Design and Development of High Efficiency and High Voltage-Gain Module Level Converter for PV Systems

Abstract

Grid-connected photovoltaic (PV) system has received a great attention as a clean source of energy thanks to the development of advanced grid-connection technologies. This utility-interactive PV systems are rapidly growing, of which the power interfaces are commonly classified as the central, string, and module-integrated inverters or microinverters. The performance of the conventional string or central topology is drastically reduced by partial shading effect or any mismatch condition. The module-integrated inverter is superior to other technologies of PV converter in terms of obtaining the highest maximum power point tracking (MPPT) accuracy on each PV module during partial shading and clouding effects. Hence, the module level inverter is more-popular in small scale PV system like residential and commercial rooftop PV system. The challenge for module-integrated inverters lies in efficiency, voltage gain, reliability, and cost, which are not as competitive as the traditional solutions. Significant research is focusing the existing issues and trying to find the best solution. The module-integrated inverters presented in the literature can be classified into isolated and non-isolated type with respect to the presence of galvanic isolation. Isolated types are found to be more preferable in terms of reliability and transferring higher quality of power to the grid. However, the efficiency of the isolated converters degrades due to high frequency transformer and high switching losses. Therefore, increasing the efficiency of the PV converter maintaining higher lifetime and lower cost is the most critical job to form a reliable module-integrated inverter. This thesis proposes and investigates new topologies and techniques to overcome the limitations of the existing literature and to achieve high efficiency and high voltage gain isolated module-integrate inverters for PV applications. The proposed topologies optimally integrate the technology of resonant circuit, adaptive modulation scheme, and active clamping to enhance soft-switching capability and system efficiency. A voltage-doubler circuit with the flyback converter is realized to produce the high voltage-conversion gain that optimally minimize the high demand of the winding turns ratio in the flyback transformer. Furthermore, a bidirectional converter with high voltage gain is proposed to accommodate low-voltage-rated battery packs for grid support. This modular solution is based on a parallel infrastructure that enhances the overall system reliability. The operating principles, theoretical analyses, design considerations, control schemes and loss analyses are performed for all of the proposed topologies. The effectiveness of the converters is analysed by simulation and justified by experimental evaluation of a 250 W laboratory prototype. The proposed solutions have demonstrated maximum efficiency of 97.8% and 97.1% for module-integrated inverter and bidirectional converter respectively.