Transformer oil

Transformer oil viscosity

The viscosity of the oil is an important factor in cooling the engine’s transformer. The lower the viscosity, the better the oil performs the cooling process. Therefore, there is no advantage in using an oil with high viscosity.

Flash point of transformer oil

Attention to the flash point is essential due to safety issues in the transformer.

IEC 60296 specifies the closed cap method (Martin’s button) for measuring the flash point. ASTM D 3487 uses the open cap method. The results obtained using the closed cap method are 5-10c less than the results obtained using the open cap method.

The flash point is in direct contact with the light components in the oil and is highly sensitive to contaminants from light oils such as diesel and gasoline. Although the repeatability efficiency of both methods is relatively low, the use of the closed cap method is important.

Transformer oil density

Determining the oil density in cold areas is necessary to avoid ice floating on the oil surface (at low temperatures) and also to calculate the total weight of the transformer. Ice floating on the oil surface occurs when there is some insoluble water in the transformer oil that has not yet been circulated. This causes problems when the transformer is started.

Resistance to the oxidation of transformer oil

There are two types of oils on the market:

oils containing antioxidant additives (inhibited) and oils free of antioxidant additives (uninhibited).

Amount of transformer oil water

The solubility of water in the oil depends on the temperature and the amount of polar aromatic molecules present in the oil. The solubility depends on the refining method and the temperature used during the refining process. If the amount of water present in the oil is high, it causes the oil to boil heavily during the degassing operation, and the resulting boil is caused by the evaporation of the water present.

Insulation failure of transformer oil

This property is very complex and depends on parameters such as the amount of particles, the type of particles, the amount of water and the method of conducting the test. The usual method for measuring the failure voltage is the IEC 60156 standard and the ASTM D1816 standard. In these two methods, the electrodes are spherical or spherical in shape and located at a distance of 5/2-2 mm from each other. The voltage increases at a rate of 2KV/s until the failure occurs, the result will be an average of six consecutive tests.

Insulation loss coefficient or Δtg of transformer oil

Δtg is a parameter that is part of the usual characteristics of motor transformer oil. Δ tg depends on the amount of ionized and polarized molecules present in the oil. A well-refined oil will show values of Δ tg, but it should be noted that this parameter is very sensitive to contamination during the process or loading.

Interfacial tension of transformer oil

This parameter measures the force between the surface of the water and the oil. The surface tension depends on the polar groups present in the oil. New oils should have a surface tension of at least 40 mN/m.

Transformer oil acidity

In freshly refined engine transfer oil, the acidity is expected to be less than 0.03 mgKOH per g oil mass. If the acidity number of the oil is higher than the stated amount, it can affect the metal parts of the equipment and reduce the life of the oil.

Corrosion of transformer oil

In IEC 60296 this parameter is measured using a copper strip immersed for 19 hours in oil at a temperature of 140 °C. The sensitivity of copper to corrosion due to sulfur compounds present in oil is high so the negative response to the above test ensures the non-corrosion of other sensitive metals in the equipment present in the motor transformer against contact with oil.