Abstract: Sulfur hexafluoride (SF6) transformers are a type of electrical equipment recently introduced from abroad. When a fault occurs during transformer operation, SF6 gas, as the main insulating medium, may produce fault characteristic gases. Based on the gas quality indicators provided by the manufacturer, and through analysis of the characteristic gas generation principle and verification by simulation experiments, a method for detecting fault characteristic gases in SF6 transformers is proposed. Keywords: Sulfur hexafluoride; Transformer; Impurities; Detection SF6 transformers are transformers that use SF6 instead of traditional insulating oil as the main insulating medium. They have advantages such as high insulation strength, non-flammability, and explosion-proof properties, making them particularly suitable for substations in urban centers. Since traditional medium detection methods cannot adapt to changes in the insulating medium, corresponding detection methods must be proposed to monitor the safe operation of the equipment. 1 Gas Quality Indicators for SF6 Transformers SF6 transformers have special requirements for the quality of SF6 gas. For example, the Shenzhen Power Supply Bureau introduced 50MVA SF6 transformers from Mitsubishi Corporation of Japan, which uses forced circulation air cooling. Table 1 shows the gas quality indicators provided by the manufacturer. Currently, relevant technical indicators have not yet been established in China. Table 1. Gas Quality Indicators of Sulfur Hexafluoride Transformers (%) [IMG=Sulfur Hexafluoride Transformer Gas Quality Indicators]/uploadpic/THESIS/2008/1/2008010312031839991B.jpg[/IMG] 2. Generation of Characteristic Gases in Sulfur Hexafluoride Transformers Transformers may experience faults such as overheating, partial discharge, flashover, and arcing during operation, all of which manifest as thermal faults. For traditional oil-insulated transformers, gas chromatography is typically used to analyze characteristic gases such as methane, ethane, and hydrogen released from oil cracking for fault diagnosis. However, sulfur hexafluoride, as an insulating medium, decomposes at certain temperatures but readily recombines, making the detection of its decomposition products in sulfur hexafluoride gas extremely difficult. However, when a fault affects the solid insulation, sulfur hexafluoride will produce some decomposition products. For example, when overheating or discharge occurs in the windings, the solid insulation material of the conductors (such as polyester film, insulating paper, etc.) will carbonize and decompose to produce carbon monoxide and carbon dioxide, the content of which varies with the energy of the fault. When the temperature of insulating paper exceeds 100℃, the following reactions will occur: [IMG=reaction equation]/uploadpic/THESIS/2008/1/2008010312032653807X.jpg[/IMG] When the temperature is greater than 300℃, and the content of impurities such as moisture and air is high, and under the action of metals (such as copper), sulfur hexafluoride gas may produce the following reactions: 2SF6+Cu=CuF2+S2F10 SF6+2Cu=2CuF2+SF2 2SF6+5Cu=5CuF2+S2F2 S2F10=SF4+SF6 2SF2=SF4+S 2S2F2=SF4+3S It may also hydrolyze into low-fluorine compounds and hydrofluoric acid: SF4+H2O=SOF2+2HF SOF2+H2O=SO2+2HF 2SF2+H2O=SOF2+2HF+S 2S2F2 + H2O = SOF2 + 2HF + 3S Therefore, faults can be identified by detecting these decomposition products. 3. Simulated Overheating Fault Test To simulate overheating of the sulfur hexafluoride transformer windings, we used the experimental setup shown in Figure 1 to measure the decomposition products of sulfur hexafluoride gas under solid insulation. The container was filled with sulfur hexafluoride gas at a pressure of 0.2 MPa. During the test, the temperature was controlled at 150–250 °C. Gas was removed after every 1 hour of heating, and the volume fractions of carbon tetrafluoride, carbon monoxide, and carbon dioxide were measured. The data are shown in Table 2. Table 2 shows that the overheating decomposition of solid insulation in sulfur hexafluoride gas produces carbon monoxide and carbon dioxide, and their volume fractions are directly proportional to the temperature and time of the overheating point. The carbon monoxide content after heating is 40–80 times higher than before heating, while the carbon dioxide content is not significantly different. The insulating paper begins to carbonize at 150 °C; when the heating temperature exceeds 250 °C, the insulating paper is easily carbonized. The most prominent characteristic gas is carbon monoxide. When insulating paper is heated and carbonized in an oxygen-deficient atmosphere, the main product is carbon monoxide. Therefore, detecting carbon monoxide gas is necessary as a primary quality indicator for sulfur hexafluoride transformers. [IMG=Solid Insulation Fault Simulation Device]/uploadpic/THESIS/2008/1/2008010312033969122Y.jpg[/IMG] Figure 1 Solid Insulation Fault Simulation Device Table 2 Test Results of Impurity Volume Fraction % of Simulation Device [IMG=Test Results of Impurity Volume Fraction of Simulation Device]/uploadpic/THESIS/2008/1/20080103120350603669.jpg[/IMG] 4 Detection Methods for Characteristic Gases 4.1 Hydrolyzable Fluorides and Acidity Hydrolyzable fluorides are byproducts of the synthesis of sulfur hexafluoride gas or products of arc decomposition under high voltage. These products can be hydrolyzed or alkaline hydrolyzed. Hydrolysis is carried out using the absorption oscillation method. The determination can be performed using the alizarin-lanthanum fluoride complex colorimetric method and the fluoride ion selective electrode method. The total amount of low-fluoride compounds in the gas is expressed as the mass ratio of HF. The acidity test method for sulfur hexafluoride (SF6) involves absorbing a certain amount of gas with an excess of alkali solution, followed by titration with a standard acid solution. The acidity of SF6 is calculated based on the volume and concentration of the acid solution consumed. 4.2 Carbon tetrafluoride and thionyl, etc., in SF6 gas are determined by gas chromatography using a thermal conductivity detector. For increased sensitivity, a flame photometric detector can be used. The column material can be a mixture of GDX-104 or Porapak-Q with 3X molecular sieves. 4.3 Carbon monoxide and carbon dioxide are conventionally measured by gas chromatography using activated carbon or carbon molecular sieves. Carbon monoxide and carbon dioxide are detected using a thermal conductivity detector with a sensitivity of approximately 0.01% (volume fraction), employing a dual thermal conductivity cell method. The column temperature should be programmed to accelerate peak elution. The detection chromatogram is shown in Figure 2. If a hydrogen flame detector is used after conversion with a nickel catalyst, the detection sensitivity should not be less than 0.0025% (volume fraction), but the poisoning of the nickel catalyst by sulfur hexafluoride gas must be considered. [IMG=Carbon monoxide and carbon dioxide spectra in sulfur hexafluoride gas]/uploadpic/THESIS/2008/1/20080103120402217548.jpg[/IMG] Figure 2 Carbon monoxide and carbon dioxide spectra in sulfur hexafluoride gas 5 Discussion In addition to humidity detection, gas detection tests for sulfur hexafluoride transformers (including instrument transformers) should include the detection of carbon monoxide, carbon dioxide, and hydrolyzed fluorides. This allows changes in the volume fraction of these gases to be used as a basis for judging latent faults. Since sulfur hexafluoride transformers have not been in operation for long, gas sampling, detection methods, and judgment standards are not yet standardized. Standardized test methods and quality standards should be established, and annual tracking tests should be used for monitoring to accumulate experience and provide an effective basis for safe operation.