Natural ester transformer oil performance in the cold

Natural esters always delivers on dielectric performance, even in cold temperatures

Natural Ester oil has proven to work effectively in transformers situated in cold temperature environments. It has successfully passed high voltage testing after 6 days at -40°C absolute temperature, which is rarely seen on the planet. In operation through cold temperatures, natural ester fluid is expected to perform to industry expectations assuming recommended maintenance and handling procedures are being followed.


Cold temperature and pour point

When it comes to reviewing cold temperature characteristics of fluids, some people only look at the pour point and assume that the fluid will not flow below its pour point.

This is not true. The pour point is simply a diagnostic test that compares various oils and the flow rate in a certain period of time. The pour point establishes a relative scale for context but does not determine if the fluid will work below its pour point.


Fluid breakdown voltage is maintained to at least -50°C.

Natural ester oil maintains its dielectric strength in cold temperatures to a greater extent than mineral oil. Due to its chemical make-up, natural ester oil has a high water saturation level (1000 ppm vs. 60 ppm for mineral oil at 20°C), so it maintains its dielectric strength even in very cold temperatures. While mineral oil maintains lower viscosity in cold temperatures, its ability to hold water is very low, potentially weakening the dielectric strength significantly.


Cold Start Procedure Used for Different Reasons

The state of the fluid is dependent on the volume of fluid, ambient temperature, the amount of time at that temperature, and rate of cooling. IEEE has published three standards related to no load start up procedures (C57.12.00 and C57.93.00 and C57.106.00). For any cold start situation, these procedures should be used whether it’s mineral oil or natural ester – just for different reasons:

  • At subzero temperatures, mineral oil can only tolerate a few parts per million of water before it becomes saturated. Subsequently, the dielectric strength of saturated mineral oil (not to mention free water) is poor. Energizing a very cold mineral oil filled transformer at full load yields a significant risk of dielectric failure. The cold start procedure enables the transformer to warm slowly in order for the free water to dissolve within the mineral oil.
  •  For a natural ester fluid-filled transformer that is not in operation (for example, -21°C for more than 3 consecutive days), the fluid’s viscosity does increase. While natural ester fluid’s dielectric strength is maintained, the same cold start procedure is used to enable the fluid to reduce its viscosity without hampering movement of liquid immersed components.

Using natural ester oil in your transformers

If you need more information on how you can incorporate natural ester oil in your transformer fleet, give our team a call and we'll work with you throughout the process.