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oil type transformer maintenance

The frequency of transformers maintenance depends on the importance, design features and durability of transformers. The first step towards a preventive maintenance program is testing. Testing helps us to determine when a treatment is necessary.

WHY DO WE NEED TO CHECK TRANSFORMER OIL?

Transformer oil is an insulator between the metal parts of the transformer and the wiring within the tank. Over time, the oil can be contaminated with moisture or create combustible gases. By sampling and testing that oil, we can plan repairing or replacing the transformer before a failure occurs.

Maintenance Schedule for Oil Filled Power Transformers

Maintenance or Test	Recommended Interval	Reference
Transformer physical inspection	Annually
Leakage of oil and reading of MOG (Magnetic Oil Gage) of tanks	Daily	In case of unsatisfactory oil level in the MOG, oil to be filled in transformer and also the transformer tank to be checked for oil leakage. If oil leakage is found take required action to plug the leakage. If silica gel becomes pinkish, it should be replaced.
Dielectric strength test (BDV) and moisture content (PPM).	Annually	A dielectric strength is a measure of the voltage which is conducted by the transformer oil. At times it is the presence of contaminants which conduct electricity faster than the oil which in turn lowers the dielectric breakdown voltage
Neutralization test	Annually	This test which is carried out to ascertain the level of acid which is present in the oil. When oxidation occurs in the transformer oil, it leads to build-up of acid.
Dissolved Gas Analysis Test (DGA)	Annually for 132 KV transformer, once in 2 years for the transformer below 132 KV transformer and in 2 years interval for the transformer above 132 KV transformer.	It helps to analyze the gases which are present in the transformer oil.
Bushings– check of oil level	Weekly	If it is required, the oil to be filled in the bushing up to correct level. Oil filling to be done under shutdown condition.
Bushings –visual inspection	Annually
Bushings – visual inspection and cleaning	5 years
Interfacial tension test		This is a test which is carried out to detect the presence of polar compounds such as paint, varnish and paper.
Tan & delta test	One in two years	Finally the presence of water too is measured as water can decrease the dielectric breakdown voltage of the transformer oil
Colour test		This test indicates aging, quality and the presence of contaminants in the transformer oil.
Transformer excitation current test	5 years
Insulating oil – DGA, physical, and chemical tests	Anually
Top oil and winding thermometer Inspection	Anually
Oil level indicator operation	5 years
Sudden pressure relay	5 years

oil type transformer maintenance

So, for getting high performance and long functional life of the transformer, it is desired to perform various maintenance activities. GlobeCores’s oil reclamation equipment will keep your transformer oil in the best condition, providing with oil degassing, water and solids removal as well as acids reducing. It’s a fantastic transformer life!

Dielectric strength is one of the most important indicators that describes the insulation characteristics of transformer oil. In number, this strength is equal to the relation of breakdown voltage and the distance between the electrodes.

The breakdown voltage is a characteristic that is very sensitive to the impurities and water in dielectric liquids. The dielectric strength may be drastically reduced by water supply or increased amount of foreign particles.

Pure transformer oil is usually free from water and mechanical impurities and its breakdown voltage is greater than 60 KV(kilowatt). But ,in general, the dielectric strength is a changeable characteristic of insulating oil.

Under the impact stress, mechanical impurities almost do not influence the electrical strength. It is known that the dielectric breakdown mechanism is not the same during the impact and constant stress. It is assumed that during the impact stress, the dielectric strength is higher than during the constant one. The risk of breakdown therefore, is small in the case of lightning discharges or switching surges.

If the temperature increases from 0 to 70ºC, the dielectric strength is increased as well. It may be caused by such factors as the moisture disposal or decreased oil viscosity.

Dissolved gases are no less important in case of breakdown. There are bubbles in the electrodes because of low tension of the electric field. The dielectric strength of non-degassed transformer oil falls if the pressure drops.

The breakdown voltage is improved by:

(1) degassing of dielectric liquid;

(2) impact stress; and

(3) high pressure (10 mPa (millipascal)).

The breakdown voltage depends on water concentration in the emulsion. Temperature falls or relative humidity of the air may cause the reduction of the dielectric strength or emergence of some water. Such phenomena may be also caused by the ingress of water into the oil blending process. This water is absorbed on the surface of a container.

The amount of water that enters the oil blending process may be reduced by substituting a glass container for a plastic one. In such a way the dielectric strength is increased.

It should be noted that there is no widely accepted theory of the dielectric liquid breakdown despite years of experience in this sphere. Still, there are three leading theories:

(1) The thermal theory states that a gas channel is formed by the boiling of dielectric liquids in points of high inhomogeneity of the electric field;

(2) The gas theory considers that a source of breakdown is the bubbles of gas dissolved in oil or adsorbed onto the electrodes; and

According to the (3) chemical theory, the breakdown is caused by chemical reactions passed in dielectrics under the influence of electric charge in gas bubbles.

But, there is one important point that unifies all three theories: the dielectric breakdown takes place at a steam channel realized by oil evaporation.

In order to avoid the breakdown of liquid dielectrics, it is recommended to use the equipment that is designed for drying, purifying, degassing and regeneration of oil products.

Tag Archives: dielectric strength

The schedule of transformer oil maintenance oil type transformer maintenance

oil type transformer maintenance

Dielectric Strength of Transformer Oil