Classification of Liquid Dielectrics:
In recent years, a substitute to mineral oils, other polyester oils have been developed which are extensively used in transformers in Europe and other countries. One such oil is the halogen free Penta-Etythrite-Tetra Fatty Acid. Polyester oil (PETFP oil) which has very good electrical, physical and thermal properties. It is also biodegradable, i.e. when decomposed has almost negligible toxicity and does not contribute to pollution. The Classification of Liquid Dielectrics are
1.Transformer Oil (Mineral Oil)
As already mentioned, transformer oil is the most commonly used liquid dielectric in power apparatus. It is an alniost colourless liquid consisting of a mixture of hydrocarbons which include paraffins, iso-paraffins, naphthalenes and aromatics. When in service, the liquid in a transformer is subjected to prolonged heating at high temperatures of about 95°C, and consequently it undergoes a gradual ageing process. With time the oil becomes darker due to the formation of acids and resins, or sludge in the liquid. Some of the acids are corrosive to the solid insulating materials and metal parts in the transformer. Deposits of sludge on the transformer core, on the coils and inside the oil ducts reduce circulation of oil and thus its heat transfer capability gets considerably reduced. Complete specifications for the testing of transformer oils are given in IS 1866 (1983), IEC 296 (1969) and IEC 474 (1974).
Among synthetic Classification of Liquid Dielectrics dielectrics, polyolefins are the dielectrics of choice for applications in power cables. Over 55% of synthetic hydrocarbons produced worldwide today are polyolefins. The most commonly used olefins are polybutylene and alkylaromatic hydrocarbons (e.g., alkyl-benzene). Their general characteristics are very similar to those of mineral oils.
Two aromatic hydrocarbons, benzene and diphenyl, are chlorinated to produce chlorinated aromatic compounds called askarels or simply polychlorinated biphenyl (PCB). They possess high fire point and excellent electrical properties. In recent years their use has been banned throughout the world, because they pose serious health hazards.
Silicone oils represent an alternative to PCBs but they are expensive. Even at a temperature of 150°C they exhibit high long-term thermal stability. Silicone oils are resistant to most chemicals, and are oxidation resistant, even at higher temperatures. They can be used at higher temperatures than mineral oils. Silicone oils are an acceptable substitute for PCBs in transformes despite their slightly inferior nonflammable properties.
Natural esters such as castor oil has been used as a capacitor impregnant for many years, but currently two types of synthetic esters are being used, viz. organic esters and phosphate esters.
Organic esters have high boiling points in relation to their viscosity and, therefore, have high fire points. They have a good viscosity-temperature relationship and are used extensively in capacitors.
The phosphate esters have high boiling point and low flammability and therefore are used in transformers that are to be installed in hazardous areas.
6. Latest Developments
Some new oils have been introduced in recent years. These are being marketed under different commercial names, such as high temperature hydrocarbon oil, tetrachloroethylene and perfluoropolyether.
High Temperature Hydrocarbon (HTH) oils have good electrical insulating and adequate heat transfer properties. They are chemically similar to regular mineral transformer oils, but possess higher boiling points and higher tire points. However, they have higher viscosity which reduces heat transfer capability.
Tetrachloroethylene (C2C14) is a nonflammable insulating fluid, and is used in mixtures with mineral oil. It has very low viscosity and therefore gives excellent heat transfer properties. Some typical properties of HTH oil and C2C14. Perfluoropolyether has been recently introduced in the European market with the trade name Galden HT40. It is a nonflammable oil and its boiling point exceeds 400°C. It possesses low vapour pressure, so can be used as a good heat transfer medium.