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Sequential ESC-Based Global MPPT Control for Photovoltaic Array With Variable Shading

The photovoltaic (PV) systems are often subject to shading in practical operation, which often results in multipeak power-voltage (P-V) characteristics. Most existing maximum power point tracking (MPPT) control strategies are local-optimum oriented, based on the gradient search or its variations, e.g., perturb-and-observation (P&O) or extremum seeking control (ESC). In order to deal with the multimodal P-V characteristics for PV array with variable shading, this study proposes a sequential ESC-based global MPPT control strategy, based on approximate modeling and analysis for the P-V characteristics under variable-shading circumstances. For the multipeak P-V characteristic curve, the bound of variation for the turning-point voltage is found, and based on which the initial voltage for the segmental search can be set. The local minimum of power for the previous segment is used as the start of the next segment, and thus the initial voltage can be set with consistent bound. Such a sequential scheme can thus significantly reduce the searching interval, i.e., the searching efficiency. Another parallel analysis was conducted for the staircase current-voltage (I-V) characteristic for variable-shading situation. It reveals that the current step size is proportional to the change of shading levels. The current profiles obtained by the sequential ESC-based global MPPT search can thus be used to indicate the shading distribution. Simulation study of partially shaded PV panels validates the modeling analysis and the proposed global MPPT.