Page 262 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Testing of transformer oil

Objectives: At the end of this lesson you shall be able to:
• explain the transformer oil
• name three insulating oils used in transformer
• list the important properties of a transformers oil
• state the necessity of transformer oil
• state the causes for deterioration of oil
• explain the methods of testings the oil for its parameter.

Transformer oil
It is an insulating liquid, used to cool and insulate the transformer windigs and core.A cooling liquid is also
considered as a part of the transformer.

Three kinds of cooling oils/liquids are used in transformers today.
• Mineral oil (inflammable)
• Silicon liquids(low flammable) and
• Hydrocarbon liquids (non-flammable)

The common transformer oil is a mineral oil obtained by refining crude petroleum. Clean and dry mineral oil is an
excellent insulator. Its loss by evaporation is small. But it is an inflammable liquid and readily absorbs moisture
from the air. Great care should be taken to keep the oil away from flame and moisture.

Synthetic liquids do not catch fire easily. Synthetic liquids are therefore replace mineral transformer oils of those
transformers used in
• underground mines

• refineries and hazardous location
• tunnels
• workshop and plants of metal processing theatres and cinemas etc.
Transformer oil consists of organic compounds, namely paraffin, naphthalene and aromatics. All these are hydro
carbons, hence insulating oil/transformer oil/ synthetic transformer oil known as ASKARELS and PYROCLORE
are also in use.
Properties of transformer oil

A good transformer oil should have the following properties.
1 High specific resistance so that high insulation resistance
2 Better heat conductivity, (i.e) higher specific heat.
3 High firing point, so that not to catch fire at low temperature.

4 Do not absorb moisture easily, when exposed to air.
5 Low viscosity
Necessity of transformer oil: Large capacity distribution transformers produces more heat due to losses like
core losses and copper losses, on load. It is necessary to stablize the heat within temperature class by providing
suitable insulating materials.
Transformer oil acts as a good electrical insulating material. Thus it reduces electrical break down. Transformer
oil will also act as cooling agent. Thus it brings thermal stability to all the internal parts of transformer.
Causes for deterioration of transformer oil: When the oil cooled transformers are in use, the oils of the
transformers are subjected to normal deterioration due to the conditions of the use.

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CITS : Power - Electrician & Wireman - Lesson 41-49 CITS : Power - Electrician & Wireman - Lesson 41-49