| The content of dissolved fault characteristic gas in transformer oil is the essential characteristic quantities reflecting the fault and aging status of power transformers.Photoacoustic spectroscopy(PAS)gas detection technology is an effective technology to achieve this characteristic quantity.At present,the commonly used photoacoustic spectroscopy detection method has a large gas demand,the use of electric microphones to detect the Photoacoustic(PA)signal is susceptible to interference from the electromagnetic,which restricts the good application of PAS.This thesis carried out the research of dual resonance and multi-path PAS detection of transformer fault characteristics based on all-optical demodulation.This thesis studied the generation mechanism of PA signal,analyzed the influencing factors of PA signal intensity,optimized the design of PA cell.This thesis studied the all-optical demodulation method based on Fabry-Perot interference intensity demodulation.Aiming at the problem that the working point(Q point)in this method is easily to drift and the measurement is unstable,the Q-point self-tracking demodulation method based on wavelength tuning is developed.And the study of active cavity length control demodulation method based on cavity length tuning is developed too.This thesis studied a dual-resonant PAS gas detection platform based on a cantilever FPI optical fiber acoustic sensor.This thesis also studied a multi-reflection enhanced PAS gas detection platform based on a diaphragm FPI optical fiber acoustic sensor.The detection characteristics of four typical fault characteristic gases of transformers are experimentally studied.The main tasks of this thesis are:1.In-depth analysis of the PA signal detection principle,the study of the PA signal's analytical solution taking into account the sound energy loss and various parameters affecting the intensity.The analytical relationship between the cell constant,resonance frequency,and quality factor of the first-order normal mode PA cell and the size of the PA cell is established.This thesis built a simulation model to verify the accuracy of the analytical solution of the PA signal.The influence of the buffer chamber on the PA signal is studied.With the optimized design for the resonant PA cell,the simulation calculation results show that under the premise that the PA signal loss does not exceed 20%,the gas demand of the PA cell is reduced to 20%of the original.2.This thesis studied the all-optical demodulation method of PAS based on Fabry-Perot interference intensity demodulation.Aiming at the problem of unstable measurement caused by the drift of the Q point of this method.This thesis studied the Q-point self-tracking intensity demodulation method based on wavelength tuning by stabilizing the DC component of the interference signal,and the active cavity length control demodulation method by changing the temperature of the interference cavity and tuning based on the cavity length.The laboratory built a corresponding demodulation module and conducted a stability test.3.Based on 1 and the Q point self-tracking intensity demodulation method of 2,this thesis designed a resonant PA cell with optimized buffer chamber structure,and a cantilever beam optical fiber acoustic sensor.This thesis established a dual-resonance enhanced PAS gas detection method based on cantilever beam optical fiber acoustic sensing and built a platform.With erbium-doped fiber amplifier power enhancement,wavelength modulation and harmonic detection,further improve the detection signal-to-noise ratio.The lower limit of stable detection of 4 typical fault transformer characteristic gases C2H2,CH4,CO,and CO2 is:2.0×10-3,5.2×10-2,1.1,1.4(unit:?L/L),which can meet the needs of off-line analysis of trace gas PAS laboratory.4.This thesis studied a multi-path PA signal enhancement method for PAS.Based on 1 and the active cavity length control demodulation method of 2,this thesis designed a multiple ring-shaped reflections cavity PA cell,and a membrane optical fiber acoustic sensor.This thesis established a multi-reflection enhanced PAS gas detection method based on membrane optical fiber acoustic sensing and built a platform.The lower limit of stable detection of 4 gases C2H2,CH4,CO,and CO2 is:1.4×10-2,0.15,4.2,7.6(unit:?L/L).And the ability of the platform to resist vibration and noise interference is verified through experiments,which can meet the needs of on-line monitoring of trace gas detection by PAS.The research in this thesis proposes dual resonance and multi-path all-optical demodulation method for the PAS detection of the characteristic gas in transformer oil,which is expected to effectively improve the technology of PAS gas detection. |
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