Chemical Test for Mineral Insulating Oil
Gas content - ASTM D2945, ASTM D3284, and ASTM D3612
The gas content of an insulating fluid may be defined as the volume of dissolved gas per 100 volumes of oil, at standard pressure and temperature. Some types of equipment require the use of electrical insulating liquids of low gas content. In filling electrical apparatus, a low gas content reduces foaming and reduces the available oxygen, thereby increasing the service life of the insulating oil.
The amount and kind of gases dissolved in oil can be used as a tool to aid in detecting and diagnosing faults and abnormal operating conditions in equipment.
The test is not intended for use in purchase specifications because the oil is customarily degassed immediately prior to use. The test can be used, however, as a factory control test and is more useful in evaluating the health of the transformer equipment. Overheating or arcing within the transformer will generate combustible and noncombustible gasses that will be dissolved in the oil. For dissolved gas analysis, reference IEEE Std C57.104 for more recommendations.
Polychlorinated biphenyls (PCBs) - ASTM D4059
U.S. regulations require that electrical apparatus and electrical insulating fluids containing polychlorinated biphenyls (PCBs) be handled and disposed of through the use of specific procedures. The procedure to be used for a particular apparatus or quantity of insulating fluid is determined by the PCB content of the fluid. The results of this analytical technique can be useful in selecting the appropriate handling and disposal procedures; refer to Title 40 CFR, Part 761.
Corrosive sulfur - ASTM D1275
This test is designed to detect the presence of free sulfur and combined corrosive sulfur by how the liquid affects polished copper strips in prescribed conditions. The test indicates the possibility of corrosion inside of electrical equipment resulting from the presence of sulfur-containing compounds. The source of sulfur present in insulating oil is usually the crude oil from which it is refined. The sulfur may come from rubber hoses used for oil processing or from replacement gasket materials.
Neutralization number (acidity) - ASTM D664 and ASTM D974
The neutralization number of an electrical insulating liquid is a measure of the acidic components of that material. In new oil, any acid present is likely residual from the refining process. In a service-aged liquid, the neutralization number is a measure of the acidic by-products of the oxidation of an oil. The neutralization number may be used as a general guide for determining when oil should be reprocessed or replaced. ASTM D974 is the traditional color-change indicator method of titrating the acids with a mild (0.1 N) KOH solution. ASTM D664 is a potentiometric titration method. On some service-aged liquids, the color may be so dark as to impair the ability of the technician to determine the indicator color change in ASTM D974, so ASTM D664 is used instead. The correlation between these two methods, however, has not been established.
Oxidation inhibitor content - ASTM D2668 by infrared spectrophotometry and ASTM D4768 by gas chromatography
Two synthetic oxidation inhibitors are commonly used in dielectric fluids. They are 2-6 ditertiary-butyl phenol (DBP) and 2-6 ditertiary-butyl para-cresol (DBPC). Their use provides added resistance to oxidation in systems that are partially or wholly exposed to air. The effectiveness of the oxidation inhibitor depends a great deal on the type of crude oil from which the insulating oil came. Certain new oils may contain naturally occurring antioxidant substances that may yield a false-positive indication in this test.
Oxidation stability, inhibited only (pressure vessel) - ASTM D2112 and ASTM D2440
a) Oxidation stability - ASTM D2112: This test method is a rapid test for evaluating the oxidation stability of a new mineral insulating oil that contains the synthetic oxidation inhibitor 2-6 DBPC or 2-6 DBP. The test measures the length of time required for the oil sample to react with a given volume of oxygen when a sample of oil is heated and oxidized under test conditions.
b) Oxidation stability - ASTM D2440: This test method determines the resistance of mineral insulating oils to oxidation under prescribed accelerated aging conditions. Oxidation stability is measured by the propensity of oils to form sludge and acid products during oxidation. This test method is applicable to new oils, both inhibited and uninhibited.
Water in insulating liquids: Karl Fischer method - ASTM D1533
Water may be present in insulating liquids in several forms. The presence of free water may be indicated by visual examination. The oil will appear cloudy, or separated water drops will be observed, probably on the bottom surface. The presence of free water can be remedied by filtration or other means. Dissolved water cannot be detected visually and is normally quantified by physical or chemical means. Dissolved water may affect the dielectric breakdown of insulating oil; however, its significance is determined by several factors, including the percent of moisture saturation and the amount and type of contaminants. The method cited is suitable for the determination of water in insulating oil, and depending on the conditions of sample handling and the methods of analysis, it can be used to estimate total water as well as dissolved water in insulating oil. The units of measure of water are milligram/kilogram. New insulating oil received from the manufacturer normally contains less than 25 mg/kg moisture. New insulating oil should be tested for moisture content. If necessary, applicable measures should be taken to avoid introducing high moisture-content oil into electrical equipment.
Furans in insulating liquids - ASTM D5837
Furanic compounds are generated by the degradation of cellulosic materials used in the solid insulation systems of electrical equipment. Furanic compounds that are oil soluble to an appreciable degree will migrate into the insulating liquid. The presence of high concentrations of furanic compounds is significant in that this may be an indication of cellulose degradation from aging or incipient fault conditions. Testing for furanic compounds by high-performance liquid chromatography (HPLC) may be used to complement dissolved gas in oil analysis as performed in accordance with the test method in ASTM D3612.
