The Study Of Transformer Oil-Dissolved Gases Detection Based On Optical-feedback V-shaped Cavity Enhanced Absorption Spectroscopy

Large oil-immersed power transformers especially ultra-high voltage power transformers are the hub of power transmission and transformation system. The safe service of which directly influence the security and stability of large electric network. Dissolved gases analysis(DGA) of operating transformers has been accepted worldwide as one of the most effective method to judge incipient faults of oil-immersed power transformers. The key to achieve transformer condition maintenance is online and live analyzing oil-dissolved gases of operating transformers. Nowadays, as online detection methods of transformer oil-dissolved gases have problems on complicated structure, poor stability, high failure-rate, aging and heavy maintenance workload, the State Grid Corporation has proposed definite demand for promoting researches on and applications of live detection technology. Combined with the subtopic of Special Funds for the Development of National Major Scientific Instruments and Equipment(2012YQ16000705) and the cooperative project from Engeering and Physical Sciences Research Council(EP/E018297/1), this dissertation conducted research on detection of transformer oil-dissolved gases based on optical-feedback V-shaped cavity enhanced absorption spectroscopy(OF V-shaped CEAS) technology. In-depth research had been performed on detection mechanism of trace-gas based on absorption spectroscopy, and on improvement mechanism of detection sensitivity based on V-shaped cavity. The platform for detecting transformer oil-dissolved gases based on absorption spectroscopy was established. And the gas detection characteristics of C2H2, CH4, C2H4, C2H6, CO, CO2 and H2 were studied. The major achievements were as follows:① The basic principle of gas detection based on absorption spectroscopy was studied. The four elements of the absorption line position, absorption line intensity, absorption lineshape and the effective absorption path were determined for achieving qualitative and quantitative gas analysis. From the rotational level, vibration level and vibration-rotational level of diatomic molecules and polyatomic molecules, the mechanism of possible absorption line position and relative line intensity of gas molecules had been studied for the coupling of electric dipole transitions and electric quadruple transitions. The absorption line of H2 corresponding to the coupling of electric quadruple transitions exits and the feasibility of detecting hydrogen based on absorption spectroscopy had been proved.② The spectrum parameters of transformer oil-dissolved gases(C2H2, CH4, C2H4, C2H6, CO, CO2 and H2), such as the position, intensity, and full width at half maxium(FWHM) of absorption line, were analyzed. And the position and intensity of the strongest absorption line had been determined. The effects of temperature on the intensity and FWHM of absorption line, density of gas molecules and the absorption coefficient were analyzed. The characteristic absorption lines for oil-dissolved gases(in a position of characteristic absorption line, one and only one gas has absorbing effect) were obtained.③ The questions of frequency locking in V-shaped cavity, narrowing effect of laser linewidth was studied. The detection platform of transformer oil-dissolved gases based on OF V-shaped CEAS was designed and established. The improvement mechanism of detection sensitivity and accuracy of V-shaped enhanced cavity was demonstrated. Based on normalized absorption coefficient, the gas quantitative analysis method of OF V-shaped CEAS had been derived. The selection, function and debug of the platform hardware parts including quantum cascade laser and its drive control, V-shaped enhanced cavity, piezoelectric transducer, infrared detector, and gas-mixing system were analyzed. The software modules including data acquisition, Savitzky-Golay denoising, frequency locking with piezoelectric transducer, normalized peak picking, and cavity ring-down time measurement, were constructed.④ The threshold current lowering effect and odd and even modes effect of OF V-shaped CEAS C2H2, CH4, C2H4, C2H6, CO, CO2 and H2 array detection platform were revealed. Gas detection characteristics of C2H2, CH4, C2H4, C2H6, CO, CO2 and H2 were tested. The method for improving detection accuracy based on increasement of spectrum resolution and least-square regression analysis was introduced. The method for improving detecting sensitivity based on Allan variance analysis and increasement of gas detection pressure was introduced. Highly accurate and sensitive detection of C2H2, CH4, C2H4, C2H6, CO and CO2 were achieved. Quantitative analysis of H2 were achieved with high accuracy.In conclusion, this dissertation did research on the gas detection mechanism of absorption spectroscopy and methods to improve its detection sensitivity. The absorption line position and intensity of transformer oil-dissolved gases(C2H2, CH4, C2H4, C2H6, CO, CO2 and H2) was analyzed at atomic level, and the characteristic absorption lines of oil-dissolved gases were obtained. On this basis, the OF V-shaped CEAS detection platform was established, and gas detection characteristics of C2H2, CH4, C2H4, C2H6, CO, CO2 and H2 based on absorption spectroscopy were studied. This dissertation provided theoretical basis and technical support for transformer oil-dissolved gases live detection based on OF V-shaped CEAS technology.