Inspection and analysis of transformer oil

Transformer oil testing items 1) Freezing point； (2) Water content ； (3) Surface tension ； (4) Acid value ； (5) Water-soluble acidity and alkalinity ； (6) Breakdown voltage ； (7) Flash point ； (8) Volume resistivity ； (9) Dielectric loss (10) Chromatographic analysis (11) Analysis of furfural content in insulating oil. Testing items and significance of transformer oil tests: 1. Appearance: Examining the appearance of the operating oil allows detection of insoluble sludge, fibers, and dirt present in the oil. In routine tests, a record of this item should be kept. 2 Color: New transformer oil is generally colorless or pale yellow; its color gradually deepens over time as it is in use, but under normal conditions this change occurs slowly. If the color of the oil darkens sharply, it is necessary to investigate whether there has been overloading or overheating of the equipment. If all other relevant characteristic test items meet the requirements, it can continue to operate, but enhanced monitoring should be implemented. 3 Moisture: Moisture is one of the key factors affecting the insulation aging of transformer equipment. An increase in water content in transformer oil and insulating materials directly leads to a decline in insulation performance and accelerates the aging of the oil, affecting the reliability and service life of the equipment. Strict monitoring of moisture is an essential test item to ensure the safe operation of the equipment. 4 Acid value: The acidic compounds present in the oil increase its electrical conductivity, thereby reducing its insulating properties. At higher operating temperatures (above 80°C), these compounds can also cause aging and corrosion of solid fiber insulating materials, shortening the service life of the equipment. Since the acid value in oil reflects the aging condition of the oil, strengthening the monitoring of the acid value is important for taking proper maintenance measures. 5 Oxidation stability: The oxidation stability test of transformer oil is an important method for evaluating its service life. Due to the good oxidation stability of domestic oils and the addition of antioxidants, this test is usually conducted only on new oils. However, for imported oils, especially those without antioxidants, this test should be carried out not only on new oils but also after several years of operation, in order to take appropriate maintenance measures and extend their service life. 6 Breakdown voltage: The breakdown voltage of transformer oil is used to assess the extent to which the oil can withstand electrical stress; it is a very important monitoring parameter. Generally, this value depends on the degree of contamination, but high levels of moisture or the presence of impurity particles in the oil have a significant impact on the breakdown voltage. 7 Dielectric loss factor: The dielectric loss factor is highly sensitive to determining the degree of aging and contamination of transformer oil. New oil contains few polar impurities, so its dielectric loss factor is also very low, typically in the range of 0.01% to 0.1% ； However, when oil aging occurs due to oxidation or overheating, or when other impurities are mixed in, the amount of polar impurities and charged colloidal substances generated increases gradually, and as a result, the dielectric loss factor also rises. Even when there are very few aging products in the oil and they cannot be detected by chemical methods, the dielectric loss factor can already be clearly identified. Therefore, the determination of the dielectric loss factor is a common method for the inspection and monitoring of transformer oil, and it holds special significance. 8 Surface tension: Measuring the surface tension between oil and water is an indirect but effective method for detecting soluble polar impurities in the oil that result from aging. In the initial stage of aging, the change in interfacial tension is quite rapid; by the middle stage of aging, this rate of change slows down. Moreover, the formation of sludge increases significantly; therefore, this method can also provide a reliable assessment of the trend in sludge generation. 9 Sludge: This method is used to detect the sludge deposits that are still in a dissolved or colloidal state in the operating oil; these deposits can be precipitated out of the oil when heptane is added. Due to the different solubilities of sludge in new oil and aged oil, when new oil is mixed into aged oil, the sludge precipitates out. The deposition of this sludge affects the heat dissipation capabilities of the equipment; it also causes severe corrosion of solid insulating materials and metals, leading to a decline in insulation performance, which poses significant risks. Therefore, when mixing oils in a ratio of more than 5%, it is necessary to conduct a sludge precipitation test. 10 Flash point: The flash point is an essential parameter for monitoring operating oil. A decrease in flash point indicates the presence of volatile flammable gases in the oil ； These flammable gases are often generated as a result of local overheating of electrical equipment and arc discharge, which causes the insulating oil to undergo thermal pyrolysis at high temperatures. Flame point testing can help detect equipment failures in a timely manner. For newly filled equipment as well as transformer oil that has been overhauled, determining its flash point can also help prevent or detect the presence of oils with light fractions mixed in, thereby ensuring the safe operation of the equipment. 4.11 Gas components in oil: Combustible gases in oil are generally generated due to local overheating of the equipment or decomposition caused by discharge. If the causes of the generation of flammable gases are not identified and eliminated in a timely manner, it poses a serious threat to the safe operation of the equipment. Therefore, using gas chromatography to determine the gas components in oil is very effective in eliminating latent faults in transformers. This test is an essential part of the monitoring of transformer oil during operation. 4.12 Water-soluble acids: In the initial stages of oxidation, transformer oil tends to produce low-molecular-weight organic acids such as formic acid and acetic acid. Due to their high water solubility, an increase in the concentration of these water-soluble acids in the oil (i.e., a decrease in pH value), combined with the presence of water in the oil, can cause corrosion of solid insulating materials and metals, as well as reduce the insulation properties of electrical equipment, thereby shortening its service life. 13 Freezing point 1): Based on the climate conditions in our country, transformer oil is classified into different grades according to its low-temperature performance. For example, the grades 10, 25, and 45 refer to freezing points of -10, -25, and -45°C respectively. Therefore, the determination of the freezing point is necessary for the acceptance of new oils and the mixing of oils of different grades. 14 Volume resistivity: The volume resistivity of transformer oil, just like the dielectric loss factor, can be used to determine the degree of aging and contamination of the transformer oil. Water, contaminating impurities, and acidic products in the oil can all contribute to a decrease in resistivity. com/redirect.php?goto=findpost&pid=2216537&ptid=414777]1# OTAC

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