If the primary feeds are in phase and rotation, why isn't secondary?

[zzzztttt]

I have three 15 KV X 480 Volt transformers fed by three 15 KV feeders from the same 15 KV switch line up. Here's the problem: The primary feeders of the three transformers are in phase and rotation and the secondary sides are not. All three are straight from the factory with no field mods. What I found is that the voltages measured between phases from Transformer One and Transformer two are roughly 277V. Interestingly enough, the measured voltages between phases "A" and "C" from Transformer One to Transformer Two measures about 554V. The voltages measured between Transfomer Two and Transformer Three are right at "zero," with voltages from phase "A" to "C" being right about 480V. I am trying to pinpoint the problem and fully understand the possibilities so I can go back to manufacturer for a fix.

 

[Skogsgurra]

You probably noted that 554 = 2x277. Wouldn't that indicate a 180 degree error in one of the transformers?

Were they made twins? And born at same time?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Jerry123]

Are these dry type transformers? If so open the covers and look at how the primary connections between the coils are connected. I've had this problem before when replacing a failed unit with a different manufacture in a main tie main 480 volt switchgear. I had the same voltage measurements 277/277/544. We had to reconfigure the primary connections on the coils to swap the polarity. Swapping the polarity of each coil of the primary side, changed the polarity on the secondary side.

 

[waross]

Be careful if one of the transformers is a shell form. The coils on the center leg may be reduced to save iron. This may lead to an incorrect connection. If primary is incorrectly connected on the center leg of such a transformer the center leg of the core may be driven into saturation.
If the transformers are core form, please disregard this post.
respectfully

 

[whycliffrussell]

Skogsgurra is correct. Look at the wiring for the individual phases:

H1 H2
X1 X2

OR

H1 H2
X2 X1

Additionally, you can test the 'polarity' of the phases. The Megger TTR20 will identify normal (in phase) and reverse polarity of the single-phase transformers.

 

[sslobodan]

It was long time ago when I learned about transformer construction so this what i write should be taken with reserve, but transformers can have angle rotation between primary and secondary side due to a construction, . that is why you need 2 exactly the same transformers or you have short circuit. Your problem is very simple when they were taking out the transformer endings, they haven't paired them as it should so you got the phase angle difference as Skogsgurra intelligently saw in first place that is giving you such voltage difference. I will see If I have exact explanation here in books I have with me, and if not, the book with right answer is some 4 000 km far away from me, back home

 

[sslobodan]

Here I have it there is remark about transformation that n=U1/U2*e(exp(jk30)
k is the number of times that values on low voltage side are late for high voltage side. If you take that first transformer has k=k1 second transformer has k=k2=k1+6 that would make the difference between phases 180 degrees and voltage difference between same phases 2times nominal voltage.