Use of Vegetable oil as dielectric medium in Power Transformers

Electric transformers while in operation produce heat due to iron and copper losses (although stray losses are also there). The heat thus produced must be carried away swiftly to avoid excessive temperature rise in various parts of the transformer such as winding and insulation. The cooling medium used must prevent excessive rise in temperature in any portion of the transformer and should avoid formation of “hot spots” within the transformer.

Mineral oil is normally used as an insulating and cooling medium in power transformers. The oil covers the core and coil assembly completely and fills small voids in the insulation to enhance the transformer performance. 

Advantages of Vegetable Oil:

Over the years mineral or silicone oil has been used as insulating and cooling medium in the transformers. Vegetable oil such as rapeseed, sunflower oils etc. are bio-degradable and have a much higher flashover point, are environment friendly and less inflammable. Vegetable oil has higher flash and fire point when compared to mineral oil. Similarly the dielectric strength is also higher. 

Properties of  Vegetable Oil-based Envirotemp Insulating fluid:

The flash point and dielectric strength of “Envirotemp FR3” vegetable oil-based insulating fluid is 330^oC and 56 kV at 25 ^oC whereas for mineral oil the values are 147 ^oC and 45 kV respectively.  Transformers using vegetable oil will require lesser fire safety systems. Since transformers with vegetable oil are better in terms of fire hazard protection, hence can be used in environment sensitive and densely populated areas.

First EHV class Power Transformer with Vegetable oil:

Siemens has successfully produced and commissioned, in 2014, the world’s first EHV class power transformer that uses vegetable oil as the dielectric medium. The transformer, which is a 380/110 kV power transformer, uses nearly 100 tons of vegetable oil and is commissioned in Bruchsal-Kandelweg substation in Germany. 

Although using vegetable oil in power transformers is not new. Siemens have produced and commissioned more than 30 transformers that use vegetable oil as a dielectric medium up to 69 kV class transformers with individual capacity of 30 MVA.