Author: Lyu, Jiahua
Title: Numerical impedance extraction of Litz wire using a multi-level PEEC method
Degree: Ph.D.
Year: 2023
Subject: Electric lines
Electric resistance -- Mathematical models
Wireless power transmission
Mobile geographic information systems
Hong Kong Polytechnic University -- Dissertations
Department: Department of Building Environment and Energy Engineering
Pages: xviii, 144 pages : color illustrations
Language: English
Abstract: Electrical and electronic devices are integral to our daily lives, from powering our homes to operating the smart devices we rely on. In recent years, there has been a growing trend toward using inductive connections in industrial and consumer elec­tronics, as opposed to traditional conductive connections. Inductive power transfer (IPT) offers greater convenience and has been applied in consumer electronics, elec­tric vehicles, and robots. IPT is preferred in specific harsh industrial environments due to safety considerations, as there is no risk of electric shock from exposed wires or connectors in wet or dusty.
Litz wire is an electronic component commonly utilized for IPT couplers operating in the frequency range from dozens of kilohertz to megahertz, as it helps minimize the power loss caused by eddy current effects through its twisted structure. These frequency-dependent effects result in an increase in resistance, and consequently, the power transfer efficiency is affected at high frequencies. Thus, it is essential to calcu­late the impedance of the Litz wire accurately while designing couplers for IPT.
In industrial electronic applications, selecting the appropriate Litz wire coupler can be challenging due to the high requirements for accuracy and speed in calculations. Even slight errors in impedance assessment can lead to difficulties in evaluating the efficiency and designing compensation circuits. Estimating the impedance of Litz wire presents a challenge due to its complex structure, consisting of hundreds of strands twisted together. Analytical and numerical methods are commonly used to calculate the impedance of Litz wire in existing research works.
However, existing analytical methods are unable to accurately detect the impedance due to imprecise characterization of the wire’s complex structure, leading to errors caused by wire parameters. Although numerical methods provide better accuracy, they can be computationally costly for complex Litz wire structures and may strug­gle to model structures with hundreds of Litz wires. Additionally, using numerical methods in optimization algorithms can be challenging due to the significant number of evaluations required. Further research is needed to develop more accurate and efficient algorithms, such as novel analytical or numerical methods that can accurately capture the complexity of the Litz wire structure or employ intelligence algorithms to aid in modeling and optimization.
These challenges can be addressed by focusing on two main approaches. The first ap­proach involves developing more advanced optimization methods that integrate with the partial equivalent element circuit method (PEEC), which is a commonly used numerical method for Litz wire assessment. Although the PEEC method is more accurate than analytical methods, it can be time-consuming and require significant computational resources to solve the impedance. Therefore, developing more intel­ligent optimization methods can help reduce the computational burden while main­taining accuracy. The thesis first proposes a solution based on adopting intelligence optimization frameworks into the traditional PEEC method.
• A multi-level optimization framework named multi-fidelity optimization em­bedding adaptive trust region (MF-TR-SAO) is first proposed. MF-TR-SAO enables FastLitz, a tool based on the traditional PEEC method for Litz wire calculation, to be optimized for Litz wire design. The proposed optimization framework has proved to be more efficient than the commonly used optimization algorithms and, together with better optimization results.
The second approach involves exploring alternative methods for modeling the Litz wire structure, which can lead to more simplified and efficient numerical calculations. For example, using simplified lumped-element models or using the symmetry of the coupler can significantly reduce the complexity of the PEEC method, making it easier to optimize the IPT coupler design. The thesis contributes to the approaches by proposing novel PEEC methods for the impedance extraction of Litz wire and coil, respectively.
• A novel approach for Litz wire assessment based on the PEEC method is pro­posed. The proposed method outperforms other existing methods in terms of efficiency through a novel meshing scheme and multi-level simplification method. Our approach can accurately simulate electromagnetic fields while minimizing the computational resources required.
• A numerical algorithm based on the PEEC method named LitzImp is proposed for the Litz wire coil impedance calculation. The method could deal with the effect of the external magnetic field. The algorithm provides different simpli­fication schemes according to the symmetric structures of circular coils and rectangular coils. The accuracy is verified by experimental measurement, and the results are compared with FEM and analytical methods. The comparison proves that our proposed method has higher accuracy. LitzImp has also proved to be dozens of times faster than the existing PEEC-based method.
Apart from the contributions to the algorithms for Litz wire design, some investigation has been done on the uninsulated twisted wire (UTW). The difference between Litz wire and UTW is that there is an insulation layer outside the single strand. However, there is still a lack of research on the performance of UTW. This thesis contributes to the analysis of UTW by the PEEC method. The contribution is:
• The concept of uninsulated twisted wire (UTW) was first proposed for power electronic applications. The work introduces a novel and effective method to calculate the impedance of UTW based on different circuit connections of the novel PEEC method, which offers high accuracy and simplicity. The study also examines the impact of different twisting patterns of both Litz wire and UTW. The results of this investigation provide valuable insights for the selection of suitable wires in power electronic design.
Overall, the work first achieves the optimal design of the Litz wire simulated by a traditional PEEC method. Then, the novel PEEC methods are proposed for the effective impedance calculation of Litz wire and coil, respectively. In the end, the concept of UTW is presented, and its performance is also demonstrated by the novel PEEC method, which has a different circuit connection scheme from Litz wire. The work in the thesis might have significant potential for a wide range of applications in the field of electromagnetics and power electronics.
Rights: All rights reserved
Access: open access

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