Research On Electromagnetic Characteristics And Applications Of Superconducting Electrode Capacitors

Superconducting materials have some merits of small conduction loss,big critical current density,and so on,due to their excellent electromagnetic properties and great application potentials.In recent years,superconducting application technology has made brilliant progress with the development of high-temperature superconducting materials.In principle,superconducting electric devices have lots of advantages including low operation loss,big power capacity,small device size,and so on.However,their advantages are usually offset by cooling costs in real-world applications.These cooling costs come in a large part from the power loss of current lead wires which mainly includes conduction loss and leakage heat loss.If all required superconducting devices are integrated into a cryogenic adiabatic container to form an “all superconducting system”,lead wires across different environments can be reduced as much as possible,thus avoiding the unnecessary part of relevant power loss essentially and promoting the economy of superconducting application technology.Currently,superconducting materials are applied mainly in the form of inductance coil to most kinds of superconducting devices.From the angle of basic circuit elements,in order to form an all superconducting system,existing superconducting devices which are generally inductive,tend to call for the cooperation of capacitive superconducting devices,namely superconducting electrode capacitors(SECs).Due to the merits of creating new functions with existing superconducting devices and avoiding the power loss of some lead wires,the systematical study of SECs would make great sense to upgrade the functionality as well as economy of superconducting application technology.Based on this consideration,this thesis researches the electromagnetic characteristics of SECs and the applications of SECs to wireless power transfer,power line communication and contactless measurement,by means of theoretical analysis,simulation calculation,and experimental test.Chapter 1,the topic introduction of this thesis presents superconducting application technology including the evolutions of superconducting materials and superconducting electric devices.Secondly,it introduces the development and status of SECs,and the research of superconducting wireless power transfer(SWPT).The content of major study and the arrangement of following chapters are described at the end.Chapter 2 presents the material choice of SECs,and then analyzes their electromagnetic characteristics theoretically,including the relevant equivalent circuit model,impedance characteristic parameters,influence of applied environment magnetic field,and basic characteristics of hybrid SEC.Chapter 3 carries out a testing research on several types of SECs including superconducting electrode fixed capacitor(SEFC),superconducting electrode variable capacitor(SEVC),and hybrid SEC.Respective study contents consist of relevant prototype fabrication and system setup as well as measurement curves and result analysis.Considering that an SEC tends to work in an environment of strong DC magnetic field while cooperating with other superconducting devices,its characteristics are investigated in an applied DC magnetic fied.Chapter 4 analyzes and studies the applications of SECs to SWPT technology.Depending on the resonant frequency requirements of being fixed or tunable,two types of superconducting LC-resonators are developed by using the abovementioned SEFC and SEVC,respectively.It is found by comparing the experiment results between the SEFC and normal electrode contrasting capacitor that the SEFC can improve the SWPT efficiency further.To solve the shortcircuit problem between a SEVC and a superconducting coil,a forming mode of tunable superconducting LC-resonator is proposed.The experiment results also confirm that the SEVC can provide a frequency-tuning functionality for SWPT technology.Chapter 5 proposes a superconducting power line communication(SPLC)technology based on SECs.According to two typical structures of normal line traps,a single-frequency superconducting line trap(SLT)and a wideband SLT are developed,respectively,based on SECs.Experiment results present that two SLTs have low resistances at power frequency;while the single-frequeny one can offer higher blocking impedances at high frequency,the wideband one can offer a stable stop band without the influence of variable temperatures.In addition,hybrid SECs can be utilized as coupling capacitors to transfer the carrier signals wirelessly through bilayer metal walls,thus achieving the wireless cooperation between cryogenic HTS devices and outer normal devices.Relevant experiment results confirm the feasibility of SPLC technology based on SECs.Chapter 6 proposes a contactless measurement technology based on hybrid SECs.For the contactless measurement of voltage and current,a voltage transformer and a current transformer are developed,respectively,based one hybrid SECs.Relecant experiment results verify the feasibility of the proposed technology.This technolog can also derive a type of liquid nitrogen insulated switchgear,thus achieving the wireless cooperation between superconducting power grid and normal relay protection technology.In addition,the hybrid SECs of voltage transformers can also serve as the coupling capacitors of SPLC technology mentioned in Chapter 5,which can strengthen the interaction between two technologies to form a transboundary-wire-free,full-superconducting,and highly-integrated high-frequency protection scheme for power grid.