Abstract

Ever-increasing fossil fuels consumption in recent decades has emitted tremendous amounts of greenhouse gases, a big part of which cannot be absorbed by natural processes happening in nature. These gases have increased earth temperature by absorbing extra radiations from sunlight and turning them into heat. Global warming has had terrible effects for all creatures around the world and can threat life on Earth in future. Utilization of green or renewable energies during the recent years is getting more popular and can be a solution to this serious problem. A big source of these pollutants is transportation sector. Electrification of transportation can noticeably reduce greenhouse gasses if the electricity is obtained using renewable energy sources. Otherwise, it will just shift the problem from streets to fossil fuel power plants. Electric vehicles (EVs) were introduced around one century ago; however, they were replaced by internal combustion engine cars over time. Nevertheless, recently they are getting more interest because of their superior performance and clean operation.
Solar electricity which can be obtained using photovoltaic panels is one of the easiest ways as long as sun is available. They can be easily mounted on the roofs of buildings or roof tops of parking slots generating electric power to charge the battery pack of the EVs while providing shade for the cars. Since solar energy is intermittent and variable, power grid should be involved to ensure enough power is available. Conventional solar chargers inject power to the grid and use grid as the main source because of its reliability and being infinite. Hence, they use grid as a kind of energy storage system. This approach can lead to problems for grid stability if solar panels are utilized in large scales and comparable to the grid.
In this work, a solar powered grid-tied EV/PHEV charger is introduced which uses all the available power from PV panels as the main energy source and drains only the remaining required power from the grid. The proposed configuration provides great flexibility and supports all the possible power flows. To design an efficient system the load should be known well enough first. A comprehensive study has been done about behavior, characteristics and different models of different chemistries of batteries. Specific phenomena happening in battery packs are outlined. A novel maximum power point tracking (MPPT) technique has been proposed specifically for battery charging applications. A specific configuration involving DC link coupling technique has been proposed to connect different parts of the system. Different possible topologies for different parts of the proposed configuration have been considered and the suitable ones have been selected. Dual active bridge topology is the heart of this configuration which acts as the bidirectional charger. A detailed state space modeling process has been followed for the power converters and various small signal transfer functions have been derived. Controllers have been designed for different power converters using SISO design tool of Matlab/Simulink. Different modes of operation for the charger including constant current mode (CCM) and constant voltage mode (CVM) have been analyzed and appropriate cascade controllers have been designed based on required time domain and frequency domain characteristics. Finally, simulation tests have been conducted and test results have been graphed and analyzed for different modes of operation, all possible power flows and various voltage and current set points.