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Understanding the Breakdown Voltage of Transformer Oil: Importance and Benefits

Transformer Oil

Breakdown Voltage Of Transformer Oil

To understand the “Breakdown Voltage” (BDV) of transformer oil, we first need to get introduced to dielectric strength. Dielectric strength is defined as the ability of the oil to withstand electrical stress without breaking down. Here, breaking down refers to the failure of insulating properties.  

Now, let us move on to our main question and try to understand the BDV of transformer oil. 

What is BDV test of transformer oil

What is BDV test of transformer oil

Oil stored in drums loses its dielectric strength and must be tested and treated before use in electrical equipment. The breakdown voltage test of transformer oil is crucial for the smooth operation of the transformer. 

A periodical BDV testing of the transformer oil ensures the quality and healthiness of the transformer oil. Breakdown voltage is determined by observing the voltage at which a specific gap separates sparking strands between two electrodes immersed in oil. 

A low BDV value indicates that the oil contains moisture and conducting substances. As per the International Electrotechnical Commission (IEC), the minimum BDV value of transformer oil should be 30 kV.

Factors affecting the BDV value of transformer oil

Factors affecting the BDV value of transformer oil
  • Water is the most prevalent contaminant in transformer oil. Moisture transports charge carriers, reducing the dielectric strength of the oil. 
  • Acids, for example, are aging byproducts that deliver charge carriers via dissociation. They are also surface-active, which reduces surface tension. As a result, they promote bubble evolution as dielectric strength decreases.
  • Bubble evolution is also influenced by pressure. The breakdown voltage rises as the pressure rises. The breakdown voltage should decrease at pressures lower than atmospheric pressure.
  • Dry cellulose fibre particles lower the breakdown voltage by promoting bubble formation. 

Benefits of Regular BDV Testing of Transformer Oil

The Breakdown Voltage is the primary indicator of the health of the oil. It is a popular and important transformer oil test because it: 

  • helps determine the essential electrical properties of transformer oil
  • determines whether a specific oil is suitable for future use.
  • Informs whether filtration or regeneration is required.
  • help reduce oil costs and increase component life
  • prevent failures and maximize safety
  • Preventing equipment fires
  • Maintaining transformer reliability

Difference between breakdown voltage and rated voltage

The breakdown voltage refers to the limit voltage of the capacitor; above this voltage, the dielectric in the capacitor will fail. The rated voltage is the voltage that the capacitor can withstand over time and is less than the breakdown voltage.

Capacitors function properly at rated voltage and safely at voltages no higher than the breakdown voltage.


The breakdown voltage is the voltage at which the dielectric fails. It indicates the health of the transformer oil. Overall, the breakdown voltage of transformer oil is important because it allows the transformer to operate smoothly and prevent failures. 

BDV results are better in dry and clean oil than with moisture and other conducting impurities.

Air-insulated switchgear vs gas-insulated switchgear

Air insulated switchgear vs Gas insulated switchgear

Air insulated switchgear vs Gas insulated switchgear

Air-insulated switchgear (AIS) and gas-insulated switchgear (GIS) are two types of equipment employed in the electrical power distribution and transmission industry. While AIS is commonly used in low and medium-voltage systems, GIS is used in high-voltage systems. 

In this article, we discuss their benefits, applications and differences to provide a thorough understanding of AIS and GIS. 

What is air-insulated switchgear?

Air-insulated switchgear is a secondary power distribution device and medium voltage switchgear that helps redistribute the power of a primary power distributor powered by a high voltage distribution transformer. 

AIS controls, protects and isolates electrical equipment in power transmission and distribution systems. The equipment is designed to be operated in the air at atmospheric pressure, which is why the name “air-insulated.”

What is air-insulated switchgear

Advantages of AIS

  • It is a primary choice for areas with extensive space.
  • Due to low construction and switchgear costs and good design, the system is financially viable.
  • Less time spent on construction makes it suitable for immediate installs.
  • All the equipment is visible, making maintenance simple. Faults are simple to identify and fix

Applications of AIS

  • Power generation stations 
  • Airports 
  • Railway stations 
  • Shopping malls 
  • Oil and gas 
  • Mining 
  • Marine
What is gas-insulated switchgear

A gas-insulated switchgear is a composite device enclosed in a solid metal frame comprising electrical devices like transformers, circuit breakers, bus cars, surge arrestors, etc. These components are housed in shielded chambers with barrier devices and submerged in sulfur hexafluoride gas (SF6).

The main uses of a GIS substation are switching, separating, converting, monitoring, and distributing electrical energy in power systems. SF6, an inert gas with high insulating qualities, chemical stability, and thermal stability, is used as the primary component of a GIS device.

Advantages of GIS

  • The earthed metal enclosure provides the attending staff with a secure working space. 
  • Because the insulation system is less disrupted by compartmentalized containment of the live sections, the system is very dependable.
  • Minimal maintenance due to efficient design and protection from the weather.
  • Rapid assembly as a result of thorough pre-assembly

Applications of GIS

  • Power transmission
  • Railways
  • Grid integration of renewable energy production facilities
  • They are favored in big cities because there is limited room available in cities.
  • Valleys, as well as mountain regions
  • Underground substations
  • Substations on sea or lake
  • HVDC-based terminal substations for transmission
  • Industrial complexes

Differences between air-insulated switchgear and gas-insulated switchgear

AIS (air-insulated switchgear) GIS (gas-insulated switchgear) 
It has a metal-clad system that employs air insulation.It uses the gas sulfur hexafluoride for insulation. Sulfur hexafluoride is much heavier than air and offers great extinction behavior.
Three-position draw-out circuit breakers are used in the metal-clad AIS.GIS systems use fixed-mounted circuit breakers. 
More economical for low/medium voltage systems.It is expensive as compared to AIS.
More maintenance cost over a lifetime Less maintenance cost 
The AIS breaker is removable for maintenance & troubleshoot The breakers here are ‘sealed-for-life.’ 
AIS systems cover more space GIS systems cover less space 
More installation time Less installation time 
Visual inspections of AIS systems should be done every year to every two years.GIS switchyards require significantly less maintenance


Gas-insulated switchgear is mainly used in industries with high power demand and limited space to install equipment. However, it can be more expensive in the long term, requiring more specialized tools and skilled professionals to install and maintain.  On the other hand, air-insulated switchgear is a more common and economical alternative for low and medium-voltage systems. get redirected herepersonnaliser coque de telephonepersonalised phone case