Natural Ester Oils for Transformer Cooling

Natural Ester Oils or Esters are used in cooling Transformers. These ester based oils are created by a reaction between a tri-alcohol and fatty acids. Natural Oils have higher flash and fire point. They are chemically stable. They are also biodegradable.

 Another important advantage is the water solubility. Natural ester oils have a water solubility which is 20 to 30 times that of mineral oils. Thus water is drawn from the insulation and absorbed in the oil. This increases the life of the insulation.    They are thus considered an effective alternative to mineral oils which are environmentally risky and prone to fire accidents.

The downside is that Natural Ester Oils are more expensive. They also have high viscosity which results in slower flow through the transformer and results in reduced heat transfer. Natural Ester Oils are also prone to oxidation which reduces the lifespan. Natural ester oils are used in temperate climates. At cold climates, the flow rate is reduced. They are used in distribution and Traction Transformers.

Synthetic Ester based oils are artificial oils made by select ingredients. These oils can be tailored to meet the exact requirements of the application. Synthetic ester oils can be made to resist oxidation. They can be used in cold as well as temperate climates. 

Silicone Coolant in Transformers

Transformers generate tremendous amounts of heat. This heat can be cooled by air in case of small transformers. In large transformers, more substantial cooling media are needed. Mineral oil is widely as a cooling medium. Another medium of cooling transformers is Silicone Transformer Oil.

Silicone Transformer Oil has high flash point. Hence, it can be placed closer to buildings as the risk of fire is reduced. Silicone fluid is also self extinguishing. The heat and smoke produced in the event of a fire is minimal. It is not hazardous and environmentally friendly. It can be recycled. Its base is polydimethylsiloxane is a chemically inert material. 

It does not form sludge and down not break down over its lifecycle. The Transformer has a longer life with reduced maintenance. 
The downside is that Silicon Transformer Oils is not biodegradable. 
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Oil Leakages in Transformers

Oil Leakages are a frequent issue in Transformers which if not properly addressed can result in serious accidents such as fire or explosions. Oil can also cause accidents due to slipping.  

Besides, Transformer oil is an environmental pollutant. Spillage of Transformer oil can result in severe penalty for environmental violations.  

Oil Leakages can be detected visually most of the times. However, some minor leakages are not always detectable by the naked eye. Special techniques such as the application of special fluids which indicate oil leakage by changing colour.

Once an oil leakage is detected, it needs to be arrested and the point of leakage should be plugged. Welding the leakage would require switching off the transformer and draining the oil. 

Transformer oil leaks can be rectified by the application of special polymeric compounds and putties which cure and seal off the leakage.  

Leakages can be prevented by ensuring that the elastomeric components of the transformer such as the gaskets, O rings, etc are replaced at the scheduled replacement intervals. The Transformer should be properly painted and any damage to the painting should be properly rectified.  

In addition to oil leakages, in transformers filled with Nitrogen the leakage above the oil surface can result in the leakage of nitrogen. The leakage of the inert gas can be confirmed by applying a soapy solution and observing the bubbles. 


Regeneration of transformer oil

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. 

Significance of Acidity in Transformer Oil

Acidity in Transformer oil is measured by the TAN (the Total acidity number). Acids cause the breakdown of oil which results in sludge formation. 
Thus, measuring the acidity of Transformer is an important parameter. The acidity is expressed in Total Acidity Number (TAN) which is the milligrams of Potassium Hydroxide (KOH) required to neutralize the acid present in one gram of transformer oil.
Oils which have a high value of TAN will have to be treated.
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Sludge in Transformer

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.
See Also: Transformer oil Deterioration

Nitrogen in Transformers

Oil filled Transformers usually have a conservator which maintains the level of oil. The conservator has a layer of air on top of the oil. This air communicates with the atmosphere through a breather containing a desiccant,usually, silica gel.  

In some transformers, the tank of the transformer is filled with a layer of nitrogen above the oil in which the windings and the core are immersed. Nitrogen, being an inert gas, reduces the risk of a fire hazard. It also protects the transformer oil and the transformer insulation from reacting with oxygen in the air and deteriorating.  

In transformers containing nitrogen, the nitrogen is led into the space above the conservator through a pressure reducing valve. When the oil level in the transformer rises and the transformer breathes out, the nitrogen inside is released into the atmosphere. When the oil levels fall due to a reduction in ambient temperature or the load, fresh nitrogen is released into the conservator through the cylinder and the valves. 








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.