Transformer oil is usually used in less severe conditions compared to motor oil.
Used motor oil cannot be practically regenerated without some hard processes (high temperatures and pressure, solvents etc), since they are used at elevated temperatures.
The chemical basis of the oil is subjected to strong destructive forces in such conditions; besides, the presence of a range of additives in the oil complicates purification, since the additives form stable associations with the oil.
Transformers of various voltage categories widely use mineral oil as a cooling and insulating medium. Transformer oil is the main insulating material and defines the dielectric strength of the whole unit.
The main requirement to transformer oil is high dielectric strength (breakthrough voltage) and insulation ability (dissipation factor). The oil is used in strong electromagnetic fields, high temperature (50-90°С depending on transformer design and cooling system), in contact with oxygen etc. Therefore, beside high dielectric strength, the oil must be highly stable to oxidation and gas dissolution; another important parameters is low dielectric loss factor. With time, transformer oil ages to due heat, chemical and electrical processes, reducing its performance due to the changing chemistry of the oil.
A distinction of transformer oil use is the influence of electromagnetic field. The field accelerates oxidation of oil. Oxidation of transformer oil in an electric field of 1 MV/m in laboratory conditions showed 18-20% increase in sediment and 6-10% increase of acidity compared to oxidation without the field. Electric field not only accelerates oxidation of oil, but also changes the nature and composition of the resulting oxidation products. In particular, water formation is 4-5 times higher than that without the electric field. Formation of water due to oxidation of hydrocarbons in the oil is one of the reasons of moisture contamination during transformer operation.
One of the most important properties of transformer oil is oxidation stability, i.e. its ability to retain its performance parameters in long term operation. Special additives are used to improve oxidation stability, its efficiency is based on the ability to interact with the free peroxide radicals, which are formed as the result of radical chain reaction of hydrocarbon oxidation.
The oxidation process is promoted by heat, varnish, moisture, metals (especially copper), and other materials which come in contact with the oil. Especially harmful to the oil is the sludge remaining in transformer due to insufficient cleaning when the oil is changed.
Oil oxidation can be divided into two phases:
The initial oxidation phase is known as induced. The changes occurring in the oil at that time cannot be detected by regular analysis methods. The stability of the oil gradually decreases. In the second phase, oil performance drops, the color changes from light yellow to brown or turbid. Acidity and ash content grow, low molecular acids appear, which destroy winding insulation and cause metal corrosion; subsequently, sludge forms which can block oil channels and disrupt transformer cooling.
In the process of aging, the oil accumulates gaseous products which form as a result of local overheating in the transformer or electric discharges in insulation. These are hydrogen, hydrocarbon gases (methane, ethane etc) and carbon oxides. Small amounts of gas are usually not as dangerous as other aging products, since the gases solve in the oil and have little impact on its performance. But as the oil becomes saturated with gas, or when the gas is formed intensively in local volumes, the rate of gas generation may exceed gas solution and gas bubbles form, sharply reducing the dielectric strength of the oil, with the possibility of strong partial discharges or even insulation breakthrough.
The dielectric loss factor is degraded by asphalt and resin compounds, formed by oil oxidation and present in the oil as fine particles or colloids. The colloids accumulated in the oil during use are:
- winding varnish and oil oil sludge;
- soap formed by the reaction between oil aging products and the metal in the transformer;
- acidic sludge, which contains now metals, such as acids, including asphaltogenic acids, poorly soluble in oil, carbenes and other oxidation products.
Low molecular substances (peroxides and acids) solved in the oil have virtually no impact on its dissipation factor, but if the substances are colloidal, they are the main factor of oil conductivity.
As the concentration of moisture and other impurities in the transformer oil grows, the dissipation factor worsens and the dielectric strength is reduced.
With the growing amount of insoluble oxidation products, sediment forms on the transformer parts, degrading the dissipation of heat from the core and other hot parts. The sediment reduces the dielectric strength of solid insulation and destroys it, also causing corrosion of metal parts, reducing the dielectric strength of oil due to corrosion products. Non-organic impurities (atmospheric dust) enter the transformer during oil change and have little effect on equipment operation.
The nature of the sediment varies. Sometimes it is dark, asphalt like substance on the windings, other times it is sticky oily sludge.
When oil is oxidized by molecular oxygen, acidic and neutral oxidation products form in the oil beside oxyacids. These are carboxylic acids and phenols.
Enlargement of sludge particles is due to the fact that the particles are charged, and, like any colloidal particles, they acquire induced dipole momentum in the electrical field. Other similar particles, as well as other dipole molecules turn the oppositely charged pole towards the original particle, forming a shell around it. Thus, relatively large solvated complexes form.
Short service life of oil may be due to production faults, accelerated oxidation due to the influence of transformer construction materials, faulty design of transformers and incorrect operation conditions. New and purified oil transportation may cause contact with air and solution of air and water in the oil, i.e. increase of moisture and total gas content in the oil. Hence the strict quality requirements to the oil used in electrical equipment of various voltage class.
This information on the changing properties of transformer oil during use offers understanding of the processes due to the influence of air, electric field, temperature and the reaction of oxidation products with the transformer construction materials.