Theoretical And Experimental Studies Of Current Sensing Based On Giant Magnetostrictive Materials And Fiber Grating

With the growth of the capacity of the power system, the role of relay protection system hasbecome increasingly prominent, current measurement is the key to relay protection, energymetering, system monitoring, system analysis in the power system, the measurement accuracyand reliability is directly related to the power system safe, reliable and economic operation. Thetraditional electromagnetic current transformers have drawback of immunity to electromagneticinterference, oil flammable and explosive, insulation structure complex, high cost, smalldynamic range, which constrained their development and application to some extent. Fiber opticsensor has a small size, light weight, high sensitivity, immunity to electromagnetic interference,sensing and transmission at the same time, easy making and buried inside the material,high-precision measurement of structural stress, strain, temperature and magnetic field. In thispaper, a current sensing system based on giant magnetostrictive materials and fiber grating isdesigned and the system model is established too. The giant magnetostrictive materials havelarge magnetostrictive, wide frequency bandwidth, fast response speed. The influence oftemperature, prestress, bias field to the system’s sensing characteristics is analyzed, and theexperimental study of the sensing system is also carry out.The magneto-mechanical-thermo coupling characteristics of giant magnetostrictivematerials are analyzed according to the experimental data of the giant magnetostrictive material.A nonlinear magnetostrictive model which can describe the magneto-mechanical-thermocoupling characteristics of giant magnetostrictive materials has been proposed according toelastic Gibbs free energy, Wiss ferromagnetic theory, thermo-dynamics relations andJiles-Atherton model. The proposed model gives the mathematical relation among themagnetostriction, magnetic field, magnetization, stress and temperature for magnetostrictivematerial.The parameters referred to the model can be determined through experimental methods.The model can predict magnetization, magnetostriction, magnetic susceptibility, piezomagnetic coefficient curve with the applied magnetic field under different temperature and prestress, andcan also to reflect the saturation characteristics and “flip effect” under the influence of differentprestress. The proposed model can be used to calculate the magnetostrictive characterization ofgiant magnetostrictive materials in different temperature and under different prestresses. There isa good prospect in engineering practice.The magnetization and magnetostriction characteristics of Terfenol-D rods under differenttemperature and prestresses were tested by use of multi-parameter magnetic test system. Theresults show that the effect of prestress on magnetization process and the saturationmagnetostriction is significant, prestress causes the saturation magnetostriction increasesignificantly, and the effect of temperature on magnetization process and magnetostriction in thelow magnetic field is very weak, but significant in saturation field. The comparison between thecalculation results of the nonlinear model and the experimental results shows that the proposedmodel can be used to calculate the magnetostrictive characterization of giant magnetostrictivematerials in different temperature and under different prestresses. The experimental results havea certain significance of the theoretical study of the giant magnetostrictive material and providebasis for the sensor and transducer’s design, optimization and application.A high-sensitivity current sensor is designed based on magnetostriction effect and fiber-loopcavity ring-down technique. A mathematic model of the magnetic field sensor is presentedaccording to the Jiles-Atherton model, quadratic magnetic domain rotation model and thedemodulation principle of fiber-loop cavity ring-down technique. The model quantifies thecoupling mechanisms among the current，magnetic field, strain and the ring-down time of thefiber loop. The strain variation of a Terfenol-D rod, the ring-down time of the fiber loop and thesensitivity of the sensor with the magnetic field are analyzed and simulated under differentprestresses. Results show that the sensitivity of the sensor is very high and the measurementrange of the magnetic field sensor can be changed by changing the prestress. The study result canprovide a theoretical foundation for the development of new high-sensitivity current sensors.An AC current sensing system is designed based on giant magnetostrictive material andfiber Bragg grating.The model of AC current sensing system is presented according to strainsensing mechanism, nonlinear model of giant magnetostrictive materials, demodulation principleof fiber Mach-Zehnder interferometer. The model is able to reflect the coupling mechanisms among the current，the center wavelength of fiber Bragg grating and output intensity of fiberMach-Zehnder interferometer. For different bias current, the operating characteristics of ACcurrent sensing system under the influence of prestress are theoretically analyzed and simulated,results show that the bias current and prestress will affect the operating characteristics of ACcurrent sensing system based on giant magnetostrictive material and fiber Bragg grating,therefore, the influence of the bias current and the prestress needs to be considered in the designand application of AC current sensor and the suitable value of bias current and prestress shouldbe selected.