Transformer oils perform several functions. They operate as liquid insulation and heat dissipation medium in oil filled switches, transformers, power cables and high voltage capacitors. Besides, they also act as arc extinguisher.
Transformer oil is made by purification of petroleum oils, made in turn from crude oil. The crude oil originates from various sources, and the types of crude oil may vary in terms of chemical composition, influencing the oil products.
During operation, transformer oils accumulate oxidation products and other contaminants, which reduce their performance and lifetime, as well as the lifetime of the whole transformer system.
Purification (regeneration) of transformer oils facilitates removal of water, solid particles, acids as well as other undesirable components (asphalt, tar, nitric and sulfuric compounds, certain hydrocarbons). Let us review the operations, performed during regeneration of transformer oils.
Industry literature recommends the following sequence of methods: mechanical, heating, physical and chemical, followed by chemical. It should be noted that the last group of methods is only used if the previous have unsatisfactory effect. The use of chemical methods requires more complicated equipment and is rather costly.
Water is probably the most dangerous contaminant in electric insulation oil, since even small amounts of water can lower the transformer’s dielectric strength. It can enter electric insulation material in the process of aging or from the environment.
Water can be present in oil in the form of droplets, solved or dispersed. There is an equilibrium between each state, depending on the presence of stabilizing additives, atmospheric pressure and ambient temperature.
To remove free or dispersed water, filtration and centrifuge separation are recommended. If the water cannot be removed by separation, it should be removed by vacuum processing or adsorption.
The latter method makes use of solid porous materials (adsorbents), which capture contaminants.
Adsorption purification may be performed in several ways: percolation, contact or counter-current.
Regeneration of one pass offers the highest efficiency and regeneration rate. In this case, the oil from GlobeCore’s CMM plant is supplied to a clean tank. Dielectric strength after regeneration increases to 70 kV, while acidity is decreased ten times.
If one pass of purification does not yield results, circulation is used. In this case the oil from the CMM goes back to the original tank. The increase of dielectric strength directly depends on the number of passes of the oil through the unit and increases gradually from the original number to a certain limit. The latter depends on the unit’s capacity.