Theories And Methods Of On-line Monitoring Of Dissolved Gases In Transformer Oil Based On Resonant Photoacoustic Spectrometry

Dissolved gases in transformer oil are important characteristic values which reflect the types of incipient faults of the transformers, and on-line monitoring of dissolved gases has fine prospects in the application to on-line evaluation of the running state of the transformers and on-line forecast of their residual lives. As an optical detection technique, photoacoustic spectrometry (PAS) need not consume gas and separate gas, and has some advantages including high sensitivity, high selectivity, and big dynamic detection range. This dissertation investigates the applications of PAS to analyze dissolved gases in transformer oil.The mechanism of the photoacoustic effect in gas, namely, the heat production and the sound generation, is analyzed in detail. The distribution of the sound field and its amplitude in the standard cylindrical photoacoustic cell are researched. And the wave equation and the amplitude of the photoacoustic signal are also given on the base of analyzing the sound energy loss of the actual photoacoustic cell. These theories provide guidance for the establishment of the photoacoustic gas detection system, especially for the design of the photoacoustic cell.Gaseous infrared absorption properties are the bases of analyzing gas by infrared optical methods. A line-by-line integration model for the gaseous infrared absorption properties is given in this dissertation. The gaseous absorption coefficients for specific wavelength at different temperatures and pressures can be calculated by using this model and gaseous line parameters of HITRAN2004 database. As for the specific absorption lines of CH₄, C₂H₆, C₂H₄, C₂H₂, CO and CO₂, the laws that the peak absorption coefficients vary with the pressure and temperature are also analyzed.The first longitude resonant cell which can satisfy the need of on-line monitoring of gas in oil is designed on the basis of analyzing detailedly the operating principles and the features of the non-resonant, Helmoholtz, and resonant photoacoustic cells, and analyzing the relationship curves between the geometric parameters of the resonant cavity and the characteristic parameters, namely, resonant frequency, quality factor, and cell constant. In the meantime, by analyzing the advantages and disadvantages of the application of incoherent sources and coherent sources in the photoacoustic gas detection systems and the near infrared spectra of CH₄ and C2H2, an experiment-setup for on-line photoacoustic monitoring of dissolved gases in oil based on the array source which is composed of several distributed feedback diode lasers is established. And a method of photoacoustic quantitative analysis for gas based on the least square regression is proposed, experimental results show that it is valuable in the quantitative analysis for a photoacoustic gaseous detection system with good selectivity.On account that the cross-correlation method has the disadvantage of high signal to noise ratio threshold, a chaotic detection method based on the cross-correlation and the Lyapunov exponent is given. Simulation analyses show that the proposed method can detect weak sine signals in strong noise, which can not be detected by the cross-correlation method, and that it is superior to the cross-correlation method and the chaotic detection method in precision.