What is liquid dielectric ?
If we used liquid as a insulating material in electrical equipment , it should be moisture free and also have good dielectric properties. In the transformer oil is used as the insulating purpose. Always select dry liquid because presence of water in transformer oil which may reduce its electrical strength very quickly.
Highly purified liquids have dielectric strengths as high as 1 MV/cm. liquids whose specific electric resistivity is more than 1010 ohms-cm. In an electrical field, both liquid and solid dielectrics are characterized by their polarization and dielectric losses; in strong fields, breakdown occurs.
Petroleum oils are the commonest insulating liquids.
- Mainly liquid dielectric material are used in high voltage equipment for two purpose are following
- Heat dissipation
- There are some other factor must be considered for selection of liquid are
- Heat dissipation capacity
- Saving of space
- Low cost
- Environmental influences
Classification of liquid dielectric
Transformer oil :-
Most commonly used liquid dielectric in power system equipment is transformer oil. it has very good heating capability temperature of 95°C and decrease with ageing Transformer oil is made of mixture of hydrocarbons which include paraffins, iso-paraffins and aromatics. The transformer oil become darker due to the impurities which are reduce heat transfer capability. due to acidic impurities, corrosion will occur on metal parts of transformer.
- Esters :-
natural esters such as estor oil has been used as a capacitor impregnate, but currently two types of synthetic esters are
Organic esters – high boiling and good viscosity temperature relationship.
Phosphate esters – have high boiling point and low flammability.
Synthetic hydrocarbons :-
characteristics is similar to the mineral oils. 55% of synthetic hydrocarbons produced world wide today are Polynesians. The most commonly used olefins are poly-butylene and alkylaromatic hydrocarbons.
Chlorinated hydrocarbons :- two aromatic hydrocarbons, benzene and diphenyl are chlorinated to produced chlorinated aromatic compound called askarels or simoly polychlorinated biphenyl. It has excellent electrical properties and processes high fire point.
Silicone oil :-
it represent an alternative to PCBs but are expensive. At temperature of 150°C , they exhibit high thermal stability.
Characteristics of liquid dielectric:-
Electrical properties :-
- permittivity is Low
- dissipation factor is Low
- conductivity is Low
- High resistivity
- Low dielectric loss
- High dielectric strength
- High dielectric constant
Mechanical properties :-
- mechanical strength is High
- Material should be homogeneous so local stress may be avoided
- Should not be deteriorate the material in contact with it
- Non inflammable
- No oxidation when exposed to atmosphere and moisture
- Thermal conductivity is High
- Thermal strength is High
- Thermal expansion is Low
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