Transformer oil deterioration, regeneration and equipment derating.

Transformer Oil Deterioration

The Oil inside power transformers have a vital role to play in the transformer's functioning. The function of the transformer oil is two-fold, to provide cooling to the transformer windings and to provide insulation. However, over a period of many years, the transformer oil deteriorate owing to many factors. This deterioration causes a change in the physical and chemical properties of the oil.

Some of the reasons for transformer oil deterioration are

Oxidation of the oil.

The transformer breather permits the entry of air into the transformer, although it filters the moisture. The air which flows inside the transformer oxidizes the oil and forms a sludge of hydrocarbons. This process, though, usually occurs gradually over a period of many years. The sludge thus formed hinders the cooling of the transformer and causes heating. The sludge, sometimes, blocks the cooling ducts of the transformer. Higher temperatures inside the transformers, in turn, cause further sludge formation.

Thermal Decomposition

At high temperatures, the organic compounds in the transformer oil break down due to a phenomenon known as pyrolysis. This results in the formation of unwanted carbon compounds, sludge, etc.

Moisture contamination

Under ideal conditions, the oil in a transformer is protected against the entry of moisture by means of the silica gel filter in the breather. The silica gel changes color from blue to pink when it gets saturated with moisture. If the silica gel is not renewed in time, moisture may pass through the filter contaminating the oil.

Transformer Oil Regeneration

Transformer oil regeneration refers to the treatment of old transformer oil. Oil Gets contaminated due to the entry of moisture and the formation of sludge. The contaminated oil in the transformer Regenerated. Regeneration involved degasification, dehydration and filtration. The old transformer oil is regenerated by passing it through columns containing fuller's earth. Fuller's earth is a type of clay which removes the impurities in oil when it is passed through it.
The impurities are thus removed without the use of any chemical. The oil which is purified can be reused in the tank. After a certain number of times of cleaning, the Fuller's Earth can be reactivated.
The oil is drawn from the transformer, purified and sent back to the tank. Thus the transformer need not be taken offline.

Effects of sludge in Transformer Oil 

Sludge in Transformer is formed as the oil breaks down. Sludge attacks the cellulose of the transformer windings causing it to deteriorate. It reduces the oil circulation inside the transformer.
Sludge also forms a layer on the transformer winding and reduces the heat transfer. These lead to higher transformer winding temperatures.
Sludge thus has to be removed. The transformer oil can be replaced or filtered.

Transformer Temperature Rise Ratings

When a temperature is at no load, its temperature is slightly greater than the ambient temperature. When the transformer is loaded the temperature rises. The temperature rise rating of a transformer gives the maximum value to which the temperature of the transformer would rise.
Dry type transformers are usually available in three standard temperature rises, 80C, 115C or 150C. Liquid filled transformers have ratings of 55C and 65C. These values are based on a reference value of 40C.
For instance, a transformer with a temperature rise rating of 80C will reach a maximum temperature of 120 C (40+80) during operation.
The lower the temperature rise rating, the better is the ability of the transformer to withstand momentary overloads. Thus a transformer with a temperature rise rating of 80C will have a better overloading capacity than a transformer with a temperature rating of 120C.
The temperature rise rating gives and idea of amount of heat produced and the amount of heat removed. Transformers with lower temperature rise ratings use windings with lower resistivity.
Transformers with low temperature rise are used in special applications such as in underground installations, air conditioned buildings

Reduction in Transformer Capacity due to Aging of Core

The core of a Transformer is made of a number of steel sheets which are placed one on top of another.  These sheets are laminated to prevent losses due to eddy current.

During the operation of the transformer, these sheets get deformed temporarily due to the magnetic flux.  This phenomenon is called magnetostriction.  Over long periods of time, due to repeated movement, gaps form between the surface of the sheets.

These gaps affect the magnetic circuit of the transformer circuit.  This causes a reduction in the flux of mutual inductance.  The transformer current increases by about 10 percent.

The gaps in the core sheets also distort the flux lines and cause an increase in the leakage flux.  This leakage flux causes eddy current heating in metallic components of the transformer such as the tank and other fixtures.  This causes the current to increase by another 5 percent.

This causes an overall reduction in transformer capacity.