PhD Oral Exam - Nil Rajeshkumar Patel, Electrical and Computer Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

The world is rapidly shifting towards electrification transportation due to proliferating price of fossil fuels (petrol or diesel) and awareness of global warming. Electric vehicles (EVs) have emerged and consistently supplant conventional internal combustion (IC) engine-based automobiles to decrease greenhouse gas emissions which bringing environmental and economic benefits to society. The EVs that are on road majorly comprise of two-wheeler, E-rickshaw, intralogistics equipment, trio, golf carts, short range mobility vehicle, which come under the category of low power EVs and passenger buses, loading trucks, electric trucks, which fall under the category of high power EVs. Batteries are the primary source of energy to power these EVs. Therefore, to recharge these batteries a battery charger is required. State-of-the-art research on EV battery chargers puts increased emphasis on enhancing the power density and efficiency of the power conversion.

The conventional EV battery chargers are developed with two cascaded power conversion stages. The first stage converts the AC voltage to a DC voltage by employing power factor correction (PFC). The second stage is an isolated DC-DC converter stage. These two-stages are interconnected by DC-link capacitors. This two-stage battery charger suffers from low overall efficiency due to two different power conversion stages and higher component count. Also, these battery chargers have complex control system, low input power quality, and inflexible charging options. At the same time, the power density is limited due to the inevitable presence of intermediate DC-link capacitors. Generally, high-value electrolytic capacitors are selected for the DC-link. More importantly, the battery charger is placed close to the internal combustion engine under the hood in the case of a plug-in hybrid EV (PHEV), where the ambient temperature is more than 150ºC. The electrolytic capacitors are most susceptible to failure at high ambient temperature, thus the reliability of the conventional two-stage EV battery charger is low in a high-temperature environment. Moreover, the existing research on battery chargers is mainly based on voltage-fed power converter topologies, where the feasibility of current-fed power converter topologies has received very limited attention. Considering all the abovementioned drawbacks, this thesis work proposes and studies a family of novel current-fed isolated single-stage bidirectional power converter topologies with novel soft-switching modulation techniques for EV battery charging applications to address the shortcomings of the conventional two-stage battery chargers.

In this thesis, at first the proposed novel bidirectional single-phase single-stage matrix-based AC-DC PFC configuration. The proposed topology achieves PFC and DC voltage regulation with a single-stage which offers a lower component count and higher efficiency. In addition, the converter power density and reliability are improved due to the elimination of the intermediate DC-link capacitor. The converter enables zero current commutated (ZCC) without any active clamp circuit or passive snubbers, which significantly reduced switching losses, footprints, and cost. The proposed modulation strategy and control technique are presented for promising soft-switching operation of all semiconductor devices throughout the range for all modes with bidirectional power flow capability. The operation of the proposed converter is demonstrated in the developed converter hardware prototypes, enabling grid-to-vehicle (G2V) and vehicle-to-grid (V2G). The idea has then been extended to a bidirectional current-fed push-pull single-stage topology. This converter configuration has a compact structure with only a single inductor topology with two 4-quadrant switches which enhances the overall system efficiency. In detail, the thesis outlines the steady-state operation and design equations of these converters. Validation of the analysis, design, and performance is supported by simulation results obtained from the PSIM 11.04 software and experimental results from the 1.5 kW lab-prototypes. Finally, to enhance the charging flexibility to mitigate the range anxiety, a bidirectional current-fed dual active bridge (CFDAB) isolated DC-DC converter has been analyzed and designed for V2V charge transfer. A novel charge transfer technique has been implemented using the CFDAB. The proposed secondary modulation technique naturally clamps the voltage across the low voltage side current-fed devices with ZCC. The same concept has been widened to a current-fed push-pull (CFPP) isolated topology. These converters have been analyzed and studied to combat range anxiety, and which have the advantages of simpler gate control requirements, high power density, and lower conduction losses. The converters steady-state operation and design equations are reported in detail. The simulation results from PSIM 11.04 software and the experimental results from 1.5 kW proof-of-concept laboratory hardware prototypes are provided in order to validate the report analysis, design, and performance.