| Extrinsic Fiber Fabry-Perot Interferometer(EFPI)sensors are widely used in the research of partial discharge ultrasonic signal detection of power equipment because of their compact structure,strong anti-electromagnetic interference capability and high detection sensitivity.In this paper,the detection characteristics of EFPI sensor and its high sensitivity design method are studied to design an external EFPI sensor that can be applied to the outside of the tank wall to detect the partial discharge ultrasonic signals;the silicon diaphragm EFPI sensor with ultra-high sensitivity and high consistency is designed and prepared by using Micro-Electro-Mechanical System(MEMS)technology in conjunction with the high sensitivity design method of EFPI sensor.The narrow-band detection characteristics of the sensor are clarified based on the multi-beam interference principle and intensity demodulation principle of the EFPI sensor,combined with the basic structure of the diaphragm type sensor.Using the theory of fluid dynamics,a computational analysis is carried out for the problem of the reduction of the intrinsic frequency of the diaphragm caused by the additional mass,and the correlation between the reduction rate of the intrinsic frequency of the diaphragm applied to the insulating oil medium and the density,thickness and radius of the diaphragm medium is obtained and verified by simulation calculations and experimental studies.In order to improve the detection sensitivity of EFPI sensor for partial discharge ultrasonic signal in insulating oil medium,the interference spectrum,diaphragm size and directivity response characteristics of the sensor are studied,showing that the sensor interference spectrum and diaphragm size jointly restrict the sensor detection sensitivity,but with the increase of detection sensitivity,the directivity response characteristics of the sensor become more obvious;according to the actual situation of sufficient space inside the power transformer,the EFPI sensor with paraboloidal focusing characteristics is designed to realize the physical structure enhancement of the sensor detection sensitivity,and the experimental results show that the paraboloidal structure prepared by Plexiglas can improve the sensor detection sensitivity by one time.In view of the fact that the EFPI sensor cannot be installed inside the power transformer that has been put into production,an external laminated EFPI sensor that can be installed on the outside of the oil tank wall is proposed.The prepared sensor has a first-order resonance frequency of 107 k Hz.The detection sensitivity is similar to that of the built-in EFPI sensor and the piezoelectric(PZT)sensor.Combined with the partial discharge ultrasonic signal propagation characteristics of the liquid-solid-liquid three-layer composite interface,the external oil cavity coupled EFPI sensor is designed and prepared with a first-order intrinsic resonance frequency of 113 k Hz and a parabolic polyacoustic structure.When the sensor diaphragm is located near the parabolic focus,the detection sensitivity is similar to that of the PZT sensor.For the problem of poor consistency of EFPI sensor,the silicon diaphragm EFPI sensor is prepared using MEMS technology,the thickness of the sensor diaphragm is5?m,the effective constraint radius is 0.23 mm,the first-order intrinsic resonance frequency is 117 k Hz,the resonant peak drift of the sensor interference spectrum is15.3pm per 1? temperature increase,and the effective dynamic pressure test range is about ±245Pa,the sensor output signal characteristic is 18.62 m V/Pa,the consistency and sensitivity of the sensor are tested by using the plate-plate electrode partial discharge test platform and the real model partial discharge test platform based on80 k VA transformer.The result shows that the sensor has high consistency in the similarity of interference spectra and detection sensitivity,and the detection sensitivity of the partial discharge ultrasonic signal is at least 24 times that of the PZT sensor. |
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