Internal Energy Balance of a Modular Multilevel Cascade Converter Based on Chopper-Cells With Distributed Energy Resources for Grid-Connected Photovoltaic Systems

This paper presents a methodology to control the internal energy balance of a modular multilevel cascade converter (MMCC) with distributed energy resources. The converter is characterized by a high voltage multi-terminal DC link connected to a three-phase MMCC with distributed photovoltaic arrays. Each modular arm structure is composed of bidirectional cascaded chopper cells with floating capacitors connected to the photovoltaic array through a DC/DC converter. Based on the power flow equations developed for the individual cells, a control methodology is applied to maintain the internal energy balance of the MMCC under power imbalance between the cells. The operation at different levels of irradiance is discussed to allow the operation with individual cells with zero energy generated from the photovoltaic array. Zero power cells are used to maintain three-phase output voltage and stable MMCC operation. Power balance between the arms is achieved through of the AC circulating currents in the MMCC arms without affecting the DC link and AC output currents. This converter topology and control methodology is suitable for hybrid renewable energy systems such as wind-solar power systems. Comprehensive experimental results are presented to validate operating principles and control methodology.