AC loss modelling in high temperature superconducting coils for EV wireless power transfer application

Abstract

With the rapid development of high temperature superconducting (HTS) technology, new second generation (2G) HTS materials are promising to replace the traditional materials in the power transmission industry. Since the 2010s, the topic of using HTS materials in electric vehicles (EVs) wireless power transfer (WPT) systems has received widespread attention from research groups around the world. When cooled down to a particular cryogenic temperature, superconductivity offers near-zero resistance to electric current. Meanwhile, using superconducting material in different power applications can improve the overall effciencies. In the WPT system, HTS coils could achieve high quality factors and power densities compared to traditional resonant coils such as copper and litz wire coils, which lead to a decreasing in coil size and weight. However, the optimal working frequency for the EVs WPT is signi cantly higher in terms of kilohertz level. HTS coils working under high frequency transport current could generate AC losses, thus increasing the cooling power and reducing the overall effciency of the WPT system. This thesis aims to study the AC loss characteristics and coil design optimisations for HTS coils working at EVs WPT frequency range when subjected to various input conditions.

At the outset of this thesis, the first chapter reviews the EVs WPT systems and its current research combining the HTS technology. Subsequently, a review of the superconducting theory and the AC loss fundamental are provided in the second chapter. Following the literature review, the fi nite element modelling method for the HTS coated conductor (CC) simulated in this thesis is presented. The fi nite element modelling method used in this thesis is developed based on a set of equations referred to as the H formulation to solve the magnetic fi eld component. To perform a thorough investigation of the AC loss characteristics and the electromagnetic properties in the HTS coils at EVs WPT frequency range, the multi-layer numerical model, which considers all layers within the HTS CC is constructed in this thesis. As a starting point, the two-dimensional (2D) HTS tape model is built. The single tape model is then extended to the 2D axisymmetric HTS coil model. The simulation results from the HTS tape and coil models are validated by analytical calculations and the published measurement data from the literature at both low and high frequency ranges. Subsequently, the model-building process of three commonly used HTS coil con figurations and the simulation results concerning the transport loss, magnetisation loss and total loss under high frequency transport current, external magnetic field and combined are presented and discussed in detail. In particular, the weak turn analysis within different coil con figurations and the transition frequency study under these conditions are investigated. Consequently, an efficiency model is also proposed to evaluate the performance of three HTS coil confi gurations under WPT conditions, followed by a discussion regarding three coil optimisation designs.

Finally, two AC loss mitigation methods are studied and analysed. The AC loss characteristics and the turn loss distributions for three HTS coils at different inter-turn spacings and tape widths are simulated and discussed. Further, the impact of these two loss reduction methods on the performance of the WPT system is estimated and evaluated based on the efficiency model. The outcome of this research can be used to provide information for designing HTS coils with lower AC losses for EVs WPT applications.