Dynamic Modeling and Analysis of Buck Converter based Solar PV Charge Controller for Improved MPPT Performance

Abstract

Maximum power point tracking (MPPT) control is a key functionality in solar photo-voltaic (PV) based power generation systems that enhances the efficacy of energy extraction. A pulse-width modulated (PWM) power converter connected to the PV is designed to perform this essential function. A variety of MPPT control schemes are available in literature, many of which are voltage based techniques wherein the PV bus voltage is controlled in closed loop to the required level that achieves MPP tracking. However, a suitable plant model of the PV fed converter system that facilitates the design of such a PV voltage control loop for MPPT purpose, is not fully covered in literature. In this paper, a small-signal model is developed for a buck converter based charger system fed by a PV source, which is a nonlinear active source. Based on this model, the relevant transfer-functions are analytically derived. The control to converter input voltage transfer-function $\widetilde{v}_{i n} / \widetilde{d}$ thus obtained, is useful in the systematic design of voltage controller bandwidth, that facilitates the selection of perturbation period of a typical perturb & observe (P&O) MPPT controller. Such an approach for control design is verified in this work on a PV system with energy storage, which is typically used in dual-mode or standalone PV applications. The P&O MPPT control is validated on a buck PV charge controller system employing a 36 V battery bank. Experiments conducted on a 1 kW hardware prototype verify the accuracy of the proposed analytical transfer-functions and the performance of the PV-charger system.