Finite-Element Multiphysics Simulation Analysis On GMM-FBG Current Transformer

Employing the ferrite material as the magnetic conduction loop,a simple structure current transformer is designed by combining the super magnetostrictive material(GMM)and the fiber Bragg grating(FBG)sensing technology,in which the magnetic-strain characteristics of ferrite-GMM current detecting configuration and FBG wavelength modulation are investigated with the considerations of temperature and GMM characteristic size through multiphysics field finite element simulations and analysis.The static harmonic magnetic field analysis is carried out by calculating the magnetic field,hysteresis and eddy current loss in GMM which are induced by alternative current through ferrite-magnetizer and varying with the characteristic geometric size of GMM component.The finite element force field simulations are performed to analyze the intrinsic resonant frequency and dynamic frequency-response characteristics of GMM rods with various geometric sizes according to the modal analysis theory.The effect of GMM characteristic size on its resonant frequency is elucidated and the frequency bandwidth of detecting current is improved by optimizing the design structure of GMM rods in combination with the electromagnetic field analyses.The stability of amplitude-frequency response are evaluated to avoid resonance of GMM-FBG current transformers during current detection.Through the magnetostatic field and solid mechanics coupling multiphysics simulations,the magnetostrictive strains in GMM rods of the current-detecting ferrite-GMM constitute which are generated by DC current excitation are analyzed with a steady state method.The variations of magnetic flux and magnetostriction of GMM rod during the quasi-static rise of DC current are simulated to demonstrate the sufficient and stable magnetostrictive performance deriving from the uniform strain field in the dominant region of GMM rod.Through the finite element analysis of electromagnetic field and temperature(thermotics)coupling simulations,the influence of thermal strain and magnetization suppression caused by altering temperature on GMM magnetostriction and refractive index of FBG center in GMM-FBG current transformer are analyzed to reveal and verify the underlying mechanism of the temperature drifting of the Bragg-wavelength signal modulated by GMM-FBG constitute.The soft magnetic ferrite as high resistance magnetism-conducting material is exploited in magnetizer circuit design to avoid the eddy current loss that is inevitable in traditional metal silicon steel materials.Furthermore,the magnetic flux induced by measuring current can be more efficiently assembled to improve magnetic field injection into GMM rod than utilizing silicon steel as magnetizer material.The residual magnetic moment after saturated magnetization,which is selectable due the multifarious of ferrite materials,make it feasible to provide a sufficient bias magnetic field for GMM working in linear region and thereby prevent the double frequency phenomenon in the detection of alternating currents.On both sides of the air interval gap which is designed for GMM free magnetostriction between GMM rod and ferrite-magnetizer,the GMM terminal and the magnetic pole surface of magnetizer are designed to be hemispherical convex and concave shapes respectively,leading to ameliorated magnetic flux introduction into GMM rod and reduction of magnetic leakage.Therefore,the magnetic flux density into GMM will be further enhanced to promote the measurement sensitivity of GMM-FBG current transformer.This dissertation presents a simple optimization schemes for GMM geometry and magnetizer materials based on the simulation analysis of electromagnetic field and mechanical response characteristics of ferritemagnetizer-GMM assembly unit.Temperature-altering-caused variations of GMM characteristic performance and thermal strain in GMM rod are investigated to explore the temperature-drift mechanism of FBG Bragg wavelength.The present research provides theoretical basis and experimental direction for developing GMM-FBG current transformers with high sensitivity and stability.