Design of a Misalignment Tolerant Control Scheme for a Bidirectional Inductive Power Transfer System for Electric Vehicles

In modern society the increasing energy demand, the depletion of fossil fuels, and the environmental polution which comes with it, are one of the biggest global problems. Therefore the transition towards a renewable transport and energy system is becoming more and more important. Electric vehicles are a big part of this transition and therefor the use of them should be promoted. However drawbacks such as operating range and charging time are obstacles for this transition. To help in this transition wireless charging has been investigated in recent years. In this thesis the design and implementation of a misalignment tolerant control scheme for a bidirectional inductive power transfer is discussed. This control scheme allows the transformer coils to be misalignment while ensuring that the maximum power transfer efficiency of the inductive link is tracked. To do this the resonance frequency of the primary or secondary current is tracked. Furthermore two dc/dc converters are used before the inverter and after the rectifier, here one is used for controlling the power while the other is tracking the maximum power transfer efficiency point (MEPT). In order to calculate this MEPT, the coupling of the transformer coils is calculated using the primary (or secondary, depending on the direction of power) dc link currents and voltages. The first part of thesis is a literature review including an investigation of the dynamics of a series-series resonant tank, part of this analysis is about the bifurcation phenomena. Since for control purposes it is important to determine the conditions for bifurcation free operation and the effects of bifurcation. Next, the best control scheme for controlling the output power is discussed and finally the proposed misalignment tolerant control scheme is proposed including how to estimate the transformer coupling and how to track the resonance frequency. It was found that using the MEPT control scheme bifurcation is always avoided. The second part of this thesis about the dynamic modelling and the design of the controllers. This dynamic model is comprised out of three different models, two of which are the dc/dc converters and one is the inductive link (specified from inverter input to rectifier output). These models are then combined in order to get the frequency response of the entire system. Based on this model the voltage controllers are designed. The third and final part of this thesis is about the practical implementation of the needed hardware en the results obtained using the MEPT control scheme. In the end an improvement in efficiency of 5\% was achieved at optimal alignment, up to 23\% increase under 8 cm misalignment. The total system efficiency at optimal alignment was 80\%.