Transformer oil is used in the oil filled transformer and in some other system such as high voltage capacitors, fluorescent lamp ballasts, circuit breaker etc.
Basic requirement of transformer oil is that it should be stable at high temperatures with excellent electrical insulating properties and suitable cooling property.
Standard for transformer oil
ASTM D3487 – 09 is the Standard Specification for Mineral Insulating Oil Used in Electrical Apparatus. ASTM- American Society for Testing and Materials.
How transformer oil is used in the liquid type transformer
Core and windings of liquid cooled power transformer is submerged with transformer oil.
Function of transformer oil
Following are the function of transformer oil
- As Electrical insulation media
- As Cooling media (transfer heat to the wall of tank/ conservator).
- Suppression of arcing & corona.
Types of transformer oil
- Highly refined mineral oil.( Mostly used)
- Silicone-based oil, fluorinated hydrocarbons (high fire resistant but costly then mineral oil)
- Pentaerythritol tetra fatty acid, natural and synthetic esters (low flammable, environment friendly, high moisture tolerance than that of mineral oil)
- Vegetable-based oils ( Unsuitable for cold environment)
Transformer oil temperature requirements as per standard
As per ANSI (American National Standards Institute), the maximum temperature permitted for oil is of 90°C
Why Maintenance & inspection of transformer oil is important
Modern transformer manufacturing technique is so much improved that internal inspection of transformer become less important. Maintaining the transformer oil from deterioration serves the purpose of transformer preventive maintenance. Also by inspecting the transformer oil, transformer condition can be defined.
Required characteristics of transformer oil
- High dielectric breakdown
- Low viscosity -resistance to gradual deformation by shear stress or tensile stress.
- Well refined and free of materials that they may corrode the metallic parts
- Be free of moisture and polar ionic or colloidal contaminants
- To have a low pour point (the temperature at which a liquid lost its flow characteristics- become semi-solid)
- Low flash point (lowest temperature at which a liquid vaporizes to create ignitable mixture in air).
How Deterioration of the transformer oil occurs
Absorption of the moisture from the air and foreign particles that get into the oil and start to cause oxidation.
Oil suffers a series of chemical reactions such as the decomposition and the polymerization that produces particles that are not dissolved in oil and that are collected in the coil and windings. These particles are called sediments. The sediments do not affect directly the dielectric breakdown, but the deposits that are formed on the winding hinder its normal refrigeration.