In this article we will look at “oil polishing”: the meaning of the term, when this process is needed and how it is done.
What is transformer oil polishing
Oil is an essential part of transformer insulation system. Apart from its insulation function, it also removes heat from the core, taking some of the strain off the other parts of the transformers. This causes the oil to gradually age and accumulate impurities. Since the amount of oil even in one transformer is often tens and even hundreds of tons, operators like to use the oil to its fullest potential before a very costly oil change. Purification to remove impurities helps restore the performance of used oil and extends its service life.
There comes a point when the regular purification equipment, such as filtration units, can only remove part of the impurities. The condition of oil at this point is far from the ideal, since it now contains the products of aging and contaminants that make further use of oil difficult. Filtration does not return the original color of the oil and leaves its primary performance parameters (such as the dielectric strength, acidity, dissipation factor etc) largely unaffected and below the specifications. In this case oil polishing is necessary.
How polishing works
The actual process of oil polishing depends on its condition and type of impurities remaining after the preceding purification. If the previous process left only solid particles after the oil passes coarse filter, polishing involves simply a fine filter, or several filters for many stages of particle removal.
If the oil is contaminated with oxidation and aging products, its color has changed and the purity of the oil is out of specs, polishing involves the use of adsorbents The well developed surface of these materials capture impurities from the oil. Oil passes through columns which contain the adsorbent.
Transformer oil polishing equipment
- the types of adsorbent are different. The CMM-R machines are used with fuller’s earth, while the UVR units use special clay;
- the method of sorbent restoration is different. The adsorbent in the CMM-R is restored (reactivated) inside the machine. The UVR requires additional equipment for adsorbent reactivation;
- operation with online transformers. The CMM-R machines can be operated in direct connection to an energized transformer without the need to dry the oil or disconnect the transformer. The UVR does not have this capability.
As oil passes through the adsorbent, the latter captures the products of aging and oxidation. The polishing process restores the performance parameters of the oil and its color: from dark brown to clear amber.
The results for equipment with fuller’s earth
Below are the benefits of polishing transformer oil with fuller’s earth:
- color change from turbid brown to clean and transparent;
- acidity reduction from 0,63 to 0,01 mg KOH/g;
- reduction of corrosive sulfur content;
- dielectric strength increase from 11 to 73 kV.
Polishing is the finishing stage of oil regeneration, which brings all of the important performance parameters of the oil in compliance with operational requirements.