Paralled Transformers
Paralled Transformers
(OP)
I have a customer that is trying to install a "maintenance bypass" transformer in parallel with his existing backup generator and normal utility feed.
The new transformer is fed from the same utility connection point as his existing transformers. All of the transformers are cast coil construction connected Delta-Wye, with 480Y/277V secondaries. The output phase rotation of the new transformer has been verified as equal to the existing 480V grid.
The problem is that the output phase voltages of the new transformer are "not in phase" with any of the existing (there are four) transformers. For example the voltage from A(new) to A'(existing) is 275V, A to B' is 277V, and A to C' is 551V. Swapping primary H1, H2, and H3 connections does nothing except change the phase rotation
I have asked for wiring diagrams of the new and existing transformers to see if there is a problem with additive or subtractive winding connections or if the transformer is really built as a wye-wye.
Any other thoughts?
The new transformer is fed from the same utility connection point as his existing transformers. All of the transformers are cast coil construction connected Delta-Wye, with 480Y/277V secondaries. The output phase rotation of the new transformer has been verified as equal to the existing 480V grid.
The problem is that the output phase voltages of the new transformer are "not in phase" with any of the existing (there are four) transformers. For example the voltage from A(new) to A'(existing) is 275V, A to B' is 277V, and A to C' is 551V. Swapping primary H1, H2, and H3 connections does nothing except change the phase rotation
I have asked for wiring diagrams of the new and existing transformers to see if there is a problem with additive or subtractive winding connections or if the transformer is really built as a wye-wye.
Any other thoughts?






RE: Paralled Transformers
So two of the phases are 60° out, and one is 180° out, giving you 277 and 2*277 voltages difference.
You need to reverse the polarity of the secondary connections forming the 'star' of this unit.
RE: Paralled Transformers
RE: Paralled Transformers
Just check your vector-diagram on the nameplate. Dyn1, Dyn5 and Dyn9 transformers are in one group, and Dyn3, Dyn7 and Dyn11 transformers in the other group. Most probably all your transformers will be Dyn11 transformers, and then one a Dyn1 transformer, or vice versa. By changing connections on either side of one transformer, you can swap it from one group to another. See also FAQ238-1090.
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RE: Paralled Transformers
RE: Paralled Transformers
It should be easy to do with a cast coil design, but be sure to get the manufactuer's blessing.
RE: Paralled Transformers
IMPORTANT: once this transformer is changed to match your requirements, ENSURE that the NAMEPLATE be re-done to reflect the new configuration, or future troubleshooters will be cursing your name forever.
RE: Paralled Transformers
RE: Paralled Transformers
RE: Paralled Transformers
Essentially what you are doing is turning a Dy11 (+30 degrees) into a Dy1 (-30 degrees) by reversing the phase sequence. Not a problem as long as you can swap two leads on the low voltage side to get the phase sequence back.
RE: Paralled Transformers
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