic array through a DC/DC converter. Based on the analytically developed power flow relations in the arms, a control methodology is proposed to maintain internal energy balance and fully independent control of the floating DC capacitor voltage in each cell, allowing the operation with asymmetric power generation at different levels of irradiance. The operation with individual cells with zero power generated from photovoltaic array is discussed to maintain the internal energy balance without distortions in single-phase and three-phase currents. This topology and control methodology is suitable for hybrid power generation systems, such as wind-solar power generation. Comprehensive experimental results are presented to validate operating principles and control methodology.