Electrical Engineering ⇒ Topic : Different type of insulating materials

INSULATING MATERIALS

'Electrical insulating materials' are defined as materials which offer a very large

resistance to flow of current, and for that reason they are used to keep the current in its proper path along the conductor.

'Thermoplastic materials' are those which soften on the application of heat, with or without pressure but they require cooling to set them to shape.

'Thermosetting materials' are those plastics which require heat and pressure to mould them into shape.

Insulating varnishes' ate generally classified according to composition as "oil varnishes" and "spirit varnishes".

The electrical insulating films comprise a group of thin flexible insulating materials made from various polymers.

The function of insulating liquids is to provide electrical insulation ana neat transfer.

Transformer oil serves two purposes :

• It transfers heat by convection from winding and core to the cooling surfaces.
• It maintains the insulation of the windings.
• Presence of even a trace of water in transformer considerably.

'Sludge' formation produces the following effects :

•  Rate of heat transfer is reduced.
• Ducts are clogged.
• Increase in operating temperature results.

The various tests which are carried out on transformer oil are given below :

• Moisture test
• Acidity test
• Sludge resistance test 
• Electric strength test.

If the gas is used in applications under high pressure which may cause liquification.,

chemical instability and corrosion problem may be encountered. Therefore insulating gases are normally used under conditions such that the liquification of gas does not occur.

Insulation resistance is the resistance between two conductors (or systems of conductors) usually separated by insulating materials. It is the total resistance in respect of two parallel paths, one through the body and other over the surface of the body.

The potential gradient at which breakdòwn occurs is termed as dielectric strength.

Dielectric constant (Permittiuity) is the ratio of the electric flux density in the material to that produced in free space by the same electric force.

Dielectric hysteresis is defined as the lagging of the electric flux behind the electric force producing it so that under varying electric forces a dissipation of energy occurs, the energy loss due to this cause being called dielectric hysteresis loss.

Ageing is, in effect, the wearing out of an insulating material by reducing its resistance to mechanical injury