V-Shaped Cavity-Enhanced Raman Spectroscopy And Characteristic Gases Measurement Of Power Transformers

The concentration of transformer faults characteristic gases（N₂、O₂、CO₂、CO、H₂、CH₄、C₂H₆、C₂H₄、C₂H₂）is the key indicator to reflect the fault and aging state of an oil-immersed power transformer.Research on the advanced detection method for multigas measurement with high sensitivity and accuracy is of great significance to ensure the safe and reliable operation of oil-immersed power transformers.As a spectroscopy gas sensing method,Raman spectroscopy can measure multigas simultaneously with a single-wavelength laser.Besides,Raman spectroscopy has the advantages of high selectivity,anti-aging,no cross-interference between different gases,and no consumption of samples.However,the application of Raman spectroscopy for gas sensing is seriously restricted by its insufficient sensitivity and accuracy.In this thesis,V-shaped cavity-enhanced Raman spectroscopy for transformer characteristic gases measurement is studied.The optimal parameters of the V-shaped cavity are calculated;A gas detection apparatus based on V-shaped cavity-enhanced Raman spectroscopy is developed.The Raman characteristics of the main characteristic gases of transformers are studied;A multi-factor calibration method with internal standard gas for quantification was established.The details are as follows:1.The cavity-enhanced Raman spectroscopy based on the V-shaped cavity for multigas measurement is developed.The optical stability condition and the resonance condition of the V-shaped cavity are calculated.The mathematical model of Raman scattering intensity output from a V-shaped cavity is established.The optimum parameters of a V-shaped cavity are calculated,including the cavity length,the curvature radius of cavity mirrors,the reflectivity of cavity mirrors,etc.The optical-feedback frequency-locking method for the V-shaped cavity is studied.The influence of the feedback phase of feedback light on the intracavity laser intensity is obtained.The effect of the feedback rate on the Raman resolution was studied.2.Based on the theoretical calculation of V-cavity for Raman spectroscopy,a gas detection apparatus based on V-shaped cavity-enhanced Raman spectroscopy using to measure the transformer faults characteristic gases is designed and developed.The power gain factor of 2200 is achieved.The gases（N₂、O₂、CO₂、CO、H₂、CH₄、C₂H₆、C₂H₄、C₂H₂）are measured qualitatively,and the Raman spectra are obtained.The characteristic Raman peaks of each gas are determined.With 200 s exposure time and 1atm total pressure,the detection limits for gases（N₂、O₂、CO₂、CO、H₂、CH₄、C₂H₆、C₂H₄、C₂H₂）are 3.9,2.8,0.76,2.0,0.79,0.25,0.32,0.35,and 0.27μL/L respectively.3.The partial pressure characteristics,laser power characteristics,and temperature characteristics of Raman peaks of the transformer faults characteristic gases（N₂、O₂、CO₂、CO、H₂、CH₄、C₂H₆、C₂H₄、C₂H₂）and internal standard gas（SF₆）are acquired.The intensity（peak height or peak area）of characteristic Raman peaks of gases have an excellent linear relationship with the gas partial pressure and the laser power.The scattering intensity and the Raman shift of characteristic peaks of gases decrease linearly with the increase of temperature approximately.Based on the experiments of mixed gas,we found that the change of gas composition does not affect the partial pressure characteristics,laser power characteristics,and temperature characteristics of every single gas.4.A multi-factor correction quantitative method with internal standard gas is established.The detection characteristics（detection limits,accuracy,repeatability,etc.）of the V-shaped cavity-enhanced Raman spectroscopy gas detection apparatus are acquired.The quantification model for gas measurement by Raman spectroscopy is established.The influence of laser power fluctuation,temperature change,detector performance fluctuation,and optical path change are eliminated.The accuracy of nine gases is higher than 95%.The measurement of the transformer faults characteristic gases by V-shaped cavity-enhanced Raman spectroscopy with high accuracy is realized.In conclusion,this thesis provides the foundation for online monitoring of the transformer faults characteristic gases by cavity-enhanced Raman spectroscopy.Besides,the V-shaped cavity-enhanced Raman spectroscopy developed in this thesis is also suitable for other trace gases measurement,which expands the application of Raman spectroscopy in the field of trace gas detection.