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Transformer Startup in Colder Temperatures

Are transformer startup procedures the same in colder climates? Do I need to warm up my transformer? Learn how to startup your transformer in colder climates.

Written by:
Ben Gulick & Nathan Stenzel

April 2, 2024

A Maddox transformer undergoing startup procedures

Are the startup conditions the same for transformers in colder climates? Do I need to warm up my transformer? 

Yes, colder climates merit more care at startup.

In normal temperatures, transformers below 69kV do not need any warm up time at no load. They can run at full load upon startup. 

In colder climates, the rules change a bit. We will take a brief look at what changes and the best way to handle those changes. To start, let’s define colder than normal temperatures.

How Cold Is Too Cold?

Temperatures below -20 °C (-4 °F) are unusually cold. The IEEE standards consider such temperatures unusual service conditions for energizing transformers. The main concern in this colder climate is the transformer’s oil

Most fluids do not perform the same in very cold climates. They lose the strength of certain characteristics. Transformer startup processes must account for these issues.

What to Check Before Startup: Water Saturation, Dielectric Strength, and Viscosity

Water Saturation & Dielectric Strength

Colder temperatures reduce the saturation point of transformer oils. Saturation point is the point where a fluid can no longer contain any more moisture. At this point, water becomes free floating in the fluid. This in turn reduces the dielectric strength of the oil. Take an oil sample to measure the moisture content of the fluid. Each fluid will have a specific limit for moisture content. Be sure you know which type of fluid your transformer has and what its limits are. Review the sample data and verify the moisture levels are acceptable for the fluid type.

Fluid Viscosity

Some fluids become more viscous than others in colder temperatures. Proper cooling cannot take place when the fluid is too viscous to move through the tank. As a result, the fluid may need a longer soak time to warm up. 

The fluid around the transformer core warms up first at startup. The fluid closest to the tank wall will take the longest to heat. It may take some time before all the fluid moves freely throughout the tank. This process could take days in some cases if temperatures are particularly cold. The size of the transformer and the volume of oil affect the time needed under no load.

A diagram of a pamdount transformer in cold temperatures with cold oil by the tank and heat coming off of the core.

Here is the basic process for energizing a transformer in colder climates.

Recommended Cold Startup Procedure

For temperatures below -20 °C, we recommend energizing and holding at no load (soaking) for at least 8 hours. Then, apply the load in 10% to 20% increments. Allow at least 30 minutes between each load increment. This method follows the industry recommendations outlined in IEEE C57.93-2007.

Soak time varies with ambient temperature, transformer type, and fluid. As mentioned earlier, some transformers may need days to soak before loading.


Energizing transformers in colder climates requires more patience and attention to detail. Fluid type, volume, and temperature affect startup procedures. Yet, the basic principles remain the same. Reach out to one of our technical experts for startup details on your Maddox transformer.

Maddox padmount transformer loaded on truck

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