Transformers . . Are they Hot! ! !

During one of my power quality audits, I visited a customer that had three transformers running very warm. Temperatures: 170F-190F. (75C-88C). It was obvious as soon as I walked into the small room that there was excessive heat being generated. The room had a ventilator, which was running. As I touched each unit, I found that the middle unit was too hot to place my hand on it for more than a second or two. I proceeded to open each unit and take some temperature and voltage/current measurements. Guess what I found?

First, the transformers were not being overloaded in KVA, but their harmonic content or Total Harmonic Distortion (THD) was high. The hottest transformer was registering upwards of 55% THD on all three phases. The other two units were registering between 22% and 35% THD. But I have been in places where I have measured THD values of 80% and the transformers were not running as hot as these units were!

So . . what gives?

The harmonic spectrum my test equipment had displayed was distorted or non-linear, and phase-to-phase imbalance was over 40%. The imbalance resulted in increased stress on the units, which was adding extra heat, excess vibration, and excess wear. The solution was to balance the transformers across all phases and to increase the air circulation within the room. An air conditioner would also be helpful. Changing the transformers to a higher K factor rating would extend the lifetime of the units.

What is K-factor?

K-factor defines the non-linear load a transformer can tolerate without overheating or damage. The basis for K factor is seen in the ANSI C57.110 standard. This standard identifies the method for correctly de-rating transformers for non-linear loads. Hence, a basis for allocating a value to quantity of heating effect of non-linear (harmonics) loads on the transformer. The heating effect of harmonics is proportional to the square of the current and the square of the harmonic. A K- factor rating of 13 will handle 3 phase non-linear loads well. Larger units may be required for single and 3 phase combinations.

Were these units K-factor rated? There was no label on the front of the units. So I would have to assume that they were not K-factor rated.

Adding up all of the imbalance inputs, I could readily understand that the cause of the excessive heat was due to harmonics and the imbalance loads. I recommended that the loads be balanced, increase cool air flow, or replace the transformers. To cool down the transformers, one could install muffin fans on the units to force outside air through the units. Obviously, if the units are replaced, they should be replaced with the proper K-factor rated transformer. The customer had the choice.

So what does this teach us. First, transformers run hot, some hotter than others. Pure nonlinear loading on non K-factor rated transformers will add excessive heat and stress, shortening the life of the transformer and risking a electrical fire. Second, all customers must be made aware that if they let a heat/stress situation exist on their transformers, THEY WILL EVENTUALLY FAIL! OFTEN IN A FIRE!

Third, a secondary source of equipment problems can be due to something as simple as a phase to phase current imbalance. Phase to phase imbalances are very common many facilities. Very few people pay any particular attention to this, and it is usually the easiest and cheapest thing to fix.

Oh . . one last thing! If you find yourself in a room with very warm transformers, and there appears to be no air circulation, be quick to identify to the customer that he is risking fire to his facility and/or transformer damage. He may also be risking a long time delay in repair due to the room’s isolation. Have the customer consider a temperature monitor for the room. These are cheap, easy to install units.

Until next time, so long from sunny North Carolina.

Back to Technical Articles