Transformer oil is a common liquid dielectric used in many types of electrical equipment.
It is most often used to increase the dielectric strength and cool transformer windings etc.
One of the most important parameters of transformer oil is its dielectric strength. The minimal allowable dielectric strength is often strictly regulated.
In general, the dielectric strength of transformer oil depends on its chemical composition and the presence or absence of impurities.
If the oil is well purified, a breakdown can only occur due to collision ionization.
A breakdown involves sharp current increase. This phenomenon is to be avoided if at all possible, because although it does not reduce the oil’s general dielectric properties, its quality is still reduced.
The dielectric strength of transformer oil drops sharply if the oil contains water, gases and particulate matter. Research indicates that water is the main factor among the above. Water droplets are usually uniformly distributed in the oil without mixing. Since water is a polar liquid, and the oil is neutral, electricity causes attraction of polarized water molecules, forming chains with high electrical conductivity. Breakdowns occur in those chains.
The critical importance of water content is illustrated by this: a 0.01% increase of water content reduces the dielectric strength of the oil by a factor of 3.
Gas bubbles in the oil facilitate ionization processes, due to the different dielectric strength of the gas and the dielectric fluid. Ionized gas inclusions grow in size and cause local overheating of transformer oil. A gas channel forms, serving as a breakdown path.
Metal particles, soot and fibers from winding insulation also have a noticeable effect on the dielectric strength of the oil. For instance, insulation fibers can accumulate moisture, block insulation gaps and cause breakdowns.
To improve the dielectric strength of transformer oil, it is dehydrated, degassed and filtered to remove particulate matter using special GlobeCore oil purification systems.
Acids, resins and other impurities form in transformer oil during use due to electric field, heating and contact with oxygen. The oil ages and its dielectric strength is reduced, along with the reliability of the whole electric system. To prevent failures and protect the expensive equipment, transformer oil must be analyzed before and during use. The dielectric strength test is the most important part of such analysis.