Sliding mode control of DC/DC switching converters for photovoltaic applications

Abstract

The maximum power point tracking (MPPT) is one of the most important features of a system that process the energy produced by a photovoltaic generator must hold. It is necessary, in fact, to design a controller that is able to set the value of voltage or current of the generator and always ensure the working within its maximum power point. This point can considerably change its position during the day, essentially due to exogenous variations, then sunshine and temperature. The MPPT techniques presented in literature and adopted in commercially devices operate a voltage control of the photovoltaic generator and require careful design of the control parameters. It is in fact complex obtain high performance both in stationary that strongly variable conditions of sunshine without a careful choice of some parameters that affect in both conditions the performance of the algorithm for the MPPT. In this thesis has been addressed the analysis of an innovative current-based MPPT technique: the sensing of the current in the capacitor placed in parallel with the photovoltaic source is one of the innovative aspects of the proposal. The controller is based on a nonlinear control technique called ”sliding mode” of which has been developed an innovative model that allow to obtain a set of conditions and enable the designing of the controller with extreme simplicity. The model also allow to demonstrate how the performance of this MPPT control tecnique are independent not only from the characteristics and operating conditions of the photovoltaic generator, but also by the parameters of the switching converter that implements the control. This property allows a significantly simplification in the designing of the controller and improve the performance in presence of rapid changes of the irradiance. An approach to the dynamic analysis of a class of DC/DC converters controlled by a sliding mode based maximum power point tracking for photovoltaic applications has been also presented. By referring to the boost and SEPIC topologies, which are among the most interesting ones in photovoltaic applications, a simple analytical model is obtained. It accounts for the sliding mode technique that allows to perform the maximum power point tracking of the photovoltaic generator connected at the converters input terminals. Referring to the previous approach, a correction term allowing to have an increased accuracy of the model at high frequencies has been also derived. The control technique proposed has been implemented by means of low cost digital controller in order to exploit the potential offered by the hardware device and optimize the performance of the controller. An extensive experimental analysis has allowed to validate the results of the research. The laboratory measurements were conducted on prototypes of DC/DC converters, boost and SEPIC, carried out by Bitron SpA. There are a considerable experimental tests both in the time and in the frequency domain , both using source generator in laboratory than photovoltaic panels. The results and theoretical simulations have found a large validation through laboratory measurements. [edited by author]