Transformer Oil & its Evolution

The article gives a glimpse of types of transformer oils and its tests. - S Prabhakar Karthikeyan, Sarvesh Rathi, Narala Anudeep Reddy  

Transformer Oil & its Evolution - Electrical India Magazine on Power & Electrical products, Renewable Energy, Transformers, Switchgear & Cables
Transformer Oil & its Evolution

Transformer oil is a mineral insulating or cooling oil derived from crude petroleum. It is a mixture of various hydrocarbons consist partly aliphatic compounds (open chain compounds) with the general formula – CnH2n+2 and CnH2n.

Types of Oils Used

Mineral oil and Synthetic oil are the majorly used transformer oil.

Mineral Oils

These 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

Synthetic oils which 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.

Figure 1

Transition from Conventional Oils

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.

Mineral oil is hazardous directly for human and the environment. This is mostly available as the by-product of refining crude oil to make gasoline and other petroleum products. This mainly consists of alkanes and cycloalkanes. And its poor biodegradability makes it a potential long term pollution for the environment. And also these are classified under carcinogenic substance by the World Health Organization.

Figure 2: Use of Nanoparticles

Alternative to Mineral Oil

There are few alternatives to these mineral oils. Some of them are Pentaerythritol tetra fatty acid, natural and synthetic esters.

Some of the advantages when compared to mineral oil are:

  • Low volatility
  • High fire point, so that it can be used in high-fire-risk places
  • Lower pour point
  • Greater moisture tolerance
  • Improved function at high temperatures
  • Non-toxic
  • Bio-degradable.

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. From the figure 1, 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.

Use of Nanoparticles

In most of the applications, nano-technology is the ultimate answer for its sensitivity. There had been some literatures on nanoparticles of titanium oxides and iron oxides in transformer oil applications. The maximum enhancement of BDV has been observed by using these particles which is achieved by adding different type of oxides into the promising ester fluids to make it even better than before by which the efficiency can be increased. Many oils contain certain aromatic compounds (closed chain or ring compounds) related to benzene, naphthalene and derivatives of these with aliphatic chains. Good transformer oil must insulate and prevent flash over on the exposed parts within the equipment and it must effectively transform the heat from the core to the radiating surface. The transformer oil with high dielectric strength is always considered for application in which it is used.

Transformer Oil Tests

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).
Figure 3: Dielectric Strength Test

Dielectric Strength Test

An increasing AC voltage of rated frequency is applied to the electrodes which are immersed in the testing oil (with the gap of 2.5 mm), approximately at the rate of 2 kV/Sec, starting from zero up to the value which produces breakdown. The test kit will have provision for automatic switching off of the supply voltage within 0.02 second. The test shall be repeated six times on the same cell filling and the arithmetic mean of the results is noted as the electric strength or BDV (Break Down Voltage) of the oil under test. The test shall be conducted in a dry place free from dust and voltage applied every time after disappearance of any air bubbles. The time intervals shall be five minutes, if the disappearance of air bubble cannot be observed.

Figure 4: Transformer Oil Treatment

Transformer Oil Treatment

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.


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. Infrared spectroscopy, gas chromatography or thin layer chromatography is used to detect the presence of DBPC in the oil.