Control of a cascaded H-bridge multilevel converter for grid connection of photovoltaic systems

In this paper a control method for cascaded H-bridge multilevel converters for grid connection of photovoltaic systems is analyzed. The use of the multilevel converter introduces a series of advantages: improved power quality (lower current ripple), smaller filters, reduced switching frequency, no need of boost dc-dc stage and possible elimination of step-up transformer, all of which have a positive impact on the system efficiency. However it requires a more sophisticated control method specially due to dc-link voltage drifts produced by circulating power in the converter. The proposed control method is based on traditional voltage oriented control with a cascaded dc-link voltage control and grid current control loop. Two extra stages are added to this traditional control scheme, one to introduce maximum power point tracking for each module or string, and another to control the dc-link voltages drift due to circulating power in the converter. The latter is performed by the addition of a simple feedforward control strategy directly in the modulation stage. The proposed control method is modular and easy to adapt for any number of converters in series. Simulation results are presented to support the theoretical analysis.