Transformer output

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GlennG

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I have a transformer in a building I'm working in, it's 3 phase, 30kva, 120/208 on secondary side. My voltage readings to neutral are L1- 112, L2- 134, L3- 135. Phase to phase seems about right. My question is does those voltage readings mean the transformer could be going bad? Is there some kind of range that that voltage needs to be within? I know there is a range for voltage drop on feeders and branch circuits but is there something similar for transformers? The high voltage and difference is concerning to me. Any code references would help also. Thanks for any input, transformers have always been a little foreign to me.
 
A preliminary question: were these measurements taken while the transformer was loaded, and if so, did you measure currents (provide numbers)?

Asking because a heavily loaded winding will drop the output voltage.

Another question: did you measure primary voltage and/or current? Provide numbers if yes.
 
GlennG said:
Yes the transformer was partially loaded, some lighting and heat(oil fired) nothing substantial. I didn't take current readings, I overlooked that.
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Okay.

Your voltages are within tolerance individually, but should be closer in comparison, especially if the transformer is only lightly loaded.

Before suspecting the transformer as going bad, you need to take measurements on the primary side. The problem could originate further upstream and may not be the transformer.
 
I wasn't sure if it had any relevance but his measurements were L1, L2 and L3 with no mention of the other reference point.
 
infinity said:
I wasn't sure if it had any relevance but his measurements were L1, L2 and L3 with no mention of the other reference point.
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Actually noted reference...
GlennG said:
I have a transformer in a building I'm working in, it's 3 phase, 30kva, 120/208 on secondary side. My voltage readings to neutral are L1- 112, L2- 134, L3- 135. Phase to phase seems about right. My question is does those voltage readings mean the transformer could be going bad? Is there some kind of range that that voltage needs to be within? I know there is a range for voltage drop on feeders and branch circuits but is there something similar for transformers? The high voltage and difference is concerning to me. Any code references would help also. Thanks for any input, transformers have always been a little foreign to me.
Click to expand...
 
140321-1400 EDT

GlennG:

If all the phase angles are close to correct, then your line-to-line voltages cannot be about the same.

I get 232 V line-to-line for the two 134 V line-to-neutral legs.

Then for the other two line-to-line voltages I get 213 V.

This derives from 67+112 = 179, and 116 for two legs of a right triangle and the hypotenuse is the sq-root of 179^2 + 116^2 = sq-root of 45,497 = 213 V. Check my math. 232 and 213 are quite different.

.
 
gar said:
If all the phase angles are close to correct, then your line-to-line voltages cannot be about the same.
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Looks like another opportunity for me to promote providing all possible voltage measurements when troubleshooting.:D

all Line - Line
all Line - Neutral
all Line - Ground
all Neutral - Ground
 
jim dungar said:
Looks like another opportunity for me to promote providing all possible voltage measurements when troubleshooting.:D

all Line - Line
all Line - Neutral
all Line - Ground
all Neutral - Ground
Click to expand...
...and that includes both primary and secondary (but not primary to secondary :p).
 
Could be a bad capacitor at one of the POCO's transformers feeding the service. Had this happen at a facility I managed. At least, the POCO said that's what the problem was.
 
Looks like you have a high negative sequence phase rotation, 2 of the phase out by 9 deg or so.

Like previous post stated, likely blown PF correction cap somewhere or somebody really is unevenly loading with low PF load.
 
Is this voltage being measured at the transformer or at some point further downstream? A bad neutral could give you those kind of readings as well, exact voltage measurements in that case would depend on connected loads at the time of measurement. The further downstream you are taking measurements, the more potential failure points there may be.
 
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