Transformer Oil

Transformer oil or insulating oil is an oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled transformers (wet transformers),[1] some types of high-voltage capacitors, fluorescent lamp ballasts, and some types of high-voltage switches and circuit breakers. Its functions are to insulate, suppress corona discharge and arcing, and to serve as a coolant.

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Transformer Oil

Transformer oil is most often based on mineral oil, but alternative formulations with different engineering or environmental properties are growing in popularity.

Transformer oil's primary functions are to insulate and cool a transformer. It must therefore have high, dielectric strength, thermal conductivity and chemical stability, and must keep these properties when held at high temperatures for extended periods.[2] Typical specifications are: flash point  140 °C or greater, pour point  −40 °C or lower, dielectric breakdown voltage 28 kV (RMS) or greater.[3]

To improve cooling of large power transformers, the oil-filled tank may have external radiators through which the oil circulates by natural convection. Power transformers with capacities of thousands of kVA  may also have cooling fans, oil pumps, and even oil-to-water  heat exchangers.[4]

Power transformers undergo prolonged drying processes, using electrical self-heating, the application of a vacuum, or both to ensure that the transformer is completely free of water vapor before the insulating oil is introduced. This helps prevent corona formation and subsequent electrical breakdown under load.

Mineral Oils are the petroleum products, like Naphthenic based transformer oil and Paraffinic based transformer oil. Naphthenic based transformer oils are known for their heat distribution, which is one of the main problems with transformer. This also has a good flowing feature under low temperature and it is wax-free. These type of oils are better for low-temperature. Even though it oxidises easier, the product formed by this process (i.e sludge) is soluble. Hence, it won’t obstruct the cooling system of the transformer. Paraffinic based transformer oil is obtained from paraffinic crude oil using solvent separation methods. This is known for its good thermal and oxidation durability and good high temperature viscosity feature. Because of its high viscosity index due to the presence of wax, though the oxidation rate is lower than the naphthenic oils, the precipitant or the sludge is formed due to the oxidation. This might become an obstruction for the heat dissipation. Since the rate of oxidation is low, cost effectiveness and availability, this oil is widely used in India.

Synthetic oils are generally silicone based were popular in the middle of 70s. This is generally used in the fire-prone area because of its fire-retardant properties. It also has few problems of low heat dissipation and high moisture absorbing capacity. It is also costlier than mineral oil.

Petroleum based oils are actually very effective as a transformer oil. However, due to high flammability, a small leakage can easily catch fire. This is one of the reasons why synthetic oils are used in fire-prone areas. And also the fire codes require these transformers to be non-flammable or a dry type transformer, if they happened to use it inside the residential buildings.

Silicone based oils are even less flammable but they are not only expensive than esters but also less bio-degradable. Researches are going on in the usage of vegetable based oils like coconut oil. But it is found as unsuitable for the cold climatic condition and also for voltages over 230kV. One can also say that the dissipation factor is reducing over the period of time which is not as usual as in the case of other transformer oils.

The following tests are performed to determine the quality  of transformer oil

  • Dielectric Strength
  • Moisture
  • Acidity
  • Interfacial Tension
  • Dielectric Dissipation Factor Test for
  • Corrosive Sulphur in Oil
  • Test for Oxidation Stability
  • Specific Resistance (Resistivity)
  • Flash Point
  • Pour Point
  • Viscosity
  • Sludge Test
  • Dissolved Gas Analysis (DGA).
  • Dielectric Strength Test

Solid materials and water molecules are removed from the transformer oil using centrifugal separators. Apart from the above process, de-aeration, filtration and dehydration are also carried out to enhance the quality of the transformer oil. In small transformers, purification of oil is done directly by removing the oil and cleaning the equipment. Once cleaning is completed, oil is transferred using filter plants. For large transformers, without removing the oil, it is made to circulate through the purifier. This process is done without energising the transformer. Inhibitors

To extend or delay the process of oxidisation, substances like Ditertiary Butyl Para Cresol (DBPC) are used. This process is called ‘inhibiting an oil’. By this process, the life of the oil can be extended by three to four times of the actual period.

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