Investigations of High Efficiency Wireless Power Transfer Systems (WPTS) for Electric Vehicles (EVs)

Abstract

Over the last decade, fossil fuel prices have significantly increased due to the dependency on hydrocarbon energy sources for transportation and electricity generation. In order to solve power generation issues, most governments in the world have heavily promoted the installation of roof top solar photovoltaic (PV) in domestic low voltage and commercial high voltage distribution networks. In addition, Electric Vehicles (EVs) have been introduced to substitute the hydrocarbon fuelled transportation which is required to provide high mileage and affordable prices. Currently, EVs have been charged with the utilisation of plug-in AC and DC chargers to charge their battery bank. To expand their range, EVs are required to have larger energy storage batteries, which leads to higher costs and limits their adoption in society. Furthermore, plug-in chargers require manual operation to connect to EVs, which may create health and safety issues such as electric shock and fire. Wireless Charging Systems (WCS) have the potential to minimise both these major concerns by offering frequent charge while the EV is in stationary or dynamic modes. Frequent charge to the EVs at the car park, traffic signal and on the roads brings indefinite charging options which can dramatically reduce the battery bank size. However, improvements in some of the challenging factors such as health and safety, power levels and power efficiency requires further investigation to create a user-friendliness of the WCS for EVs. This thesis deals with the investigation of concerning issues which are limiting the development of Wireless Electric Vehicle Charging Systems (WEVCS) from becoming a part of the electrified transportation system. Currently available wireless power transfer technology for the EVs has been studied including wireless transformer structures with a variety of ferrite shapes. WEVCS are associated with many health and safety issues, which have been discussed with the current developments in international standards. Two major applications; static and dynamic WEVCS, have been explained with up-to-date progress from research laboratories, universities and industries. A variety of laboratory prototypes have been developed with the help of calculation and simulation methods, and verified with experimental techniques. Firstly, High Frequency Wireless Planar Transformers (HFWPT) are used to investigate the flux leakages and other electromagnetic compatibility (EMC) problems which are associated with the wireless charging system’s efficiency. The HFWPT was designed using the bifilar winding concept on a PCB. An LLC resonant converter has been designed to improve the conversion efficiency with a maximized air gap. Assisted by a near-field scanner, the magnetic field has been analysed with and without a magnetic ferrite core at resonant frequency. The magnetic ferrite core in this arrangement is used to minimize flux leakages and to increase the magnetizing impedance. In addition, EMC computer modeling and simulation techniques are employed to investigate the magnetic flux distribution and associated EMC problems such as stray fluxes and hot spots. A finite element method (FEM) has been used to calculate the magnetic field. The effect of the planar magnetic ferrite cores on magnetic flux distribution has been investigated by using three designs. The first design has the ferrite core only at the primary side, the second draft has a planar core on the primary and secondary side, and the third design has a U-shape magnetic core for the primary and secondary side. A new proposed design is introduced to minimize flux leakages and reduce hot spots, in order to improve the flux distribution and to increase the magnetizing impedance. Poor considerations of leakage flux between the primary and secondary coils may cause complications for persons with a pacemaker or any other life supporting electronic devices. Two scenarios are investigated via computational simulation. Firstly, a simulation of a person with an electronic biomedical implant device standing beside a car during the charging process through the WCS is investigated. Secondly, a person is walking or standing over the primary coil area of the WCS when the system is not in charging mode. Both of these scenarios include an investigation of four different versions of magnetic core configurations, to examine the outer magnetic flux distribution as well as the power distribution of the WCS by using a FEM simulation. Another concerning issue is the lower power transfer efficiency of WCS for EVs in comparison to the plug-in due to the poor coupling between the transmitter and receiver charging pads. In order to solve the problem, in-wheel WCS for EVs have been introduced with a concept proven laboratory prototype, which can operate in static and dynamic applications. The coupling coefficient is dependent on the thickness of the tire rubber and the transmitter installation height underneath the road surface. A variety of scenarios have been applied to study the in-build steel belt (IBSB) tire effect on the wireless power transfer for the static and dynamic cases. FEM simulation has been performed to investigate the magnetic flux distribution and leakage fluxes due to IBSB in the vehicle’s tire. Finally, the Wireless Vehicle to Grid (W-V2G) concept has been presented to solve future instability issues on the distribution networks created by unscheduled feedback power from renewable energy sources (RES). In addition, W-V2G can provide a platform to transfer power wirelessly in both directions: grid to vehicle and vehicle to grid where the EV’s battery can be a back-up of additional energy storage to reduce the peak demand energy requirements. A 3.7 kW wireless transformer for a single phase W-V2G prototype, and a high efficiency compact filter inductor for a D-StatCom inverter in the three phase 30 kVA W-V2G have been built with the utilisation of calculation and simulation methods. Both prototypes have been constructed and validated with experimental methods. Currently, a 3.7 kW W-V2G prototype is under development, and will be finished in future with complete systems analysis and results.