Transformer oil is the main liquid dielectric that has been widely used in many electrical devices.
Most often, it is used to increase the electrical strength and cooling of windings of power transformers and other apparatus.
The most important characteristic of a transformer oil is its electrical strength. Usually the minimum possible values of this parameter are strictly regulated by the rules for the operation of electrical equipment.
In general, the electrical strength of the transformer oil depends on its chemical composition and the presence (absence) of mechanical impurities.
If the oil is thoroughly cleaned of foreign matter, then the breakdown in it can occur only because of impact ionization of the molecules.
Breakdown is characterized by a sharp increase in current. This phenomenon is highly undesirable, since after it the transformer oil regains its insulating properties, but its quality still deteriorates.
If we talk about the electrical strength, then it drops sharply when there are moisture, gases and mechanical impurities in the oil. Studies show that among the listed factors, the greatest impact is still water. Usually, its droplets are evenly distributed throughout the volume of the oil and do not mix with it. Since water is a polar liquid and pure transformer oil is neutral, when the voltage is applied, polarization of water molecules takes place with the formation of chains of increased conductivity. It is for these that an electrical breakdown occurs.
On the critical effect of moisture can be judged only one figure: its presence in the oil in a volume of only 0.01% leads to a decrease in electrical strength of 3 times.
The presence of gas bubbles in the oil contributes to the intensive development of ionization processes, which is due to the different electrical strength of the gas and the liquid dielectric. Ionized gas inclusions increase in size and lead to local overheating of the transformer oil filtration. As a result, a gas channel is formed, along which breakdown occurs.
Metal particles, soot and scraps of winding insulation fibers also significantly reduce the electrical strength of the oil. For example, insulation fibers can absorb moisture and are then able to overlap insulating oil gaps and form channels through which breakdown occurs.
To increase the electrical strength, the oil is dried, degassed and cleaned of mechanical impurities at special GlobeCore oil purification plants.
During operation in transformer oil, contaminants such as tar and acids are formed. Their appearance is due to the influence of the electric field, heating and oxygen of the air. There is an aging of oil and a reduction in its electrical strength. The loss of insulation properties reduces the overall reliability of the electrical apparatus. To prevent emergencies and breakdown of expensive equipment, it is necessary to test transformer oils both before refilling into the equipment and during its operation. At the same time, the determination of electrical strength is an important part of the general test of energy oils.