Insulating oil

Insulating oil. “Insulating oil”, “dielectric oil” or simply “transformer oil” is an oil that is generally used in electrical equipment and that exhibits dielectric properties that are characteristic and essential to resist the passage of electric current. Contained in most transformers and is a by-product of petroleum distillation .


[ hide ]

  • 1 Features
  • 2 Functions
  • 3 Advantages
  • 4 Applications
  • 5 Composition
  • 6 Insulating oil analysis
  • 7 Analysis of gases dissolved in oil
  • 8 Break in insulating oil
  • 9 Theoretical considerations
  • 10 Sources


The inhibited semi-synthetic insulating oil (type 2) naphthenic base brand VENOCO, provides greater oxidation stability than the mineral insulating oils known in the market, offers a great capacity for heat dissipation and has excellent physical , chemical and dielectric characteristics, being also fully compatible with all types of mineral insulating oil.

Insulating oil for transformers

Extends the useful life of the oil in the equipment due to its minimal acid and sludge formation during service. There are two properties that contribute to giving dielectric characteristics to insulating oil: “resistivity” and “dielectric strength”. The first is the specific resistance that a dielectric material offers under conditions of moderate voltage, the second is the ability to prevent arcing between two electrodes or between the electrical source and ground, under conditions of high electrical potentials.


The basic functions of oils in transformers are of two types:

  • Physical

The physical function is that of cooling, by dissipating the heat generated during the operation of the unit. The electrical function is to act as a dielectric (insulating) medium to prevent arcing between two conductors with a high potential difference.


  • Greater oxidation stability.
  • It has excellent fluidity at low temperature.
  • It is free of moisture and particles.
  • High electrical resistance and great thermal stability.
  • High flash point.
  • They do not contain any type of corrosive sulfur


  • Safe in high power and voltage transformers.
  • Recommended as an insulator in all kinds of electrical transformation and power devices, such as starters, switch circuits and fuses.
  • Power switches.
  • Automotive starter coils.


  • Naphthenic mineral oil
  • Synthetic component based on Branched Heavy Alkylate.
  • Antioxidant: 2,6 Di, tert-butyl-4-methyl-para phenol (BHT).

Insulating oil analysis

Insulating oils are essential components of a large number of electrical equipment, in particular for power and measurement transformers. The evaluation of the state of the insulating oil in service is carried out according to the following control indices:

Insulating oil analysis

  • Appearance and color.
  • Contained in water.
  • Neutralization index.
  • Dielectric loss factor.
  • Breakdown voltage.
  • Quantity of particles that by size are counted.

Analysis of gases dissolved in oil

One of the diagnostic methods that provides an early indication of abnormalities in its functional behavior and allows determining the measures to be taken before the equipment suffers major damage is based on chromatographic analysis of the decomposition gases of the insulating oil by heating excessive of certain points of the transformer or by electric shocks inside it.

Depending on the temperature of the hot spot and the energy of the discharges, the proportions in which the different decomposition gases are produced are different.

Due to thermal or electrical stresses, insulating oils give rise to the following decomposition gases.

  • Hydrogen
  • Methane
  • Ethane
  • Ethylene
  • Acetylene
  • Monoxide
  • Carbon dioxide
  • Oxygen
  • Nitrogen

By determining the content of each gas, the global assessment and the relationship between the concentrations of the different gases and their evolution, it is possible to know not only the existence of a defect, but also its type and its importance.

Rupture in insulating oil

Insulating oil in power transformers serves as a heat transfer medium and as a liquid insulator, two reasons for its insulation properties:

  • Power transformers are usually operated under aging conditions. Thus the moisture content in the oil increases, the products of aging or deterioration dissolve and the particles disperse.

The transformers are operated under new environmental conditions, where pressure drops and highs occur . The security and maintenance service needs a careful investigation of these influences.

Theoretical considerations

Insulating liquids derive their dielectric strength from the highest density compared to gases. The rupture process begins with a microscopic bubble, an area of ​​long distances between corpuscles, where ions and electrons can initiate avalanches.

These microscopic bubbles are originated by current impulses in an electrode. The next current pulse injects charge carriers into the bubble, leading to current amplification and ultimately rupture].

From these considerations, it is necessary to:

  • Moisture leads to charge carriers and therefore decreases the support of the dielectric strength.
  • Oxidation by products such as acid also carry charge carriers through dissociation. Additionally, they are active on the surface, reducing surface tension. Thus they maintain the evolution of the bubble following a decrease in dielectric strength.
  • Pressure also influences the evolution of the bubble. With an increase in pressure, the breakdown voltageshould increase. For pressures below atmospheric pressure the breakdown voltage should be reduced.

The particles will move in areas of high tension depending on their permittivity in relation to that of the oil. A decrease in breakdown voltage is expected if they are highly conductive or wet.


by Abdullah Sam
I’m a teacher, researcher and writer. I write about study subjects to improve the learning of college and university students. I write top Quality study notes Mostly, Tech, Games, Education, And Solutions/Tips and Tricks. I am a person who helps students to acquire knowledge, competence or virtue.

Leave a Comment