Reverse fed 480/277-2300 delta secondary

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hillbilly1 said:
I’ve got audio of one of those such wired transformers screaming. (208 to 480 back fed) It was recently connected, customer complained about being so noisy. My guy wired it the same way the old one was. (It was replaced because it was overheating and noisy) when I seen the photo of the connections, I knew what was wrong. Had tech remove the primary bond to XO, tech turned it back on, he had to check the output to see if it was still working because it was so quiet! Transformer was no longer overheating.
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Interesting example, but I don’t believe this is the case with the OP. The OP didn’t mention anything about excessive noise or overheating, so the fact we have a primary neutral on the X0 with no issues seems to suggest it’s unrelated to your example.
 
I think the issue is, you are almost creating an autotransformer by connecting XO, unbalanced loading would cause unequal voltage drop in the windings, so I believe it causes the excessive heating. It seems to be even worse when done to single phase transformers. (480/240 to 240/120)
 
Xptpcrewx said:
Interesting example, but I don’t believe this is the case with the OP. The OP didn’t mention anything about excessive noise or overheating, so the fact we have a primary neutral on the X0 with no issues seems to suggest it’s unrelated to your example.
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The op hasn’t said what the end load is, so it may be balanced such as a water pump or lift station, so that maybe the reason it’s working fine. But then again, since he/she is raising the question, I wonder if they are having issues.
 
hillbilly1 said:
The op hasn’t said what the end load is, so it may be balanced such as a water pump or lift station, so that maybe the reason it’s working fine. But then again, since he/she is raising the question, I wonder if they are having issues.
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I agree and while I’m not discounting your experience, I think it’s best to provide a sound technical reason why not to connect a primary neutral to X0.
 
LarryFine said:
Google it.
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Burden of proof is on you. You’re the one telling us it should be connected the same as a delta. I provided some counter examples of utility and bi-directional applications. I stand by my conclusion (in post #17) unless you or anyone else can provide sound technical facts.

Using a primary neutral has a lot to do with the core construction, banked units, ferro-resonance and how the supply system is grounded. As far as I know, it’s not a hard rule to connect the primary neutral or not. If you “Google” it you will find transformer connections utilizing a primary neutral.
 
hillbilly1 said:
The op hasn’t said what the end load is, so it may be balanced such as a water pump or lift station, so that maybe the reason it’s working fine. But then again, since he/she is raising the question, I wonder if they are having issues.
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The load is a movable bridge, after the 2300 is stepped down. There hasn't been any issues. Contractors are replacing the 2300V cable, and I had questions the contractors couldn't answer. Mainly, ground fault detection for the secondary.

After looking at the primary connection, I was thinking it was connected incorrectly with the neutral to the xo.
 
jminer99er said:
The load is a movable bridge, after the 2300 is stepped down. There hasn't been any issues. Contractors are replacing the 2300V cable, and I had questions the contractors couldn't answer. Mainly, ground fault detection for the secondary.

After looking at the primary connection, I was thinking it was connected incorrectly with the neutral to the xo.
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I can almost guarantee it's not balanced also
 
jminer99er said:
The load is a movable bridge, after the 2300 is stepped down. There hasn't been any issues. Contractors are replacing the 2300V cable, and I had questions the contractors couldn't answer. Mainly, ground fault detection for the secondary.

After looking at the primary connection, I was thinking it was connected incorrectly with the neutral to the xo.
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jminer99er said:
It moves several times a day
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That’s not that frequent, 20-30 minutes at a time?
 
Forgive my ignorance, I take it the ground fault detection would be installed at the gear 100 feet away, and not on the actual secondary cable leads at the xfrmr?
 
jminer99er said:
Forgive my ignorance, I take it the ground fault detection would be installed at the gear 100 feet away, and not on the actual secondary cable leads at the xfrmr?
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I am glad you made me pull out my code book. The NEC is actually silent on ground detectors for ungrounded system over 1000-VAC (unless its for portable or mobile equipment). This seems like a really big loophole as far as requirements go. I would still recommend having something. As far as location goes, this is generally achieved with a PT nearest to the supply.
 
Xptpcrewx said:
I am glad you made me pull out my code book. The NEC is actually silent on ground detectors for ungrounded system over 1000-VAC (unless its for portable or mobile equipment). This seems like a really big loophole as far as requirements go. I would still recommend having something. As far as location goes, this is generally achieved with a PT nearest to the supply.
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Ahh, maybe it isn't required?
 
jminer99er said:
Ahh, maybe it isn't required?
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At least not by the NEC, but perhaps by standard engineering practice for design. The IEEE Buff Book talks about this. Overall ungrounded systems are discouraged.
 
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