Course EP-3: Transformer Cooling Materials

The materials used for Liquid Field Type transformer cooling are known as the dielectric fluids which are used to cool the Transformer windings and provide optimal performance. 

Dielectric fluids’ Cooling manner: see fig.1

1- From the bottom of the tank where the dielectric fluid is at its lowest or “bottom” temperature, the fluid flows vertically up the winding ducts and is heated by the windings.

2- At the top of the tank, where the fluid is at its highest or “top oil temperature”, it exits the main tank and enters a series of radiators or cooling fins.

3- It then flows downward through the radiators, where it is cooled, and reenters the main tank at the bottom.

fig (1): Dielectric Fluids' Cooling Process

  • In self cooled transformers this cycle is governed naturally by convection. Natural convection can also be assisted by a series of fans directing air against the radiators increasing the rate of heat transfer and subsequent rate of cooling in the windings.
  • In some large power transformers it is also possible to have a level of forced oil circulation where a pump assists in the circulation of the fluid. This generally provides a lower top oil temperature and more uniform temperatures within the windings.

However, i will show the cooling system methods of transformers in this course later.

Dielectric fluids types:
Today there are (4) generally accepted fluid types offered in the market:

  1. Mineral Oil.
  2. Silicone.
  3. Beta fluid®.
  4. Envirotemp®. 
While each has good properties as a dielectric fluid, there are attributes unique to each that may make one a better choice over the others depending on the user’s needs.

1- Mineral Oil:
  • It has good performance and low costs. Mineral Oil is generally considered as a top choice in outdoor installations where its low first cost is of prime concern 
  • Its flammable nature is understood and accepted. 

  • Mineral Oil is considered to be a “Flammable” fluid by Factory Mutual, and as such has certain restrictions imposed on its use and containment. 

2- Silicone:
  • It was the fluid of choice when a Factory Mutual approved “Less-flammable” dielectric fluid was desired. 
  • It has a relatively high fire point and is generally considered to self extinguish when the source of a fire is removed. 

  • Silicone is not miscible with conventional mineral oils and should not be mixed with other fluids. 
  • Silicone does contain Methylpolysiloxanes which can generate Formaldehyde at around 300 degrees Fahrenheit. Formaldehyde can be a skin and respiratory sensitizer, eye and throat irritant, and is believed to be a potential cancer hazard. 

Silicone has been used for many years in both outdoor applications and indoor areas. When used indoors, it has been my experience that transformers are generally in vaulted, contained areas.

3- Beta fluid® :
  • It meets NEC and Factory Mutual requirements for a “Less-Flammable” dielectric fluid. 
  • It is a blend of petroleum oils and is 100% hydrocarbon so, Beta fluid® is fully miscible with conventional mineral oil and may be used to retrofill or top off these units. 
  • Beta fluid® has high dielectric strength, stability, and is non-toxic. 

  • its fire point is significantly lower than either Silicone or FR3™. 

4- Envirotemp® or FR3™ : see fig.2

  • It is produced by most manufactures today. 
  • The product is a soy-based, fire-resistant fluid that meets NEC requirements for a “Less–flammable” fluid and is listed by Factory Mutual and UL as such. 
  • “Because Envirotemp® FR3™ fluid is derived from 100% edible seed oils and uses food grade additives, its environmental and health profile is unmatched by other dielectric coolants. Its biodegradation rate and completeness meets the U.S. Environmental Protection Agency (EPA) criteria for “Ultimate Biodegradability” classification.
  • It extends insulation life by a factor of as much as 5-8 times because it has the unique ability to draw out retained moisture and absorb water driven off by aging paper. 
  • It helps prevent paper molecules from severing when exposed to heat. These properties can result in an increase of overload ability and/or longer transformer insulation life, resulting in both lower life cycle costs and delayed asset replacement. 
  • FR3™ is fully miscible with conventional mineral oil or R-Temp®, and may be used to retrofill or top off units filled with these fluid types. 

  • it has a relatively high first cost relative to Mineral Oil and could easily add 15-30% to the transformer first cost. 

fig (2) : Prototype Transformer filled with FR3™ 

Dielectric fluids comparison: 

1- According to thermal properties:

Mineral Oil
Beta Fluid®
Fire Point
165 Deg C
308 Deg C
371 Deg C
360 Deg C
Flash Point
145 Deg C
285 Deg C
268 Deg C
330 Deg C

2- According to first cost:

Fluid Type
Relative First Cost *
Mineral Oil
Beta Fluid®
Envirotemp®, FR3™

* Note: the above Values are only approximations and the relative costs can vary depending on the volume of liquid contained in the transformer.

For more information, you can free download the following documents:

in the next Topic, I will show another types of Transformers According to Construction.

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