Research On Transformer Magnetic Field Sensor Based On Tapered Microfiber

As the core equipment of power grid transmission and distribution,the power transformer may cause huge economic losses and social impact once it failed.Traditional protection schemes were difficult to effectively respond to early faults such as transformer winding deformation.When the winding deformation or interturn short circuit fault occurred in the transformer,the distribution of the leakage magnetic field would change accordingly.Therefore,the early warning of the transformer fault could be realized through the online monitoring of the leakage magnetic field of the transformer,which provided the original data for the active protection of the transformer.The traditional electromagnetic magnetic field sensor had large volume,poor anti-interference ability,low sensitivity,and was difficult to achieve distributed monitoring.The optical fiber magnetic field sensor used light as the carrier of sensing information,which has a simple structure,high sensitivity,and is convenient for remote monitoring.It had broad application prospects.Firstly,based on the transmission characteristics of tapered microfiber and the magnetic properties of magnetic fluid,the mechanism of magnetic field sensor of tapered microfiber coated with magnetic fluid was studied.Based on the optical fiber mode theory,the sensing equation between the transmission spectrum of the sensor and the measured magnetic field was established.The results showed that the change of the refractive index of the evanescent field on the surface of the cone region would cause the linear shift of the transmission spectrum,which could be used to indirectly measure the magnetic field.The influence of fiber structure parameters on the transmission characteristics of the sensor was studied.The results showed that the sensitivity of the magnetic field sensor was related to the waist diameter.When the waist diameter was greater than the dispersion turning point,the sensitivity of the sensor decreased with the increase of the waist diameter.Due to the inherent magnetic saturation characteristics of the magnetic fluid,the dynamic range of the sensor depended on the saturated magnetic field of the magnetic fluid.The fabrication method of tapered microfiber magnetic field sensor was studied,and the sensing characteristic test experiment was designed.The above conclusion was verified in the experiment.Secondly,in order to optimize the test performance of the tapered micro fiber magnetic field sensor and eliminate the interference of temperature change on the experimental results,the temperature sensitivity characteristics of the magnetic fluid sensor were studied,and the magnetic field temperature cross-sensitivity decoupling method of the magnetic fluid sensor was designed.The results showed that the interference wavelength of the magnetic fluid coated tapered microfiber magnetic field sensor shifts with the increase of temperature,and the sensitivity to temperature could reach 1.115nm/℃.After introducing the cross-sensitivity matrix,the maximum measurement error of the magnetic field measurement was reduced from 12.4mT to 2.5mT,and the average measurement error was reduced from 5.26mT to 1.47mT,which reduced the interference of temperature factors on the magnetic field sensor and reduced the measurement error.Finally,the packaging scheme of the tapered microfiber magnetic field sensor was designed in this paper.The tapered microfiber structure coated with magnetic fluid was encapsulated on both sides with insulating cardboard to form a ’sandwich’packaging structure.The feasibility of the ’sandwich’ type packaging structure was verified by designing the sensing experiment.The test results showed that the transformer oil environment did not affect the sensing performance of the tapered microfiber magnetic field sensor,which proved the theoretical feasibility and engineering practicability of the magnetic field sensor.