Transformer oil pump
The transformer does not allow oil leakage (zero tolerance), so the oil pump uses a sealless wet motor pump (also known as a canned pump). Thanks to this design, the bearings are lubricated by pumped oil. Although the pumped fluid is an "oil", please note that during the formulation process all lubricating properties have been intentionally removed to enhance the required dielectric properties. The lack of fluid lubricity is the main reason I specify hydrodynamic fluid film bearings for these applications, and I do not recommend the use of anti-friction (ball) bearings used by some other manufacturers.
In cold weather, when the transformer is in shutdown state, the oil temperature decreases and the viscosity increases. Therefore, the pump head, flow rate and efficiency will be reduced. Of these three factors, efficiency decreases the most and therefore the power required to operate the pump increases.
When operating pumps at lower fluid temperatures, precautions affecting high motor currents, circuit breaker size/loading, and other operating parameters must be taken. To make sure the motor is running at full speed, it's a good idea to check the speed-torque curve. If the transformer has been put into operation and the internal operating temperature is normal, then there will be no problem even if the ambient temperature drops to minus 40 degrees (Celsius or Fahrenheit).
Transformer oil
Transformer oils are usually formulated from mineral oils, but can also be natural esters or silicones. All these oils have Newtonian properties. That is, shear rate is linearly proportional to shear stress. Furthermore, the viscosity of a Newtonian fluid depends entirely on temperature and does not depend on shear rate or time factors. There is an indirect relationship between liquid temperature and viscosity. As the temperature increases, the viscosity decreases and vice versa. The above properties apply to all transformer oils, whether mineral, silicone or ester-based.
Transformer oil serves many purposes, but the two most important are heat transfer and dielectric (insulation with the ability to suppress arcing). Transformer oils are designed to perform in the high-temperature environments found in operating power transformers, typically between 60 °C (140 °F) and nearly 90 °C (194 °F).