Transformer oil or Insulating oil is an oil that is extremely refined mineral oil which is always durable at high temperature and great electrical insulating properties. It is gained from fractional distillation process with the successive processing of crude petroleum. It performs two main actions.

It acts as a liquid insulation in an electrical power transformer and secondly dumps heat of the transformer as it serves as coolants. In addition to it, it also preserves the core and winding of the transformer as these are fully immersed in the oil. Due to oxidation awareness of the paper insulation made for the winding, the transformer oil protects direct contact of atmospheric oxygen.


Parameters of Transformer Oil

  • Electrical Parameters: – Dielectric strength, Specific resistance, Dielectric dissipation factor.
  • Chemical Parameters: – Water content Acidity.
  • Physical Parameters: – Inter facial tension, Viscosity, Flashpoint.


  1. Total Acidity

Total acidity also is known as total acid number (TAN) specify the way for calculation of acidity that is obtained by the quantity of potassium hydroxide which is needed to balance the hydrogen ions (H+) in one gram of oil. The total acid number amount shows to the crude oil refinery, the occasion of corrosion problems. To get the total acidity, different procedures can be used to cover:

Potentiometric tit-ration: where toluene and propanol are diffused with a sample and a little water then tit-rated with alcoholic NaOH(aq). A reference electrode and a glass electrode is absorbed in the sample and united to a voltmeter. The meter reading is plotted in opposition to the volume of the tit-rant curve comparable to the basic buffer solution.

Transformer oil acidity is never sound good for the conduct of the transformer, if at any moment, oil becomes acidified then the moisture content in the oil shall get high property levels. This shall later result bad and affect the paper insulation of the winding. Acidity raises the oxidation process in the transformer oil.

Acid presence also accelerates decomposition of iron in combination with moisture.

The 11 KOH in mg is used to encounter acidity of oil by dissolution it in every gram of oil. This is known as neutralization number.


  1. Breakdown Voltage

The definition of breakdown voltage is that when an insulator indicates to the minimum voltage which results in unit of an insulator to convert into an electrical conductor. It is usually a particular asset of an insulator that describes the maximum voltage distinction that can be put on the material before its insulator conducts. In solid insulating materials, this generally builds a weak path within the material by creating long-lasting molecular or physical alterations by the sudden high current.

The breakdown voltage of a material is not a straightforward cost. Breakdown voltage is also referred to as tolerate voltage where the possibility of failure at a given voltage is so less that it is advised at the moment of designing insulation, with confirmation that the material will not fail at this voltage.

There exist two ways for calculation of breakdown voltage of a material which includes the impulse breakdown and the AC breakdown voltages. AC voltages are the line frequencies of the mains, while with the impulse breakdown voltages, they resemble lightning strikes.


  1. Viscosity

Here we talking about fluids and the viscosity of fluids. It refers to a measure of its resistance with reference to its steady deformity by shear stress or tensile stress.

It compares to the informal notion of “thickness”; if we focus on liquids. Viscosity is brought about by high friction between bordering molecules of the fluid that are moving at different speeds. On forcing a fluid into a tube, it will move faster slowest near the walls of a container when poured out. Therefore a little stress is needed to overcome the friction between layers and keep the fluid moving. So generally, the viscosity of a fluid is highly dependent on the shape and size of its particular plus attraction across the elements.

Oil that is always used in transformer with low viscosity is necessary and very important in that, transformer oil viscosity should not alter itself if the temperature is decreased. It must be noted that all liquids get more viscous with fall in temperature. Low viscosity is one of the better ways to recommend in order to provide an environment for low resistance to a conventional flow of oil hence proper transformer cooling is required.

However, since every liquid turns into less viscosity with temperature increase, convection takes place to facilitate cooling as well.


  1. Dielectric dissipation factor (ten-delta)

Dissipation factor refers to a calculation of loss rate of an energy of a mode vibration in a careless system. It is as well referred to as loss factor. The dielectric dissipation factor is an angle in which the phase difference between the applied voltage and consequential current diverges from pi/2 rad, at the point where the capacitors dielectric encompasses only of insulating material.

We are going to discuss that the transformer ages, impurities increase, therefore decreasing dielectric resistance. Impurities will increase current flow through the dielectric. The angle delta is less than ninety degree when the impurities are more.


5.Specific Resistance

Specific resistance with reference to the electrical resistance of an electrical conductor refers to the opposition in the passage of an electric current through a conductor.

Any conductor with specified cross section has a resistance relatively proportional

to its resistivity, length and inversely proportional to its cross- sectional area, the rest of the material shows some resistance at any given point in time. A conductor’s material and temperature greatly determine its specific resistance.

Specific Resistance of transformer oil focuses on the measure of resistance between two opposite sides of one cm3 block of oil. The resistivity of oil reduces drastically when the temperature increases. When a transformer has been shut off for a long time, its oil temperature shall be the same as that of ambient. With the full load, the temperature can rise up to 90 degrees, especially at an over loaded condition.

Therefore insulating oil resistance should be able to accommodate the needs of both the high and low temperatures; that is at the ambient level.

It is highly recommended to measure the specific resistivity of transformer oil both at 27 degrees as well as 90 degrees hence covering the expected high temperatures and minimum.


  1. Flash point

The flash point of a chemical or might be any volatile material is the lowest temperature at which it can evaporate to form an ignitable mixture in air or combustible concentration of gas. This point informs us how-how hard or easy this volatile material may burn. Materials with higher flash points are less flammable than chemicals with lower flash points. Determining a flash point requires an ignition source. The flash is not dependent on the temperature of the ignition source.

This process is very significant for it as it signifies the chances of fire hazard in the transformer for that reason higher levels of temperature points of transformer oil flash point are necessary.


  1. Moisture Content

Moisture content refers to the amount of water present in a material, such as soil- soil moisture, apple, food etc. Moisture content is used in both technical and scientific areas plus the day to day life activities.

 Water is a very undesirable pollutant to transformer oil and other insulators in a transformer. Firstly, moisture is present in oil, moisture makes the oil less thick with a lower boiling point than oil. Moisture content is directly promotional to transformer oil aging.

Learn more about it here: Knowledge Base: Transformer oil / Insulating Oil