Trans Sec Voltage Intermittently Changes Value

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MyCleveland

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Cleveland, Ohio
EC friend just described a scenario I have never seen before. I am hoping someone can provide some insight on the issue.

New building with 480/277V-3P-4W service, + plus a gen set to serve specific loads via 3 ATS's.
One of the ATS feeds a 300kVA Trans (delta-Y) and sec is 208/120V-3P-4W.

Building is just starting to have owners equipment installed so not completely operational.
The only load on the tranny at this moment is approx 20-30A (3P) serving only 120V freezers.

Called to site after equipment was in place for 7-10 days because some units went into alarm.

C phase was 50V P-N and P-G
Primary side voltages were normal and not changing.
After 30 minutes or so, while continually checking all connections, he noticed the 50V value start climbing until 15 minutes went by and it went back to normal.
We are under utility power while this is happening.

3 days later he is having and elevator inspection and testing with normal and em power and everything went fine, he stated gen set ran around 1.5 hours.

Back on utility power, Approx 1 hour after test problem resurfaced, equipment in alarm, primary voltages fine, C pase to N and P-G.
After a few minutes the voltage on C phase slowly came back up to normal.

Yikes...sounds like I wired it. Any ideas what is causing this ?
 
Perhaps one of the tap connections to the primary windings is intermittent.
And so when that tap is open circuited, the leg of the transformer core for the C phase output is only getting flux from the common magnetic paths it shares with the other two phases. That would then result in the lower voltage you are seeing on the C phase.
 
Thank you both for the responses.
‘I was leaning towards “synchro” line of thinking….or something internal to the transformer itself.
Not saying at all that H1 response is not valid, just trust this crew would know to check this throughly.

Feedback this afternoon was via increasing load on tranny they were able to initiate the voltage swing.
Continually checking everything they noticed the factory installed main served from the tranny…the C phase hold down bolt was barely threaded on and the A & B were only finger tight.
All now tightened and problem has not reoccured.
They told me NO discoloration on any of the hold downs.
 
MyCleveland said:
Thank you both for the responses.
‘I was leaning towards “synchro” line of thinking….or something internal to the transformer itself.
Not saying at all that H1 response is not valid, just trust this crew would know to check this throughly.

Feedback this afternoon was via increasing load on tranny they were able to initiate the voltage swing.
Continually checking everything they noticed the factory installed main served from the tranny…the C phase hold down bolt was barely threaded on and the A & B were only finger tight.
All now tightened and problem has not reoccured.
They told me NO discoloration on any of the hold downs.
Click to expand...
Must have been Friday at the factory when they built it! LOL!
 
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