Calculating Operating Temp of Transformer at Various Load Conditions

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xxmrwhitexx

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Does anyone know of a formula (or reference chart) for calculating the expected winding operating temperature of a dry type transformer? All I can find online of course is the temp rise info at full load. We all know most real-world conditions do not involve exactly 100% loaded transformers. I am trying to create a benchmark for IR diagnostics when scanning dry type transformers.

I have the loss charts at various load conditions from the manufacturer but those don't translate directly into operating temperature.

thanks in advance for useful feedback.
 
221028-2100 EDT

xxmrwhitexx:

A transformer has core losses that are somewhat constant independent of transformer loading. These losses will cause a certain amount of temperature rise of the transformer. As such these losses determine the unloaded temperature rise of the transformer.

As an approximation the equilibrium temperature rise of a mass is proportional to the input change in power input to the mass.

So we have a base temperature of the transformer from its core losses. When we load the transformer we add internal loss from I^2 * R of the transformer. At full load we reach a temperature rise defined by the sum of the core loss and I^2 * R loss.

Suppose the transformer is designed so that total core loss equals full load I^2 * R . At zero load the rise is X degrees from ambient, and then at full load the change will be about 2 * X degrees from ambient. At 1/2 full load about 1.25 * X.

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xxmrwhitexx said:
Does anyone know of a formula (or reference chart) for calculating the expected winding operating temperature of a dry type transformer? All I can find online of course is the temp rise info at full load. We all know most real-world conditions do not involve exactly 100% loaded transformers. I am trying to create a benchmark for IR diagnostics when scanning dry type transformers.

I have the loss charts at various load conditions from the manufacturer but those don't translate directly into operating temperature.

thanks in advance for useful feedback.
Click to expand...
Be careful with making specific recommendations based on the general appearance of a one time transformer thermograph.
The heat distribution is not equal across a transformer. The hottest location will be near the center of the core, pretty much behind where the taps are, not on the outside of the windings. Likewise the external/enclosure temperatures maybe more dependent on the heat dissipation of the ambient environment than the internal temperature of the core.
 
221029-1415 EDT

A typical way to measure transformer internal rise is to ---

Measure a winding resistance at stable point with the transformer cooled down to room temperature. A preferred winding would be one most buried in the transformer. Then run the transformer at the desired load long enough to reach a stable state. Then again measure the same coil resistance.

Using the temperature coefficient for the type of wire in the winding, then calculate the temperature rise. This is an average for the transformer. To this you apply a multiplying factor, like possibly 1.1, to estimate maximum hot spot temperature.

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See also: IEEE Std C57.96-1999 IEEE Guide for Loading Dry-Type Distribution and Power Transformers chapter 6.2 Equations for calculation of temperature, load, and loss of life
 
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