| Wireless Power Transmission(WPT)technology can transmit electrical energy in a non-contact manner,with flexibility,safety and wide application advantages.It has been widely used in electronics,medical and industrial fields.Since WPT systems need to operate in the high-frequency region to ensure high transmission efficiency,there are inevitably problems such as skin effect and high-power switching frequency,resulting in increased wire resistance and switching losses.Therefore,the realization of efficient high-power WPT under low-frequency conditions has become a research topic of great scientific significance and practical value.In traditional WPT systems,the quality factor Q of copper coils under low-frequency conditions is low,resulting in weak electromagnetic coupling between the transmitting and receiving coils,and low system transmission efficiency,making it difficult to achieve high-efficiency high-power wireless power transmission.High-temperature superconducting(HTS)materials have the characteristics of high current-carrying density,zero DC resistance and low AC loss.They can be used for low-loss transmission of large currents and become ideal materials for improving the overall performance of the WPT system.This thesis focuses on the application and frequency characteristics of hightemperature superconducting coils in magnetically coupled resonant WPT systems.Through theoretical analysis,experimental verification and simulation,the relationship between the working frequency of WPT systems with different materials and the output power,transmission efficiency and superconducting coil loss have been studied.And the frequency range of high-temperature superconducting coil performance advantages was explored,and it provides a reference for the design and optimization of hightemperature superconducting WPT system frequency.The main work and results of this thesis were as follows:(1)A theoretical analysis of the WPT system in series-series(SS)topology based on circuit theory and two-port network theory was carried out to derive the expressions for the system output power and transmission efficiency.And the effects of coil resistance and operating frequency on the transmission power and efficiency of the system are analyzed.(2)The basic performance of high-temperature superconducting coils and copper coils under different frequency conditions was experimentally studied.The results show that the quality factor Q of high-temperature superconducting coils is always higher than that of copper coils.Especially at low-frequency conditions,the copper coil quality factor Q is too low to effectively transmit energy,while the superconducting coil still maintains a high Q value,so it can be used to achieve efficient low-frequency wireless power transmission.(3)Based on the two-port network and circuit model,independently designed and built WPT active power(load state)experimental system and reactive power(no-load state)experimental system,and experimentally studied the transmission characteristics of superconducting and copper WPT systems with different operating frequencies.The results showed that the use of high-temperature superconducting coils in the traditional WPT system can achieve a greater maximum output power at a lower operating frequency,and the transmission efficiency of the system was also greatly improved.This study provided a solution for WPT systems to achieve efficient high-power transmission at low frequency conditions.(4)For the high-frequency AC loss problem of superconducting coils in hightemperature superconducting WPT systems,the AC loss characteristics of hightemperature superconducting strips were simulated based on the finite element method.The results of the simulation study showed that the AC loss of the superconducting strip increases with the increase of the external field frequency when the high-temperature superconducting strip is in an alternating external magnetic field or transmitting an alternating current.When the frequency was low,the current was mainly concentrated in the superconducting layer,and the loss was mainly the AC loss of the HTS layer;when the frequency was high,the current density distribution moved toward the two ends of the strip due to the skin effect,and the current density of the copper layer increased,and the loss was mainly the Joule loss of the copper layer.Therefore,in the high-temperature superconducting WPT system,the system should operate in the lower frequency range in order to take full advantage of the superconducting coil. |
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