Power Converter Topologies for Electric Vehicle Charging Systems

Abstract

The need for the technologies of electrified transportation can not be overemphasized in the face of unprecedented levels of global carbon emissions. Over reliance on unsustainable fossil fuel supply has led to dangerous effects of global warming, greenhouse gas emissions and rise in ocean levels, amongst other issues. The transportation sector consumes 25% of total world produced energy, which makes the electrification of transportation, not just a vital focus in academic literature and a major drive in the industry, but also a significant boost to earth sustainability. However, while sales share of electric vehicles (EV) have increased from below 4% in 2020 to about 18% in 2023 and EV adoption is predicted to displace up to 5 million barrels of oil per day in 2030 as postulated by the International Energy Agency, the world wide market share of EVs remains abysmally low at just 14%. The key challenges and banes to global EV adoption are range anxiety, inadequate charging infrastructure, longer charging times, high cost of EVs, fewer available EV models, nonuniform EV standards, and low to zero sales outside major EV markets.
Shorter charging times, faster charging rates and convenience are essential requirements for accelerated EV adoption. In the literature, there are several approaches ranging from fast to ultra-fast charging system (upto 400kW) sourcing power from the combination of AC grid and DC power sources for electric-light-vehicles charging via AC port or DC port. These approaches employ multiple conversion stages, hence not cost effective and would incur higher losses. In addition, the availability of these AC and DC ultra-fast chargers is questionable and their installations in homes are not practical due to risks of grid overloading and instability. Electric vehicle supply equipment with public fast chargers account for less than 5% of all chargers, are capital intensive projects, and require long project timelines. More so, the over 85% of all chargers are private slow single-phase type-1 and type-2 chargers, which shows that EV owners value convenience and ease of access, a factor not considered in literature regarding faster charging techniques. As a result, a novel charging strategy with simultaneous use of both AC and DC ports is proposed in this thesis to increase charging rates, reduce charging time, improve mileage, integrate renewable resources and still retain the convenience and ease of access for EV owners. Also, an improved battery management system is developed to regulate the charging process and facilitate communication amongst the three entities of EV battery, AC onboard charger and the DC charger.
Furthermore, an alternative means to confront the issues associated with shorter charging times and faster charging rates is via higher power topologies such as the three-phase onboard AC chargers or offboard DC chargers. The offboard DC chargers have the fastest charging-power ratings due to lack of limitations on charging system size and weights. However, DC chargers are not readily available in today’s EV landscape since they can be found only in public charging stations which are not abundant. This makes them inconvenient and increases the frustration of EV owners that prefer easily accessible private chargers. Office places and homes now have three-phase AC chargers that can support upto 22kW, thereby improving charging rates and maintaining ease of access. Therefore, single stage three-phase AC onboard chargers using both SEPIC and Cuk-based switched inductor configurations have proposed in this thesis. The topologies support grid-to-vehicle and vehicle-to-grid bidirectional power flows, and has both boost and buck capacity. In addition, a convenience and safety-driven technique for emergency charging of EV battery using the EV auxiliary battery for short-range last resort during emergency situations have been proposed.

Date of Award	13 May 2024
Original language	American English
Supervisor	VINOD Khadkikar (Supervisor)