An ideal insulating liquid must have following properties:
- High electric strength, impulse strength, and volume resistivity.
- Low dielectric dissipation factor.
- High or low dielectric constant (depending on application).
- High specific heat and thermal conductivity.
- Excellent chemical stability and gas-absorbing properties.
- Low viscosity, low-density, low volatility, low solvent power and high flash point.
- Good arc quenching properties.
- Non-flammable and non-toxic.
Types of Liquid Insulating Materials
- Petroleum Mineral Oils
- Contaminated Oil
- Vegetable Oils
1. Petroleum Mineral Oils
The main elemental constituents of petroleum are carbon and hydrogen, together with small amounts of sulphur, nitrogen, and oxygen and trace amounts of metals such as Vanadium, Sodium, Nickel, and Iron.
Petroleum oils have different categories: Paraffinic, asphaltic and mined type.
2. Contaminated Oil
Transformer oil gets contaminated by impurities such as moisture, fibres, resins etc. Impurities in the oil may be because of imperfect purification, or they may get into the oil during transportation and storage, and while in service and finally, the oil itself can create these impurities. Impurities may be present in dissolved state, suspended state, deposited on the bottom of the tanks or on parts of apparatus immersed in the oil or floating on the surface.
Water is one of the important impurities in the oil. Oil when kept in open container, gradually absorbs water. It is a product of oil oxidation. It helps in the corrosion of metallic and insulating materials present and seriously degrades the properties of the oil. The carbon particles make the oil dark and non-transparent. Carbon particles settle at the bottom of the tank. Carbon particles are good conductors of electricity so their presence greatly weakens the insulating properties.
3. Vegetable Oils
The vegetable oils are the oldest insulating liquids used in the manufacturing of electrical machines. The vegetable oils may be classified as drying oils (linseed, Tung); semi-drying oils (corn, cotton seed, soybean, rapeseed, sesame, sunflower); and non-drying oils (castor, coconut, palm, olive, peanut). Linseed and Tung oils are used in the formulation of insulating varnishes, treatment of transformer and motor coils. Castor oil is used as plasticizers in insulating resin composition which is applied as structural and insulating materials, and coating composition in electrical equipment.