DC-DC Converters in Distributed Photovoltaic Electricity System - Analysis, Control and Design

Abstract

This thesis presents a comprehensive review on switched-mode converters in terms of dynamic behavior and practical limitations that arise from the fundamental properties of the electrical sources and loads, control engineering principles and topological properties of the converters. The main focus is on analyzing the behavior of a single converter used to interface a photovoltaic generator into a high-voltage dc link. The main objective is to introduce interfacing principles with numerous examples and a thorough discussion. The interfacing of photovoltaic generators by means of switched-mode converters has proven to be problematic according to numerous scientific publications indicating operational disadvantages and anomalies. The output characteristics of the photovoltaic generator, which are bound to varying environmental conditions, introduce design challenges. It has been recognized recently that the photovoltaic generator does not contain similar electrical behavior as conventional electrical sources, most notably due to its limited-power characteristics, yielding two distinctive operating regions. Yet, the constraints arising from the properties of the source have not been completely recognized, although the effect of these constraints can be seen from the published research results. When switched-mode converters are used to adapt individual photovoltaic modules into larger system by connecting converters in series or in parallel, severe operational limitations are observed. On the other hand, if the photovoltaic generator is substituted with a source that does not contain similar characteristics, observations may lead to misconclusions as the effect of the photovoltaic generator is not properly modeled. Therefore, claims that are not valid for actual applications with photovoltaic generators may be presented and widely accepted. This thesis presents methods to perform proper analysis of switched-mode converters implemented in distributed photovoltaic applications, by continuing previous work around the subject (Leppäaho, 2011). The dynamic models for series-connected and parallelconnected systems of interfacing converters are given, explaining the observed operational anomalies. Additionally, it is shown by a thorough review that the parallel configuration does not contain the claimed disadvantageous properties and actually provides better performance. A patented converter topology designed for the parallel configuration is presented with comprehensive analysis and practical validation. Finally, the problematics of photovoltaic interfacing is summarized under the interfacing constraints, which give guidelines for design and analysis of interfacing converters.