Lubezine Volume 8 * NOVEMBER 2013 - JANUARY 2014

Because transformer oils are designed to provide electrical insulation under high electrical potentials, any significant reduction in the dielectric strength will indicate that the oil is no longer able to perform this vital function. Acid Number Figure 1 Oil Oxidation Oil Oxygen Paper Degradation Sludge and varnish Temperature Acids and hydroperoxides Temperature Paper Metal catalysts Water Degree of Polymerisation Paper Chain scission + water Free Multi-Width Graph Paper from http://incompetech.com/graphpaper/multiwidth/ Just like lubricating oils, transformer oils are oxidised under the influence of excessive temperature and oxygen, particularly in the presence of small metal particles, which can act as catalysts. Oxidation products are usually acidic in nature and result in an increase in acid number. Further reaction of these acids with the bulk oil can result in sludge and varnish deposits. In the worst-case scenario, the oil canals become blocked and the transformer is not cooled adequately, which further exacerbates oil breakdown. Furthermore, an increase in the acidity has a damaging effect on the cellulose paper. Oil degradation by-products, such as acids and hydroperoxides, also generally have the ability to conduct an electrical charge, which in turn reduces the insulating properties of the oil. An increase in Acid Number often goes hand-in-hand with a decrease in dielectric strength and increased moisture content as shown in Figure 1. Again, just like their industrial cousins, the acid content of transformer oils is determined by Potentiometric titration with potassium hydroxide. Furanic concentration Dielectric Strength The dielectric strength of a transformer oil is a measure of the oil’s ability to withstand electrical stress without failure. Because transformer oils are designed to provide electrical insulation under high electrical potentials, any significant reduction in the dielectric strength will indicate that the oil is no longer able to perform this vital function. Some of the things that can cause a reduction in dielectric strength include contaminants such as water, sediment, conducting particles, oil degradation by-products and cellulose paper breakdown. The test method for determining dielectric strength is relatively simple and involves applying an AC voltage at a controlled increasing rate to two electrodes immersed in the transformer oil. The gap is a specified distance and when the current arcs across this gap the voltage recorded is used to determine the dielectric strength. Power or Dissipation Factor The power factor of transformer oil is the ratio of true power to apparent power and is a measure of the current leakage through the oil, which in turn is a measure of the contamination or deterioration of the oil. In a trans- former, a high power factor is an indication of significant power loss in the transformer oil, usually as a result of contaminants such as water, oxidised oil and cellulose paper degradation, but may also be any substance in the oil that either resists or conducts electricity differently to that of the oil itself and may include diesel fuel, lubricating oil and kerosene. The test is not specific in what it detects and is usually carried out at elevated temperatures as contaminants that affect the test may remain undetected at 25ºC and only reveal themselves at >90ºC. Interfacial Tension (IFT) The interfacial tension of transformer oil is related to its deterioration. Transformer oil is generally a hydrocarbon and thus hydrophobic; however, when the sample undergoes oxidative degradation, oxygenated species such as carboxylic acids are formed, which are hydrophilic in nature. Interfacial tension is the surface tension of a sample of the oil carefully floated on top of a layer of water. The more hydrophilic the oil becomes, the lower the value of the surface tension between the two liquids. November 2013-January 2014 | LUBEZINE MAGAZINE Studies have shown that there is a definite relationship between acid number and IFT, an increase in acid number generally shows a decrease in IFT, however when there is a loss in IFT without the corresponding increase in acid number, it is generally because of contamination with another hydrophilic substance not derived from oxidation of the oil. Furanics (or degree of polymerisation) The solid insulation (

Lubezine Volume 8 * NOVEMBER 2013 - JANUARY 2014 | Page 21