Detection Of Trace Acetylene Gas Concentration Based On All-optical Photoacoustic Spectroscopy

Power transformer is an important power system device.The normal operation of transformer plays a vital role in power system.The stable and safe operation of power system has a direct impact on the daily life of residents and the normal production of industrial enterprises.Therefore,online detection of the safety of power transformer preventive fault becomes very important.The characteristic gas produced in the dissolved oil is usually inflammable and explosive.The conventional detection method is of high cost and cannot be monitored in real time.At the same time,there is the risk of deflagration.The concentration of dissolved gas in transformer oil is detected by the whole photoacoustic spectroscopy,which has the advantages of high sensitivity,dynamic monitoring and safety.In this thesis,the concentration of acetylene gas produced by the fault of oil-filled transformer is detected,the relationship between the amplitude of sound pressure signal and gas concentration is theoretically established based on the mechanism of photoacoustic signal generation,and a new ellipsoid photoacoustic cell is designed by optimizing the structure of the photoacoustic cell.And compared with the traditional cylindrical photoacoustic cell analysis,using COMSOL software of thermoacoustics module respectively to ellipsoid photoacoustic cell and the traditional cylindrical photoacoustic cell are analyzed in the finite element method,the acoustic characteristics of the model is established,and the resonance frequency of photoacoustic cell,photoacoustic cell inside the cavity acoustic pressure distribution of acoustic characteristics such as the simulation research.The relationship between the resonant frequency and the geometric parameters of an ellipsoid photoacoustic cell is simulated.Therefore,the size structure of the photoacoustic cell is optimized,and an ellipsoidal photoacoustic cell with a length of 100 mm and a center radius of 5 mm is selected.We set up all-optical photoacoustic spectroscopy gas detection system,according to the acetylene gas distribution in the near infrared absorption spectrum,select 1532.83 nm spectral line as absorption spectrum,finally choosing tunable distributed feedback(DFB)semiconductor laser as light source,using the method of in Fabry-Perot(F-P)acquisition of fiber optic acoustic sensors to sound finally has carried on the experimental measurements to acetylene gas.The performances of cylindrical resonant photoacoustic cell and ellipsoid resonant photoacoustic cell in the whole photoacoustic spectrum system are compared.The analysis data verified that the signal-to-noise ratio of the ellipsoid photoacoustic spectral system for the detection of acetylene gas was 23dB,and the detection limit sensitivity was 2.17pbb;the signal-to-noise ratio of the ellipsoid photoacoustic spectral system for the detection of acetylene gas was 34dB,and the detection limit sensitivity was up to 1.47pbb,with better performance.The comparison shows that the performance of ellipsoid photoacoustic cell is better than that of cylindrical photoacoustic cell.