Physical, Chemical And Electrical Properties Of Transformer Oil

Oil-immersed transformers use transformer oil that is normally obtained by fractional distillation and subsequent treatment of crude petroleum. This oil is stable at high temperatures and has excellent electrical insulating properties. It is this oil that decides of the operation and life expectancy of the transformer it is used in. Transformer oil serves three main purposes.

·        It creates insulation, in combination with the insulating materials used in the conductors and coils.

·        It preserves the core and windings that are completely immersed in it, and acts as a coolant to extract heat from them.

·        It prevents direct contact of atmospheric oxygen with the cellulose made paper insulation of windings, thus eliminating oxidation.

Transformer oil is basically composed of four main organic compounds – aromatics, paraffins, napthenes and olefins – present in the right proportion for better insulation and stability. This oil is said to be of good quality only if it has the following properties.

Physical properties

Viscosity – This property is inversely proportional to temperature, and determines the rate of flow of the liquid. It is the measure of the fluid’s resistance to flow. Highly viscous fluids require bigger clearances for sufficient heat removal. The viscosity of transformer oil is required to be low at low temperatures. The maximum permissible viscosity level at 27°C is 27 cSt.

Density – This is the ratio of the weight of oil to its volume. The temperature of transformer oil also has a great effect on its density. With increase in temperature, the density decreases. The maximum permissible density of transformer oil is 0.89 gm/cc at a temperature of 29.5°C.

Pour point – This is the temperature at which the oil initiates its flow under the prescribed conditions. The average pour point of the oil should be -6°C.

Flash point – This is the temperature at which the oil gets vapourized. When this vapour mixes with air, it causes a momentary flash, thus the name. A high flash point of more than 140°C is required to prevent the risk of fire.

Chemical Properties

Acidity – This is the measure of organic and inorganic acids present in the oil. The value that is required to neutralize the total force of acid present in one gram of oil is known as the neutralization value. Higher this value, higher the acidity, thus higher sludge formation, which results in poor heat conduction that blocks the flow of oil in ducts and in turn impairs cooling. Therefore, the maximum permissible neutralization value of transformer oil is 0.03 milligrams KOH/gm.

Oxidation stability – Oxidization increases the neutralization value of transformer oil. In case of oxidization, this value must not go beyond 0.4 milligrams KOH/gm in any condition. Also, the total sludge after oxidization must not be more than 0.1% of the weight of oil.

Sulphur – Transformer oil always contains some traces of sulphur within. However, this corrosive sulphur should be at its minimized level or else it will lead to formation of black deposits over the copper parts of the transformer, thus affecting heat dissipation.

Electrical properties

Resistivity – Increase in temperature results in decrease of oil resistivity. Therefore, the transformer oil must have minimum resistivity of 30 x 1012 ohm-cm at 90°C and maximum resistivity of 1500 x 1012 ohm-cm at 27°C.

Breakdown voltage – This is the voltage at which the transformer oil starts losing its dielectric property and begins conducting. It is the presence of moisture, sediments and contaminating agents within the oil that decreases this property. Therefore, the breakdown voltage of transformer oil after filtration must be maintained at 60kV.

Dielectric dissipation factor – This is the value that depicts the poor quality of oil. To have good quality transformer oil, the maximum permissible value of dielectric dissipation factor should be 0.002 at 90°C.

After checking all of the above properties, you can decide on whether the oil within your transformer is of good or poor quality. However, the operating conditions of the transformer constantly affect the quality. Therefore, you must periodically test your transformer oil so that it can be filtered or replaced by fresh oil in case of any moisture or suspended particles within. For the best quality oil-immersed transformers, you can get in touch with Miracle Electronics, the leading transformer manufacturer in India, known for its top quality products since more than the past two decades!