Cascaded H-bridge multilevel converter topology and three-phase balance control for large scale photovoltaic systems

The increase in the power levels of photovoltaic (PV) energy conversion systems has resulted in new large-scale grid connected configurations that have reached the megawatt level. This substantial increment in the power levels imposes new challenges to the grid interfacing converter, and therefore results in new opportunities to be explored. This work introduces a new medium voltage multilevel scheme based on a three-phase cascaded H-bridge (CHB) converter and multiple PV strings. The proposed configuration enables a large increase of the total power capacity of the PV system, while the introduction of a multilevel converter helps to improve both power quality and efficiency and medium voltage operation at the grid side. The main challenge of the proposed configuration is to handle the inherent power imbalances that occur not only between the different cells of one phase of the converter but also between the three phases. Simulation results of a 7-level CHB PV system are presented to validate the proposed topology and control method.