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A new design approach of stable nonlinear PI controllers for stand-alone photovoltaic systems

In this paper, simple nonlinear proportional-integral (PI) controllers are proposed for a stand-alone photovoltaic (PV) system. The controller design and analysis is based on the complete accurate system model. To this end, the nonlinear model of the PV source, the detailed model description of the battery and the accurate nonlinear dynamics of the dc/dc boost converter are taken into account. For this system, a suitable PI controller scheme is developed with main advantage its independence from the system parameters and operating conditions. Furthermore, an extensive stability analysis is carried out for the closed-loop system, based on advanced nonlinear methods. As it is proven, boundedness and convergence to the desired equilibrium of all the system states is guaranteed for simple positive values of the PI gains. Finally, to evaluate the proposed controller performance, simulations are conducted for cases where rapid irradiance or load changes occur and the results are compared with advanced passivity-based control schemes.