MPP-Tracking DC-DC Converters in Photovoltaic Applications - Implementation, Modeling and Analysis

Abstract

This thesis provides a comprehensive study of switched-mode dc-dc converters in terms of dynamic characterization, implementation and operational constraints. The study concentrates on the maximum-power-point-tracking converters used in photovoltaic interfacing in order to maximize the power generation. The main objective is to emphasize the differences between the four basic conversion schemes and to analyze the dynamical characteristics of the current-fed converters. Harvesting of solar energy directly into electrical energy is known to be challenging due to the highly varying terminal characteristics of the energy harvesting unit, i.e., generally a photovoltaic generator. The environmental conditions as well as the load determine its characteristics. The operation point of PV generator has to be kept at its maximum-power point in order to maximize the harvested energy. The dc-dc switched-mode converters are conventionally used to interface the energy sources possessing constant-voltage behavior such as batteries or voltage buses but the same converters are also used as a basis for the maximum-power-point-tracking devices. It is observed, however, that such voltage-fed converters are not capable of interfacing a photovoltaic generator optimally because of its current-source nature. Therefore, an additional capacitor is usually added between the source and converter to enhance the constant-voltage properties of the source and to enable the input-voltage-based feedback control. It is known that the dual nature of PV generator enables the use of either input-voltage or input-current feedback control. It is observed, that the input-current control is prone to saturation of the controller and therefore, the input-voltage control is recommended to be used. The use of input-voltage control will, however, force the converter operate as a current-fed converter causing also significant changes to its static and dynamic properties as well as constraints related to the voltage-fed converters in photovoltaic interfacing. This thesis presents the generalized dynamic representation of a switched-mode dc-dc converter which can be applied to analyze all possible conversion schemes. The representation makes a clear distinction between the voltage-fed and current-fed conversion schemes and their stability assessment. It is also shown that the current-fed converters can be implemented by applying three different methods based either on intuition, explicit duality transformation or adding a capacitor at the input terminal of a conventional voltage-fed converter. The study includes dynamic characterizations that have not been presented earlier.