Study On SF₆ Decomposition Products Under Partial Discharge By Fourier Transform Infrared Spectroscopy

Partial Discharge (PD) is the typical flag of premature insulation defect in GIS (Gas Insulated Switchgear). Research shows that PD may cause SF₆ to decompose and different insulation defects ,discharge energy, discharge time and gas pressure may lead to different decomposition processes, rates and gaseous components ,which can be used for fault diagnosis and condition assessment of GIS .Fourier transform infrared spectroscopy(FTIR) has the merits as fellow: fast testing velocity, more detected components than other methods, high accuracy, strong anti-interference ability, long service life, the example doesn't demand anticipation and good linearity characteristic. This paper use FTIR to qualitative and quantitative analysis SF₆ decomposition components in GIS.The inverse relation is between the density of detection components and optical length by Beer-Lambert law, it was found that 0.1 meters optical length gas-cell can't detect the trace components in GIS, the effective methods is to design long optical length gas-cell, the needed one is 20 meters by calculate reckoning. An absorption cell is designed based on the White cell law, the entrance image point and the two curvature centers of object glass aren't in the same line, the three spherical mirrors in gas-cell are very important, 40 degree reflections has completed between them, which were coated with the gold whose reflectance ratio is 98%. The capacity of the cell is 4.3 liters, the optical length is 20 meters. The optimally detecting pressure is 100 kilopascal, the optimally resolution is 0.5cm-1. it was found that the resolution, signal-to-noise ratio, stability and linear character of 20 meters'cell are better than that of 0.1 meters', the former can implement the GIS real-time online monitoring.The point-to-plane electrode was used to simulate the fixed metal protrusions defect, the PD experiment was finished on the already constructed simulating trail equipments of SF₆ decomposition components, the sample interval was 12 hours, total discharge time was 96 hours, the samples were checked by FTIR and gas chromatography(GC), the conclusion are similar, but the detected varieties of FTIR are more than the later and the spectrum of FTIR can used to qualitative analysis the components'varying tendency. Two-dimensional infrared spectroscopy was used to extract the submersion information, by which SOF₄,CF₄ were recognized. Finally, the detected components by FTIR are SOF₂,SO₂F₂,SF₄,SOF₄,SO₂,CO,CF₄, the regularity of SOF₂,SO₂F₂,CO,SO₂ are strong, which can be used as distinction gas of SF₆ decomposed components.Discharge magnitude of simulating trail equipments of SF₆ decomposition components was calibrated, IR quantity was acknowledged by theoretical derivation, selected SF₆ as the IR detection background gas in this paper, obtained standard spectrum of the above 4 distinction gases and the absorption coefficient of the optimum absorption peak. SF₆ PD experiment was finished under five different discharge magnitudes, some components are very activity so that must detect the decomposition components in time. Gained the tendency chart of SOF₂,SO₂F₂,SO₂,CO with discharge time under five different discharge magnitudes, it was found that there is critical partial discharge magnitude, it is safe when the discharge magnitude is less than this value, gas production rates of SOF₂ and SO₂F₂ have rules with different discharge magnitude and time, they are enlargement accompany with increases of discharge magnitude, these rules can diagnosed partial discharge level and discharge time in GIS; the variation of SO₂,CO gas production rates are complex, the former is effected by H2O,but the single varying curve can still indicate the inner insulation condition; the later is distinction gas of solid insulation, which can judge whether there are solid insulation defect.