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Essentials of transformer oil

A good transformer oil acts as a cooling medium and dissipates heat from a transformer and provides liquid insulation in an electrical power transformer. Transformer oil is derived from crude petroleum and is composed of mineral oil and synthetic oil that are preferred for their low flammability. Transformer oils are used primarily by electricity generation, transmission and distribution companies, and transformer manufacturers and repairers who hold the largest share of the transformer oil market. The oil is formulated depending on its end use so it can withstand extreme weather conditions and give it the balancing properties needed.

Insights on market segmentation and expansion

According to Transformer Oil Market, a report by B2B research firm MarketsandMarkets, the global transformer oil market is projected to reach USD 3.0 billion by 2025 from an estimated market size of USD 2.2 billion in 2020, at a CAGR of 6.9% during the forecast period. Driving the growth for transformer oil are growing power requirements and the upgradation of ageing electric infrastructure in developing countries along with the expansion of electric grids due to growing population, industrialisation, and urbanisation. This increase in the electricity demand will also increase the number of substations, propelling the demand for transformers, and thus for transformer oil.

 The MarketsandMarkets report has specifically cited the APAC region to be the largest market for transformer oil because of the increasing urbanisation and expanding infrastructure in countries such as India and China, leading to a growth in the demand for power.

Benefits of choosing a high-performing, trustworthy, and reliable transformer oil brand

  • Offers the most comprehensive range of transformer oils, including grades conforming to international standards and benchmarks
  • Built by indigenous R&D efforts with a strong global customers base
  • Largest range of customised grade
  • Environmentally friendly, offering greater oxidation stability and lower tendency towards gassing, thus contributing to a long service life for instrument transformers.
  • Non corrosive in nature, with very low sulphur level
  • High dielectric strength
  • Reliable low temperature performance
  • Passes the 2006 Oil/Copper interactions tests as described in ASTM D 1275B
  • Offers neutral behaviour with insulating materials

Important elements for testing transformer oil

A preventative maintenance program requires testing of the transformer oil, thus establishing when remedial action is necessary. The following 5-part tests are a minimum requirement of a yearly maintenance program.

  • Dielectric breakdown: Dielectric strength is a measure of the voltage that the oil will conduct. Many contaminants conduct electricity better than oil, therefore, lowering the dielectric breakdown voltage
  • Neutralisation or Acid Number: Oxidation occurs in the oil causing the build-up of acid which will lead to the formation of sludge. This test indicates the level of acid present in the oil
  • Interfacial tension: This test points to the presence of polar compounds, which indicate oxidation contaminates or deterioration from the transformer materials i.e. paint, varnish, paper.
  • Colour: Indicates the quality, ageing, and the presence of contaminants.
  • Water: Is measured in parts per million. The presence of water will decrease the dielectric breakdown voltage.

Glimpses of the types of transformer oil


There are two main types of transformer oil in use today: Paraffin-based transformer oil and naphtha-based transformer oil.

  • Paraffin-based oil: It is not as easily oxidised as naphtha-based oil, in theory producing less sludge. However, whatever sludge naphtha-based oil generates is more easily removed than the sludge from paraffin-based oil, because it is more soluble.
  • Naphtha-based oil: It does not contain dissolved wax, as does the paraffin-based type. This wax can increase the pour point and potentially cause issues, but in warmer climates where the temperature never gets very low, this is not an issue.

Despite the apparent superiority of naphtha-based oil, paraffin oil remains the most commonly used type of oil in transformers worldwide.

Recent innovations and explorations

Nanofluids have been researched widely in the past decade as a transformer oil or nanoparticle combination for insulating and cooling interest. The advantages of nanofluids based transformer oil as compared to conventional transformer oil have been cited as follows:

  • Nanofluids have better AC and impulse breakdown performance as compared to the mineral oils, which makes it favourable to be used in HVAC and HVDC applications.
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  • The AC breakdown strength of nanofluids based transformer oil is less influenced by moisture as compared to mineral oils, thus improving the insulation life and thereby the transformer life.
  • Nanofluids have better partial discharge characteristics as compared to the mineral oil.
  • Nanofluids have a better anti-ageing characteristic as compared to mineral oil. So, it can improve the operational reliability and lifetime of high voltage transformers.
  • Nanofluids have a higher thermal conductivity than the conventional transformer oil, and they are helpful in better cooling of transformers.