Transformer Oil Functions. Transformer oils perform at least four functions for the transformer. Oil provides insulation, provides cooling, and helps extinguish arcs. Oil also dissolves gases generated by oil degradation, moisture and gas from cellulose insulation, deterioration, and gases and moisture from whatever atmosphere the oil is exposed to. Close observation of dissolved gases in the oil, and other oil properties, provides the most valuable information about transformer health. Looking for trends by comparing information provided in several DGAs, and understanding its meaning, is the most important transformer diagnostic tool.

Transformer Oil Testing

1.	Breakdown Voltage The breakdown voltage is of importance as a measure of the suitability of an oil to withstand electric stress. Dry and clean oil exhibits an inherently high breakdown voltage. Free water and solid particles, the latter particularly in combination with high levels of dissolved water, tend to migrate to regions of high electric stress and reduce breakdown voltage dramatically. The measurement of breakdown voltage, there fore serves primarily to indicate the presence of contaminants such as water conducting particles.
2.	Water Content Water may originate from the atmosphere or be produced by the deterioration insulating materials. Dissolved water may or may not effect the electrical properties of the oil. The solubility of water in transformer oil increases with increasing temp. and neutralization value. Above a certain water content (The saturation Water Content), all the water can not remain in solution and free water may be seen in the form of cloudiness or water droplets; free water invariably results in decreased Dielectric strength and Resistivity and increased Dielectric Dissipation Factor. High water content accelerates the chemical deterioration of insulating paper and is indicative of undesirable operating conditions or maintenance requiring correction.
3.	Neutralization value The neutralization value of an oil is a measure of the acidic constituents in the oil. Its value, negligible in an unused oil, increase as a result of oxidative ageing and as a general guide for determining when an oil should be replaced or reclaimed, provided suitable rejection limits have been established and confirmation is received from other tests. Oil with high neutralization value affect the winding & paper.
4.	Sediments and Sludge The test distinguishes between sediment and total sludge that is oil insoluble sludge plus sludge which is precipitated by adding heptane. Solid matter comprises insoluble oxidation or degradation products of insulating materials, fibers of various origins, carbon, metallic oxides, etc. arising from the conditions of service of the equipment. The presence of solid particles may reduce the electric strength of the oil and, in addition, deposits may hinder heat transfer, thus promoting further deterioration of insulation. Sludge consists of products formed at an advanced stage of oxidation and is forewarning of sludge deposits in the equipment.
5.	Dielectric Dissipation Factor (Tan Delta)
6.	Resistivity (Specific Resistance) These characteristics are very sensitive to the presence in the oil of soluble polar contaminants, ageing products or colloids. There is generally a relationship between DDF and Resistivity at elevated temperature with Resistivity decreasing as DDF increases. High values of dissipation factor may influence the power factor and/or the Insulation Resistance of Transformer windings.
7.	Inter Facial Tension The interfacial tension between oil and water provides a means of detecting soluble polar contaminants and products of deterioration. This characteristics changes fairly rapidly during the initial stages of ageing but levels off when deterioration is still moderate.
8.	Flash Point A low flash point is an indication of the presence of volatile combustible products in the oil. Prolonged exposure of the oil to very high temp. under fault condition may produce sufficient quantities of low molecular weight hydrocarbons to cause a lowering of the flash point of the oil.
9.	Dissolved Gasses Analysis D.G.A. is a very reliable and proven techniques to detect the internal faults in working transformers. The fault diagnosis by the gas chromatography is based upon the types & relative quantities of various hydrocarbon gases which can get dissolved in the oil under various fault.
10.	Furan Analysis in oil Furaldehydes (Furals) are chemicals formed when cellulose paper degrades due to overheating. The status of solid insulation can be measured using liquid oil insulation. Powertech offers a comprehensive analytical service for the detection of trace levels of these products in insulating fluids.
11.	De Polymerization of paper in oil Degrees of Polymerization (DP) is a test done on the paper to reveal its mechanical strength. An inverse relationship exists between the DP (paper test) and the Furals (oil test); the higher the Furals in the oil, the lower the DP of the cellulose paper. This relationship holds, however, only if the paper is degrading evenly. The DP of the paper usually varies within the solid insulation. Since it is difficult to sample paper from an operating transformer, Fural Analysis in the insulating oil can indirectly reveal the status of the solid insulation.
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