Single DC-link cascaded H-bridge multilevel multistring photovoltaic energy conversion system with inherent balanced operation

Large-scale photovoltaic energy conversion systems (LS-PECS) are currently well into the megawatt range. Therefore multilevel converters have been proposed as an attractive solution due to medium voltage operation, higher efficiency, power quality, and possibility to connect PV strings separately to each dc-link. However, the latter advantage makes the inverter susceptible to severe voltage unbalances due to the inherent power unbalance produced by the individual maximum power point tracking (MPPT) of the PV strings. This is particularly the case of the cascaded H-bridge inverter (CHB) where power cell and phase unbalances occur. In this work a new single dc-bus collector bus bar CHB is proposed for a multistring LS-PECS configuration. The operation with a single dc-bus is enabled by adding an isolation dc-dc converter stage to each power cell of the CHB. This allows inherent balanced operation of the CHB while fully decoupling the multistring PV-system from the grid tie inverter. This also greatly extends the operating range of the system compared to previous solutions. Simulation results for a 3.3kV 7-level CHB tested under dynamic conditions are presented and serve as a preliminary validation of the configuration and control schemes.