Design and Implementation of High Gain Frequency Non-isolated DC Optimizers for Modular PV Application

In order to fulfill the increasing worldwide electricity demand, renewable energy sources have long been considered as a very attractive solution. Besides the shortage in the conventional energy sources, rising impact of the global warming and the climate change create the need for an alternative electricity supply with the lowest possible greenhouse gas emissions. Among the other renewable energy sources, photovoltaic (PV) energy is argued to be one of the most promising future clean energy source for its no carbon emission, pollution free operation, low maintenance cost, lack of noise and easy installation. Therefore, the PV energy has been foreseen as a major contributor in the worldwide electricity generation in a near future. Since 2010, the more PV capacity has been installed worldwide than in the previous four decades. The high demand and the several benefits of the PV technology brings the motivation for further research to utilize the solar power more efficiently. The PV research nowadays emphasis on improvement of the system efficiency over a wide range of operating conditions with more cost-effective way than the existing system. By following the trend, this PhD dissertation looks into an efficient way of the PV power conversion by designing and developing the novel power converters to improve the energy yield of the entire PV system. This dissertation proposes high efficient, compact, cost-effective DC-DC converters for the distributed PV application. In the PV power generation, the power loss due to the partial shading is a well-known problem in the prior art. It causes power mismatches between the PV outputs and reduces the total energy yield. To address the power mismatch problem and the fluctuations of the PV output, distributed PV power generations are considered as a potential solution. The benefits of the distributed power generation and the different types of configurations of that are discussed in this thesis. Among different options, DC-DC micro-converters (or also known as DC optimizers) are selected for further research as it is one of the most promising solution. Since DC distribution networks are newly emerging technology in the PV market, DC-DC micro-converters are also useful to integrate in a DC distribution system. In this work, two DC micro-converters are proposed for targeting two different applications, one is mainly for a series-connected distributed system and another is for a parallel-connected system. Another topology of a micro-converter is proposed, which is beneficial in terms of its wide input, output voltage range. It will help to track the generated PV power in a wide operating range. In order to deal with the PV output variations and the power mismatch problems, this work proposes the design requirements of a DC-DC micro-converter for the distributed PV application. These proposed requirements can be used as a guideline for the developments of a PV converter. By considering that, a non-isolated multiplier boost topology has been selected. The detail selection procedure and the suitability of the topology for this specific application are explained in the following chapters. A 200W high frequency non-isolated DC-DC converter is designed and built for the sub-module level power conversion, mainly for a series-connected system. Since it is a high frequency converter, few measures are taken into account during the development of the converter. The different steps from design to implementation are described in detail in the thesis. The measurement result of the converter performance, efficiency measurement results at full power range and the thermal measurement results are provided here. The converter is further tested with the high band gap gallium nitride (GaN) switches in order to investigate the efficiency improvement of the converter. Next to that, a wide input range, high gain, high efficiency non-isolated converter is developed to use as a module-integrated converter for the parallel-connected system. The converter is made to interface between a PV module and a DC bus as it can directly boost from the low PV voltage to a high DC bus voltage (360V). The converter is further integrated in a real-time PV system set-up and tested under static and dynamic irradiance for the different operating conditions. Finally, a novel DC-DC converter’s topology is proposed for a wide input, output voltage range. Different novel ideas are proposed to increase the voltage gain range and the input-output voltage range of a non-isolated multiplier converter. This topology is applicable for any kind of fluctuating voltage sources. All the presented converters are proposed to facilitate the PV energy harvesting and to increase the overall energy yield of a PV system.status: publishe