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Insulating materials used in transformers

Updated on August 28, 2014

Insulation is one of the most important constituent of a transformer. The durability and stability of the transformer depends upon the proper utilization of insulating materials in it. In transformers mainly three insulating materials are used.

  1. Transformer oil
  2. Insulating paper
  3. Press board

Of the three, the major insulating material used is transformer oil.

Transformer oil

Transformer oil

As said earlier transformer oil is the major insulating material used in transformer. It is one of the important factors that determine the life and satisfactory operation of the transformer.

The transformer performs the following two functions.

1. It provides insulation in combination with the insulating materials used in the conductors and coils.

2. It also acts as a coolant to extract heat from the core and the windings.

Transformer makes use of hydrocarbon mineral oil. It mainly consists of four generic classes of organic compounds. They are aromatics, paraffins, napthenes and olefines. Transformer oil will provide better insulation when aromatics, paraffins, napthenes and olefines are present in it at a right proportion. Transformer oil is desired to have more of saturated paraffins and less of aromatics, napthenes and olefines for better insulation. For more stability, more aromatics and napthenes are necessary. To get transformer oil at insulating property and stability, there must be an optimum mix of four organic components. This can be obtained by careful refining of crude oil. Vegetable and animal oils cannot be used in transformers because they form fatty acids and attack the fibrous insulating materials.

The transformer is affected by its operating conditions. The presence of moisture or suspended particles in transformer oil affects its dielectric property. Hence transformer oil it should be tested periodically. If the oil is containing moisture or suspended particles it should be filtered or replaced by fresh oil.

Physical constants of Transformer oil

Recommended value
Thermal conductivity
0.12 W/m deg C
Specific Heat
2.06 kJ/kg deg C
Co-efficient of Expansion
0.00078/ deg C
Mean density factor
0.00065/ deg C
The above parameters are based on IS 335

Insulating paper

Insulating paper is made from the vegetable fibers. These fibers mainly consist of cellulose. The main properties of these papers are listed below:


Grammage is the ratio of mass to the area. It influences most of the electrical and mechanical properties. The recommended value for 125 micrometer thick paper is 100 gm/mt2.The maximum allowable variation is 5%.


The paper recommended in transformer use can have a density range of 0.6 to 1.3 gm/c.c.

Moisture content

Moisture content reduces the dielectric property of the paper. Since paper is hygroscopic, moisture ingression takes place with usage. The maximum allowable moisture content is 8%.

Oil and water absorption

The dielectric property of paper increased when impregnated in oil under vacuum and decreases with water content. Even though the water content is not recommended the maximum permitted water content is 10%.

Air permeability

Permeability is the rate at which the air can pass through it. The dielectric strength of paper is inversely proportional to the air permeability. The recommended value of air permeability is 0.2-0.5.

Tensile strength and elongation

Paper must be able to withstand the tension during wrapping. Recommended value of tension is 78 - 85 N-mt/gm in wrapping direction and 25 - 30 N-mt/gm.

The electrical parameters of insulating paper are indicated below.

Recommended Value
Break down voltage
7- 7.5 kV/mm (min) at 90 deg. C
Dissipation factor
0.003 (max)
10 ns/mt (max)

Press board

Press board is also made up of vegetable fibers and contains cellulose. Solid press board unto 6 mm to 8mm thick is ordinarily made. Since the most difficult insulation problem in HT transformer occur at the ends of the windings and lead outs from the windings hence moulded pressboards are widely used in these parts for insulation. Synthetic resin bonded paper based laminates are used in voltage stressed zones. The important parameters considered are density, tensile strength, elongation, conductivity, oil absorption, moisture content, compressibility etc.

A complete Electrical Engineering reference guide


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    • profile image

      amar dubey 8 months ago

      What are the type of insulators used in transformer windings

    • profile image

      ravi Trivedi 2 years ago

      How to identify shell form transformers ? what are the parameters.

      I have observed core steel used in shell form transformers when dis assembled and tested , out of used unit, it gives very irritating result in core loss, why? what makes it poor in quality.

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      Rajinder kumar 2 years ago

      Power transformer is shell tye transformer

    • goodnews11 profile image

      OSBERT JOEL C 3 years ago from CHENNAI

      Thank you Mr.KUNJESH KAMBODI

    • profile image

      KUNJESH KAMBODI 3 years ago