Dynamic Wireless Charging of Electric Vehicles

As a more convenient alternative to conductive charging technology, wireless charging is seen as a key technology drive for transportation electrification. In electric vehicle (EV) battery charging applications, wireless power is transferred through a magnetic link, so it is referred to as inductive power transfer (IPT). One advantage of IPT technology is that the charging of EVs can be fully automated. The recharge of traction batteries can start automatically when the EV stops where its receiver (Rx) coil is coupled with a transmitter (Tx) coil of an IPT charger. Apart from static charging applications, IPT technology can also be used to build dynamic charging roads where EVs can get charged in motion and the capacity of onboard batteries can be reduced. This thesis studied four challenges that should be addressed before dynamic IPT becomes mature enough for commercial use. The research topics focus on magnetic coupler design, prediction and control of transient behaviors, reduction of power fluctuation, and detection of EVs and foreign objects (FOs).

Magnetic coupler design

The key performance indicators of an IPT system include power transfer capability, power density, power efficiency, and misalignment tolerance. Due to conflicts among these performance indicators, it is indispensable to formulate the design of IPT charging pads as a multi-objective optimization (MOO) problem. By using finite element (FE) models, the magnetic field property of a coupler can be computed. However, calculating the aligned and misaligned power losses at the rated power requires not only the magnetic field property but also the compensation strategy. The compensation strategy determines the load match method which is used to calculate the optimal load condition and the rated winding currents. Therefore, compensation strategy should also be considered for the magnetic coupler design. With the magnetic field distribution known, the power losses in the AC link can be calculated through the existing analytical method.

This thesis develops a MOO method that can find the performance space from the design search space of magnetic couplers. In the performance space, Pareto fronts can be obtained under different conflicting optimization objectives. The study shows that analytically calculating the AC link power efficiency is possible when the magnetic field is accurately computed at the rated condition. More importantly, the DC-DC power efficiency of the final prototype reaches $97.2\%$ which proves that the MOO design is vital to make full use of IPT technology.

Prediction and control of transient behaviors

IPT systems require capacitive/inductive components to form resonant circuits on both sides to improve the power transfer capability and power efficiency, while the compensation components also make the resonant stage of a high order. As a result, the analytical dynamic models of IPT systems are complex and mostly impossible to solve in the time domain.

This thesis proposes a new reduced-order dynamic modeling method that describes the transient behavior of a resonant stage from the energy point of view. The order of the resultant dynamic model is one-fourth that of conventional ones for SS compensated IPT systems. Also, a MPC controller is designed based on the proposed dynamic model. It is proven that simplifying the dynamic model is helpful in explaining how circuit parameters influence transient behaviors and also in facilitating the application of advanced control strategies in IPT systems.

Reduction of power fluctuation

The most obvious difference between static and dynamic IPT is the change in magnetic coupling. In DIPT applications, the magnetic coupling fluctuates from the maximum to a usable level as EVs move, so one of the main challenges of DIPT is to stabilize the pick-up power, especially for DIPT systems using segmented Tx coils where magnetic coupling changes more frequently. The conventional methods are either to overlap Tx coils or to add extra sets of the Rx sides, which are expensive in building costs.

This thesis presents the design of a segmented DIPT system using a multiphase Tx side. The Rx coil consists of two sub-windings connected in series with a relatively large spatial offset in the EV moving direction. One advantage of the proposed design is that the Tx coils are deployed loosely so the building cost can be reduced. The other advantage is that the pick-up power is seamless with a small ripple. The pick-up power demonstrates a $24.9\%$ ripple by experiments.

Detection of EVs and FOs

To minimize the Tx side power losses and magnetic field radiation, the detection of EVs and FOs should be implemented in DIPT systems. Considering the integration of the detection equipment into the charging pads, PCB coils become the most suitable candidate to sense the magnetic field for detection purposes. However, the detection of EVs and FOs are mostly discussed separately in the literature. There is a need to achieve these two detection functions within one set of PCB coils.

This thesis presents the design of detection equipment consisting of PCB coils installed onto charging pads and the detection resonant circuit (DRC) connected to Tx side PCB coils. It can be concluded that the detection of EVs and FOs can both be realized by measuring the variation of the magnetic field caused by their intrusion, and PCB coils demonstrate good performances in measuring the change of magnetic field together with DRC to amplify the detection signals.