Research On Key Technologies Of Magnetic Fluid LPFG Current Sensor Applied In Power Equipment

As the national energy industry,the power industry is the lifeblood of the national economic development.The country's power demand continues to expand as China's national economy continues to grow rapidly,and the current level of national grid operation continues to rise,so that current sensors that play a role in power measurement,system monitoring,and relay protection for power equipment are particularly important.Traditional electromagnetic current sensors that are widely used are increasingly exposed to many shortcomings such as difficult insulation,poor transient response,high cost,low safety,etc.,which restricts the development of power systems,and fiber-optic current sensors have low cost,safe insulation.The advantages of fast signal transmission are more suitable for the complex environment of the power system,and it is an excellent choice to replace the traditional electromagnetic current sensor.At present,there is still a certain gap between China's research on the key technologies of fiber optic current sensors and foreign countries,so research on fiber optic current sensors is of great significance to the development of China's power industry.In this paper,an analysis of the advantages and disadvantages of mainstream fiber optic current sensors proposes a fiber optic current sensing method that combines a magnetic fluid(MF)material with magnetron refractive index characteristics and a long period fiber grating(LPFG)with environmental refractive index sensitivity.The current sensor combined with magnetic fluid and LPFG has the advantages of small size,simple sensing structure,high sensitivity,no hysteresis effect,stable performance and high safety.In this dissertation,by studying the advantages and disadvantages of mainstream fiber optic current sensors,a new type of fiber optic current sensor is proposed,that is,a sensor structure that combines magnetic fluid(MF)materials with long-period fiber grating(LPFG).The magnetic fluid material has obvious magneto-optical effect,and its refractive index changes with the change of the magnetic field;the long-period fiber grating has the sensitivity to the environmental refractive index,and the resonance wavelength drift is affected by the environmental refractive index.The LPFG packaging structure covered by the magnetic fluid is placed in the magnetic field environment around the energized conductor,and current sensing can be realized by monitoring the resonance wavelength of the LPFG.This sensor has the advantages of small size,simple structure,high sensitivity,no hysteresis effect,stable performance,safe insulation and so on.In this dissertation,firstly,the transmission characteristics of long period fiber gratings are analyzed using coupled mode theory and transmission matrix theory respectively.The transmission spectrum of LPFG is obtained by Matlab simulation of LPFG coupled mode theory.The theoretical simulation of LPFG resonance wavelength is used to obtain the amount of LPFG resonance wavelength change and the environment.The relationship of the refractive index is that when the environmental refractive index is closer to and not greater than the refractive index of the grating cladding,the resonance wavelength is more sensitive to the environmental refractive index.Then,based on the theoretical analysis of the magnetic control characteristics of the refractive index of the magnetic fluid,from the magnetic field size,direction and ambient temperature to study the change characteristics of the refractive index of the magnetic fluid.Based on the Fresnel reflection principle,a micro-reflective sensor structure for measuring the refractive index of the magnetic fluid was designed.An experimental system for measuring the refractive index of the magnetic fluid was built.The experimental results show that: when the light transmission direction is parallel to the magnetic field,the magnetic fluid refractive index changes with the magnetic field and has the largest fluctuation range and is closer to the refractive index of the grating cladding,while the temperature has a relatively weak influence on the magnetic fluid refractive index.Then,the LPFG and magnetic fluid were combined to design and fabricate the magnetic fluid LPFG current sensing structure.The current measurement experimental system was built using the energized solenoid as the measured object.The magnetic field of the energized solenoid model was finite element by COMSOL software.Through analysis,the magnetic field distribution characteristics of the energized solenoid coil are obtained,and the position of the sensor in the solenoid is set.The sensitivity of the current sensor is about-2.3nm / A.Finally,in order to avoid the influence of resistance heating on the LPFG resonance wavelength,the temperature sensor is improved by adding FBG to the sensor structure to monitor the temperature.The temperature sensitivity of the LPFG and FBG in the current sensor is obtained by temperature experiment.The temperature sensitivity of the LPFG wavelength is compensated by the FBG wavelength drift.Experiments prove that the temperature compensation scheme improves the stability of the magnetic fluid LPFG current sensor.In this dissertation,the theoretical and experimental research on the magnetic fluid LPFG current sensor will lay a solid foundation for further practical application in the future.