Transformer Oil Failures and Maintenance

Insulation failure of transformer oil

Liquid insulated oil-immersed transformer was invented by Thomson, an American scientist, in 1887. In 1892, it was popularized and applied to power transformers by General Electric Company of the United States. The liquid insulation referred to here is transformer oil insulation.

The characteristics of oil-immersed transformer:

1) greatly improve the electrical insulation strength, shorten the insulation distance, reduce the volume of equipment;

2) Greatly improve the effective heat transfer and heat dissipation effect of the transformer, improve the current density allowed in the wire, reduce the weight of the equipment, it is the heat of the transformer body running through the heat cycle of the transformer oil, transferred to the transformer shell and radiator for heat dissipation, so as to improve the effective cooling level;

3) Because of the oil immersion seal, the oxidation degree of some parts and components inside the transformer is reduced, and the service life is prolonged.

1.     Performance of transformer oil.

Transformer oil in operation must have stable and excellent insulation performance and thermal conductivity.

Among them, insulation strength, viscosity, freezing point and acid value are the main property indexes of insulating oil. Insulating oils extracted from petroleum are mixtures of hydrocarbons, resins, acids and other impurities, which are not always stable and constantly oxidize under the influence of temperature, electric field and photosynthesis. Normally insulating oil oxidation process was very slow, if maintained properly and even use 20 years still can keep the quality of due and not aging, but the metal, impurities mixed with oil, gas, etc. will accelerate the development of oxidation, makes the oil quality bad, color darker, transparency turbidity, increased moisture content, increased acid value, increased ash content, etc., make oil properties deteriorates.

2.         Reasons for deterioration of transformer oil.

Transformer oil deterioration can be divided into two stages according to the degree of severity : contamination and deterioration.

Contamination is the oil mixed with water and impurities, which are not the products of oil oxidation. The insulation performance of contaminated oil will deteriorate, the breakdown voltage/dielectric strength will decrease, and the dielectric loss will increase.

Degradation is the result of oil oxidation, of course, this oxidation does not only refer to the oxidation of hydrocarbons in the pure oil, but the impurities in the oil will accelerate the oxidation process, especially copper, iron, aluminum metal dust.

Oxygen comes from the air in the transformer, even in the fully sealed transformer inside there is still a volume of about 0.25% oxygen exists, oxygen solubility is higher, so in the oil dissolved in the gas occupies a higher ratio.

When the transformer oil is oxidized, the moisture and heat of the catalyst make the transformer oil generate sludge. The influence is mainly manifested in: the precipitate particles are large under the action of the electric field; Impurity precipitation is concentrated in the strongest electric field area, forming conductive "bridge" to transformer insulation; The precipitate is not uniform but forms separate slender strips, which may be arranged in the direction of power lines. This undoubtedly hinders heat dissipation, accelerates the aging of insulation materials, and leads to the reduction of insulation resistance and insulation level.

3.     Transformer oil deterioration process.

The products of the main stage of oil degradation are peroxides, acids, alcohols, ketones and sludge.

Early deterioration. The peroxides generated in oil react with insulating fiber materials to form oxidized cellulose, which makes the mechanical strength of insulating fiber deteriorate, resulting in embrittlement and insulation shrinkage. The resulting acids are slimy fatty acids that, although not as corrosive as mineral acids, grow at a rapid rate and have a significant effect on organic insulation.

Late deterioration stage. Is to generate sludge, when acid erosion materials such as copper, iron, insulating paint, reaction generated sludge, is a kind of viscous and similar asphalt aggregate type conductive material, it can moderate dissolved in oil, under the action of electric field to generate fast, adhesion on the edge of the insulating material or transformer box shell, deposition in the place such as tubing and cooler heat sink, make the transformer temperature rise, dielectric strength decreased.

The oxidation process of oil is composed of two main reaction conditions. One is that the acid value in the transformer is too high, and the oil is acidic. The other is the oxide dissolved in oil into insoluble compounds, so that the transformer oil gradually deteriorates.

4. Transformer oil quality analysis, judgment and maintenance.

1) Insulation oil deterioration. Including its physical and chemical properties have changed, so that its electrical properties deteriorated. By testing the acid value of insulating oil, interfacial tension, oil sludge precipitation, water-soluble acid value and other items, we can determine whether it belongs to this kind of defect. Regeneration treatment of insulating oil may eliminate the product of oil deterioration, but natural antioxidant may also be removed in the process of treatment.

2) Insulation oil water damp, because water is a strong polar substance. It is easy to dissociate and decompose under the action of electric field, and the conductive current of insulating oil is increased. Therefore, the small amount of water can make the dielectric loss of insulating oil increase significantly. By testing the moisture of insulating oil, judge whether it belongs to this kind of defect. Vacuum Oil Dehydration Plant for insulating oil can generally eliminate moisture.

3) Insulation oil is infected with microorganisms and bacteria. For example, when installing the main transformer or lifting the core, insects attached to the surface of the insulation parts and stains left by the installation personnel are likely to carry bacteria, so as to infect the insulation oil; or the insulation oil itself has been infected with microorganisms. The main transformer generally operated in the environment of 40-80 ℃, is very conducive to the growth and reproduction of these microorganisms. Because the insulation performance of mineral and protein in microorganism and its excreta is much lower than that of insulating oil, the dielectric loss of insulating oil increases. This defect is difficult to deal with by field cycle treatment because no matter how it is treated, some microorganisms remain on the insulating solid. After treatment, the main transformer insulation will recover in the short term, but because the main transformer operating environment is very conducive to the growth and reproduction of microorganisms, these residual microorganisms will grow and reproduce year by year, so that some of the main transformer insulation decreased year by year;

4) Alkyd resin insulating paint containing polar substances dissolved in oil. Under the action of electric field, dipole relaxation polarization will occur in polar substances, and energy will be consumed in the process of AC polarization, so the dielectric loss of oil will rise. Although the insulating paint has been cured before leaving the factory, there may still be incomplete treatment. After the main transformer runs for a period of time, the insulation paint that is not completely treated gradually dissolves in the oil, so that the insulation performance decreases gradually. The occurrence time of this kind of defects is related to the thorough degree of insulating paint treatment, and a certain effect can be achieved through one or two adsorption treatments.

5) The oil is only mixed with water and impurities. Such contamination does not change the essential properties of the oil. The moisture can be eliminated by vacuum drying; For impurities can be filtered to remove;  Air in the oil can be removed by vacuum degassing.

6) Two and more than two different sources of insulation oils mixed use. The properties of oil should meet relevant regulations; The specific gravity of oil is the same, the solidification temperature is the same, the viscosity is the same, the flash point is similar; And the stability of the mixed oil also meets the requirements. For the degraded oil after mixing oil, because the oil quality has changed, the acid and sludge generated, which needs to use the chemical method of oil regeneration to separate the degraded product, in order to restore its properties.

In simple terms, when the transformer is polluted early, mainly by water, gas and impurities, vacuum drying and filtering can be carried out through the Vacuum Oil Purifier to reduce the content of water and gas and filter out metal and other impurities.  In the case of deterioration of transformer oil, which contains acidic substances, dark color, sludge and so on, it is recommended to use oil regeneration equipment, combined with physical and chemical treatment methods, restore the performance of transformer oil.

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