Dielectric oil: key functions and attributes for electrical equipment 

Dielectric oil: key functions and attributes for electrical equipment
08 February 2024

Within the industrial oils range we have dielectric oils, which are used as insulating and cooling fluids in transformers and electrical switchgear.

Functions of dielectric oil

A transformer has the main function of raising or lowering the voltage of the electric current that circulates through the electrical distribution system. The active part of the transformer, in order to carry out this function, must be immersed in an insulating fluid: Repsol dielectric oils.

The main functions of dielectric oils are thermal and electrical insulation and to cool the transformer’s metallic components, through convection phenomena.

Dielectric oils are capable of dissipating the heat generated by the resistance of the metal conductors and windings, and channeling it away. Therefore, it must have high dielectric withstand, thermal conductivity, and chemical stability. In addition, it must have good antioxidant properties as it needs to maintain its characteristics for long periods of time.

Characteristics of dielectric oil 

For the transformer to operate correctly, it is necessary to guarantee its excellent properties:

  • High dielectric withstand: is the oil’s ability to withstand the voltage between a transformer's windings, preventing arcing and electric discharges. 
  • Low viscosity: a low viscosity facilitates the oil’s circulation and as a consequence favors heat dissipation due to convection phenomena.
  • Material Compatibility: Dielectric oil is designed to be compatible with all materials used in the construction of electrical equipment, minimizing the possibility of any corrosion or damage to internal components. 
  • High flash point: dielectric oils must have a high flash point (above 135ºC) to improve safety in installations where there is a risk of explosion. If the oils have combustion points higher than 300ºC, they are considered increased safety oils, mandatory in critical facilities, such as hospitals, schools, etc.
  • Biodegradability: more and more modern formulations are focusing on this property, reducing the environmental impact in the event of any leaks or spills.  In addition, Repsol's biodegradable dielectric fluids guarantee zero ecotoxicity, in both aquatic and land systems.
  • Interfacial tension: indicates the presence of polar compounds due to deterioration of the dielectric oil or cellulosic insulating material, located inside the transformer. This is an indirect measure of the degradation phenomena of the paper/oil binomial, which can be useful when making changes or taking actions to guarantee the transformer functions correctly.  

In conclusion, dielectric oil plays a multifaceted role in the efficient and safe operation of electrical equipment, contributing significantly to the optimal performance and longevity of the devices.