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Oil filtration

During operation, transformer oil accumulate contaminants, which can form various chemicals. These substances reduce the oil’s performance and are, of course, undesirable.

Operation of the transformer becomes unstable. To prevent this, transformer oil is filtered and purified. Some of the methods are discussed in more detail below.

The first stage of transformer oil purification is mechanical. This is a superficial treatment to remove particulate matter and water. The next step is deeper purification performed in vacuum with heating.

The first two stages are, in fact, preliminary. The main process involves various chemicals.

One of the methods is purification of oil with a 98% sulfuric acid solution.

In comparison to other chemical purification methods, the use of sulfuric acid has a significant drawback. Beside reacting with the contaminants, the acid also adversely influecnes the structure of the oil, making it somewhat unstable. Additional processes are required to resolve that problem.

The nature of selective purification is evident from the title. Speical solvents are introduced into the oil to remove specific impurities.

De-waxing is another widely accepted process. In this process, oil is treated with special solvents: acetone, toluene, bensol etc, to remove solid contaminants.

It should be noted that chemical methods influence oil’s stability, but extend the oil’s service life at the same time.

It should also be remembered that any purification process should end with finishing purification, closing the cycle of oil processing and filtration. This is usually done by contact method.

This means that the oil is mixed with special materials, usually clay or bleaching earth. The materials are then mixed and heated. Heating facilitates acviation of all sorbents in the clay.

These absorbents capture contaminants. Deep filtration separates oil from the clay. When selecting adsorbent, it is necessary to pay attention to the content of moisture. It should be suffucient to make production efficient and to make processed oil compliant with specifications.

The most interesting technology today involves the use of bleaching clays (Fuller’s earth). Globecore manufactures a range of CMM type units for filtration of various oils with the use of Fuller’s earth. The advantages of the design are the ability of multiple reactivation of the sorbent, mobility, simplicity of operation and high quality of the output product.

Downtime is significantly reduced by the ability to reactivate the sorbent without the need for frequent replacement, thus increasing process efficiency.

Which kind of Transformer Oil Should my Company Buy?

A great number of manufacturers and distributors of transformer oils operate in international and national markets.

Transformer insulating oil is a dielectric liquid that is intended for cooling and insulating electric power transformers and other high voltage equipment.  Transformers remain an indispensible part of the electric power generation and transmission industry.

In scientific terms, transformer oil is a product of crude oil refining.  The performance properties of transformer oil depend heavily on the quality and parameters of the raw material.  The performance of the oil is defined by the positive and negative properties of the crude oil.  Chemical composition of oil is quite complex.  It includes the following components: paraffin (10-15%), naphtenes or cycloparaffins (60-70%), aromatic hydrocarbons (15-20%), asphalt (1-2%), sulfuric compounds (<1%), nitric compounds (<0.8%), naphtenic acids (<0.02%) and antioxidation additive (0.5 – 0.5%).

Transformer oils must be highly resistant to oxidation and must not form sludge or emulsify when coming in contact with water.  Anti-oxidation additives are an important component and can be found in virtually all types of transformer oil.  When selecting an oil for your equipment, be sure to look for oils that have good antioxidant additives included since they will inhibit the oxidation process considerably better than uninhibited oils.

The efficiency of such additives is based on the ability of the additives to react with active peroxide radicals that form as the result of hydrocarbon oxidation reaction and are the primary carriers.  The presence of additives in transformer insulating oils slows the aging of the oil.  As soon as the additive is depleted, the oil ages in about the same amount of time as an oil without the the antioxidant additives.  Without the additives, the oil aging process will accelerate.

Two of the main characteristics of transformer oil are its viscosity and density.  These have a pronounced affect on the function of the oil.  Higher viscosity may mean better dielectric strength, but may reduce the ability of the oil to transfer heat and cool the transformer.  Optimal kinetic viscosity of the oil at 20 ̊C is about 28-30×10-6 m2/second.

The oil’s “Dissipation Factor” is responsible for the oil’s dielectric properties and protects the transformer’s electrical system from possible short circuits.

These and the other characteristics of transformer oil are improved by the use of high quality crude oil, deep refining and introduction of additives that increase the oil’s oxidation stability and reduce corrosion.

When oil is significantly oxidized due to the influence of water, solids, air and other gases, and high temperatures, the oil should be replaced.  If the degradation of the oil’s properties is not detected in time, transformers can and will fail.  When oil decays, it can no longer serve as a dielectric insulator and starts to react with the metal of the transformer tank and hinders cooling and may lead to short circuits.

The following are some of the different kinds of oils that may be purchased in the local market:

VG type oil is made from paraffinic crude oil by a catalytic process with the addition of an antioxidant  additive.  It is a good dielectric and is rather stable against oxidation.  It is mostly used in higher voltage equipment.

GK type oil is made of sulfuric paraffinic oils by hydrocracking.  It also contains ionol, an antioxidant, that ensures good stability and dielectric properties of the oil.  It is another kind of oil used in higher voltage systems.

TKp oil is made from low-sulfur naphtenic crude by an acid-alkali purification process.  The oil also contains antioxidant additives.  This oil is recommended for equipment with voltage up to 500 kV.

Oil selection depends not only on the type of electric equipment you are using, but also on the individual requirements of the facility.  Most of transformer oils are universal.  In all cases, the correct selection of oil type with consideration of local climatic and physical operating conditions ensures reliable and stable operation of electric power equipment such as high voltage transformers and switch gear.