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Predictive control of a single-phase cascaded h-bridge photovoltaic energy conversion system

Grid-connected photovoltaic energy conversion systems are among the fastest growing energy systems of the last five years. Multilevel converters, and particularly the Cascaded H-Bridge (CHB), have attracted much attention for this application due to medium-voltage operation, improved power quality and higher efficiency. The CHB enables the connection of individual PV strings to the dc side of each power cell with independent maximum power point tracking (MPPT). This advantage is in turn a great challenge from a control point of view, since each cell will provide different instantaneous active power, for which modified modulation and control schemes are necessary to avoid voltage drift of the dc-link capacitors. This paper explores a model predictive control method, in which predictions for each possible switching state are computed and evaluated in a cost function, in order to select the appropriate control action. The proposed control scheme is capable of controlling the dc-link voltages to the desired MPPT reference voltage, while injecting sinusoidal current to the grid. Preliminary validation through simulation results are included for a five level CHB interfaced PV system.