Transformer oil serves as a heat dissipater for heating parts of the transformer and as insulator of energized components. To function efficiently, the dielectric fluid must have certain operational characteristics.
Color. For quantitative evaluation of transformer oil color it is viewed in through light, then compared to digital standards. The oil can be cloudy, containing sediment and suspended particles. In practice, color is not the decisive factor to stop using the fluid, but may indicate the need to additional tests.
Viscosity. Viscosity is an important parameter, since efficiency of cooling the hot windings depends on it. The less the viscosity, the better the cooling process. If the oil is used in high voltage switches, it must have low viscosity and high fluidity so that the movement of switch parts are not impeded.
Viscosity changes with temperature, so the oil with low dependency of viscosity on temperature is selected for switches.
Pour point. This temperature is more important for switches, which are often used outdoors and in low temperature. The thicker the oil, the more resistance to movement and the worse the arc extinguishing.
Ash content. This is the indication of correct oil washing. The measurement is taken once when accepting the product. Bad washing results in excess salts and soaps in the oil, which turn into ash when the oil is burned.
Corrosive characteristics. Metals in the oil can corrode. The result is soap which makes oxidation process more intensive and increase dielectric loss. Of all the transformer construction materials, copper is the most active in terms of corrosion and oxidation of the dielectric liquid.
Alkali test. This is a method of determining the degree of removing contaminants from the oil.
It is usually performed for fresh and regenerated oil, which may have been washed improperly. If so, the insulation fluid will contain soap and certain contaminants, indicated by the results of the alkali test.