Abstract
Magnetic resonant wireless power transfer (MR-WPT) has several advantages over conventional wired underground power supply methods. However, MR-WPT inevitably encounters metal conductors, which reduce the system efficiency through the eddy loss induced in the conductor by high-frequency electromagnetic waves. In this paper, the effect of resonant frequency variations caused by metal tube interference with the aim of maximizing the system efficiency is studied. According to the variation in the resonant frequency, the system efficiency is analytically derived by an equivalent circuit model. Electromagnetic simulations are carried out to further analyze the metal tube interference on the system. The results demonstrate the existence of an optimal resonant frequency that maximizes the system efficiency. Aluminum tube interference produces a lower optimal resonant frequency and higher efficiency than a 304 stainless-steel tube. When the metal tube is slotted, the optimal slot width and number of slots enhance the maximum efficiency and reduce the optimal resonant frequency and frequency drift. With slot widths of 2 and 8 mm, the system efficiency reaches ∼67% at 40.1 kHz and 55% at 48 kHz, respectively. Finally, different types of slotted tubes are fabricated, and the theoretical results are experimentally verified.
Highlights
At present, wired power supplies are typically employed for underground equipment
Several off-borehole measurement units are embedded in the soil lateral to the borehole ∼100 m underground, and these are powered by the Magnetic resonant wireless power transfer (MR-WPT) system
The TX-coil is wound on a metal tube equipped with in-hole measurement units, while the RX-coil is wound on a plastic sleeve
Summary
At present, wired power supplies are typically employed for underground equipment. this conventional approach has intrinsic defects, such as the risk of seal wear, electric shocks, short circuits, and so on. Wireless power transfer via magnetic resonance (MR-WPT) represents a promising alternative solution for power transfer and has numerous advantages compared with other WPT technologies. Recently, the applications of MR-WPT have expanded to underground fields with the development of safe and reliable charging, potentially eliminating open contacts and hanging cables. MR-WPT presents several challenges in terms of analysis and system optimization. When the system is running, the high-frequency electromagnetic (EM) waves generated by the TXcoil induce an eddy current in the tube, which reduces the coil’s self-inductance and mutual inductance and generates joule heating.11 This may lead to a decrease in the power transfer efficiency (PTE). For conventional MR-WPT systems with free space, it is possible to improve the system PTE, such as by adding a conductive shield and ferrite structure, using a multi-coil system, optimizing the design of the coil structure, establishing an impedance matching network, or applying frequency tracking control These methods require further verification, and the added control or switching circuits may significantly increase the complexity of the system and reduce the system PTE.. An MR-WPT system with an AL tube is developed and experimentally tested to verify the theoretical results
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