Neutral Current and Heat Generation

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death900

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Texas
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Electrical Engineer
We have a few transformers connected to panelboards that are experiencing around 10-15% load imbalance between the three phases. We are seeing around 50A neutral current on panelboard power meter screen. We're also seeing high temps on the transformer.

Is the neutral current turning directly into heat? Is the neutral current generating heat based on wire resistance? Looking for some info.
 
Current results in heat. I^2R It doesn't matter if it is phase or neutral current.

Transformers are rated in Deg C rise over ambient, with many rated at 80, 115 or 150degC over ambient (which is really hot). How hot are the transformer getting?
 
ron said:
Current results in heat. I^2R It doesn't matter if it is phase or neutral current.

Transformers are rated in Deg C rise over ambient, with many rated at 80, 115 or 150degC over ambient (which is really hot). How hot are the transformer getting?
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Ron, that is coil temperature not the enclosure though, correct? Even the terminations don't generally rate over 75 degrees centigrade do they?
 
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death900 said:
We have a few transformers connected to panelboards that are experiencing around 10-15% load imbalance between the three phases. We are seeing around 50A neutral current on panelboard power meter screen. We're also seeing high temps on the transformer.

Is the neutral current turning directly into heat? Is the neutral current generating heat based on wire resistance? Looking for some info.
Click to expand...
 
What KVA are the transformers ? Years ago we had a few older panels that additional loads were added over the years and tripped out the main breaker. We took ampere readings on the three legs & the grounded conductor then rearranged some circuits to balance load. Of course that often changes between summer & winter loads so we had to rearrange a few circuits to obtain a close to balanced load. Liked to use a FLIR camera rather then the inexpensive what $100 hand temperature guns to take temperature measurements. More then once recommended that a customer install an exhaust fan in small unventilated room that had dry transformers.
 
Strathead said:
Ron, that is coil temperature not the enclosure though, correct? Even the terminations don't generally rate over 75 degrees centigrade do they?
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Those numbers are the allowable temperature rise above normal 40°C ambient, for the core and coil, giving a total acceptable temperature of upto 220°C deep inside the transformer
The temperature of the exterior of the transformer enclosure is not given, but when you enclose a +200° heater inside of an enclosure you should not be surprised the enclosure is 'too hot too touch'.
 
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Thank you for the responses but i am not asking if this is normal nor concerned about the transformer overheating. We are experiencing higher than expected heat rejection on the transformer and its being presumed that its because of the high neutral current.

All i am wondering is the two questions i asked. How much heat is being generated by the neutral current? Is there a calculation?

If its a simple I^2 x R calc i can work with this.
 
I don't believe that neutral current should heat up the transformer more than any other current. And by what metric is 50A of neutral current 'high'? Give us some specs on the transformer already. KVA, primary and secondary voltage and phases, current ratings if available. No way to evaluate the impact of 50A neutral current without knowing that stuff. The I^2*R calc can't be done without the R, which is a characteristic of your transformer.
 
800kVA
480→415Y/240V
Z = 5.74%, and Z = 6.5%

Please do not misunderstand. Im not suggesting the neutral current is high or low, we are just seeing very high temps on the transformers, higher than we anticipated, and its overloading our cooling. Looking to see if balancing the load would significantly lower the heat rejection. Which was what i was wondering, how much heat rejection is the neutral current actually contributing to.

Thanks,
 
death900 said:
we are just seeing very high temps on the transformers, higher than we anticipated, and its overloading our cooling. Looking to see if balancing the load would significantly lower the heat rejection.
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What are very the high temperatures? I'm no transformer guru but I don't think that balancing the load (if even possible) would change the amount of heat generated by the current flowing through the transformer.
 
infinity said:
What are very the high temperatures? I'm no transformer guru but I don't think that balancing the load (if even possible) would change the amount of heat generated by the current flowing through the transformer.
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Yeah it is possible to balance the load but waste of time if it wont contribute to reduced heat rejection.
 
death900 said:
Thank you for the responses but i am not asking if this is normal nor concerned about the transformer overheating. We are experiencing higher than expected heat rejection on the transformer and its being presumed that its because of the high neutral current.

All i am wondering is the two questions i asked. How much heat is being generated by the neutral current? Is there a calculation?
Click to expand...
Neutral current in and of itself will not cause excessive heating in the transformer. A Wye transformer is designed to have 100% loading between any one line and neutral.

Do you think you have an additive harmonics issue?
 
Because it is a 415/240V secondary, can I guess that it is serving computer equipment? High levels of harmonics / non-linear loads will also cause extra heating in a transformer. Do you have a meter that can measure harmonics, such as THD or TDD?
 
ron said:
Because it is a 415/240V secondary, can I guess that it is serving computer equipment? High levels of harmonics / non-linear loads will also cause extra heating in a transformer. Do you have a meter that can measure harmonics, such as THD or TDD?
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Yeah we have measured high harmonics. Do you know how much this contributes to heat?

jim dungar said:
How loaded are the units?
Are these ventilated dry type indoor units or liquid filled? Can you get an infrared thermometer reading of the core assembly or the cooling fins?
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Loaded to about 80%, its a dry type.
 
death900 said:
Temp readings about 194°F
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Don't know if that is excessive at all, if that is the core. I would not be focused on the tiny amount of neutral current at all.
Have you performed a watts loss calculation for the core and the windings? Your vendor should be able to provide these values.
 
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