On-line monitoring of dissolved gas-in-oil with FTIR spectra

To overcome the disadvantages of conventional DGA (dissolved gas-in-oil) analysis using gas chromatography and other electrochemical sensors, initial researches were completed to realize on-line monitoring of dissolved gas-in-oil of power transformers using FTIR (Fourier Transform InfraRed) spectroscopy. Gas cell method is used to determine the characteristic absorption peaks of each diagnostic gas; simple and novel devices and procedures were designed in order to get measurable samples and spectra of mixed diagnostic gases with known concentration are taken using long optical path gas cell. The range of wavelength is estimated to be 3.0-13.9 μm from experimental spectra data. Hence the corresponding sampling frequency range should be in 536-4288 Hz and usable optical materials are suggested. It is concluded that a resolution of 10 cm^-1 may well satisfy the monitoring of all diagnostic gases and water content except hydrogen, and the lowest detection limit may be as low as 2×10^-8 to acetylene with a 2.4-meter-long optical length.