PhD Oral Exam - Suvendu Samanta, 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.
Wireless Inductive Power Transfer (IPT) technology promises a very convenient, reliable, and safe way of transferring power wirelessly. Recent research on IPT establishes its indispensable role and suitability in electric vehicle (EV) applications. Efficient design of both converters and IPT coils are essential to make this technology feasible for mass deployment. The existing research on IPT is mainly based on power converters derived from voltage-source inverter (VSI) topologies, where feasibility of current-source inverter (CSI) has received very limited attention. Considering certain limitations of voltage-fed converters, this research is focused on the concept study and feasibility analysis of current-fed power electronics for IPT systems, where the primary application is EV charging.
CSI leads to parallel LC resonance in the primary side of IPT. The advantages of the parallel tank networks include lower inverter device current stress, very close to sinusoidal coil current, soft-switching of inverter devices, and natural short circuit protection during fault etc. Considering these merits, a new IPT topology is proposed in this thesis, where the inverter is full-bridge CSI and the compensations in primary and secondary sides are parallel and series types, respectively. Compared with the existing IPT topology with current-fed push-pull inverter, the proposed system does not have startup and frequency bifurcation issues. However, due to weak coupling between IPT coils, the primary side parallel capacitor experiences high voltage stress in higher power levels, and this voltage directly appears on inverter devices. To overcome this, a modified IPT topology fed from a CSI is proposed, where the primary compensation is parallel-series type and secondary compensation is series type. Detailed steady-state operation, converter design, soft-switching conditions, small-signal modelling, and closed-loop control are reported for both the topologies. To verify analytical predictions, numerical simulation is performed in PSIM 10 and experimental results obtained from a 1.6kW lab-built prototype are reported.
Considering the requirement of bi-directional power flow capability to support energy injection from vehicle to grid (V2G) for future smart-grid applications, a new bidirectional IPT topology with current-fed converter is proposed. It has current-sharing feature in grid side converter and voltage doubling feature in vehicle side converter. This is the first attempt to implement bidirectional IPT with current-fed circuit and demonstrate grid to vehicle (G2V) and V2G operation. Keeping inverter output power factor lagging, ZVS turn-on of the inverter devices are always ensured irrespective of load variation. Detailed steady-state operation and converter design for both G2V and V2G modes are reported. Experimental results obtained from a 1.2kW lab-prototype are reported to verify the analysis and performances of bidirectional IPT circuit.
The last part of this thesis addresses the possible improvements on reducing the number of power conversion stages to achieve higher system efficiency, compact size and reduced cost. This is usually done by using direct ac-ac converter as the primary side converter of IPT. Existing single stage IPT topologies are derived from VSI topology. From source side, these topologies have buck derived structure; therefore, none of them draw high quality current from source. In this thesis a new single stage IPT topology is proposed, which has boost derived structure and thereby capable of maintaining unity power factor at source. Dynamic load demand, source current waveshaping and effective wireless power transfer are achieved with two-loop control method. Experimental results obtained from a 1.2kW grid-connected lab-prototype are reported to justify the suitability of this single-stage IPT topology for practical use.