Research On SF₆ Gas Humidity Sensor Based On M-POFBG

As an excellent insulating medium and arc extinguishing medium,SF₆ gas has been widely used in the insulation field of HV electrical equipment.However,the moisture content in SF₆ gas will decompose corrosive substances under the action of arc,which seriously affects its insulation performance and service life.Therefore,it is very important to detect the gas humidity in electrical equipment.At present,the moisture content detection methods used in power systems at home and abroad mainly include electrolysis method,dew point method and resistance-capacitance method.These methods are not suitable for online monitoring and have limitations on the measured environment.The fiber grating has the characteristics of small size,corrosion resistance and on-line monitoring,and the micro-structure fiber has the characteristics of flexible structure.Therefore,this paper proposes the use of micro-structured polymer Bragg grating(m-POFBG)sensor to detect SF₆ gas humidity.In this paper,based on the hygroscopic characteristics of polymethyl methacrylate(PMMA)material to micro-structure polymer fiber grating sensor(m-POFBG)sensor as the main research object to detect the humidity of SF₆ gas in high voltage electrical equipment.First of all,because the polymer PMMA is sensitive to humidity,and the polymer PMMA has the characteristics of hygroscopic expansion and dehumidification and contraction,according to the sensing characteristics of the fiber grating,the volume and refractive index of m-POFBG sensor will change when humidity changes.The relationship between the refractive index,size and humidity of m-POFBG sensor was established through the hygroscopicity and shrinkage and expansion rate of PMMA.This provides a theoretical basis for the simulation analysis of m-POFBG sensor in the following paper.Then,the equivalent refractive index method was used to establish the relationship between the relative humidity and the equivalent model of m-POFBG sensor,and the relationship between the core radius,cladding air hole radius,grating period,grating length,grating modulation depth and the central wavelength and reflectivity of the reflection spectrum of m-POFBG sensor was studied.The influence of structural parameters of microstructure fiber and fiber grating on m-POFBG spectrum is analyzed.It is proved that the radius of cladding air hole,the radius of fiber core and the grating period are related to the central wavelength of m-POFBG,the length of grating is related to the reflectivity of m-POFBG sensor,and the modulation depth of grating is related to the central wavelength of m-POFBG.It is also related to the reflectivity of m-POFBG,which provides theoretical support for the sensitivity study of m-POFBG sensor.In this paper,m-POFBG sensor is used to carry out humidity detection experiment on SF₆gas in low humidity environment.By changing the content of desiccant in glass bottle,different low humidity gas samples are obtained.The m-POFBG sensor is placed in a glass bottle to detect gas samples with different low humidity.The experiment proves that m-POFBG sensor is sensitive to SF₆ gas with low humidity,and m-POFBG sensor can monitor humidity.However,the sensitivity of the m-POFBG sensor is low in the low humidity range.In order to m-POFBG sensor is widely used in real life,it is necessary to explore the low humidity sensing performance of m-POFBG sensor.Finally,starting from the resonance condition of m-POFBG,the relationship between the sensitivity of the m-POFBG sensor and the core radius,the half-length of the air hole in the cladding,the period of the grating,the length of the grating and the modulation depth of the grating is studied.It is proved that the sensitivity of the m-POFBG sensor is related to the core radius.The radius of the cladding air hole,the grating tuning depth and the grating period are related to the central wavelength of m-POFBG sensor,but have no obvious effect on the sensitivity of m-POFBG sensor.It provides a theoretical basis for improving the sensing performance of m-POFBG sensor and designing m-POFBG sensor.