Paralled Transformers 2

[JBD]

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?

 

[tinfoil]

Sounds like the new delta-wye has been wired 60° out-of-phase with the others. Instead of being 30° 'lagging' the primary phases, it is 30° leading (or vice-versa).

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.

 

[JBD]

I agree, which is why I have asked for the winding connection diagrams. It will not be possible to change the secondary "star" connections as the transformer is solidly bussed. We will have to reconnect the H1 and H2 terminations on each coil of the primary instead.

 

[RalphChristie]

Tinfoil is correct regarding the 60° phase-shift.

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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[PWR]

With delta wye transformers, the rule to remember is that to parallel them there is only one way to connect both the primary and the secondary. If you "roll" the primary connections, you cannot "roll" the secondary to correct and vice versa. You say the secondary is bussed and cannot be re- connected. I'm skeptical of the practicality of changing the transformer internal connections to correct your situation. Assuming you can do it, is that really less trouble than correcting the bus work?

 

[rcw retired EE]

Correctng the bus work will not correct the problem. The internal transformer connections have to be modified, either on site or back at the factory. (I have tried it. Many hours but no success.)

It should be easy to do with a cast coil design, but be sure to get the manufactuer's blessing.

 

[tinfoil]

You must reverse the polarity and 'roll' either the primary or the secondary connections. You cannot solve this problem by rearranging the external connections unless all six H1 and H2 points are accessible.

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.

 

[stevenal]

Lots of advice, none of it quite hitting the mark. Tinfoil is correct that the voltage readings sound as if two different phasor group transformers are involved. Two folks claim this cannot be corrected via external connections. I have 115:12.47 kV installation that proves they are wrong. Watch the FAQ section for a graphical method of proof. Just submitted and should be approved by tomorrow.

 

[stevenal]

Looks like it's there. faq238-1154

 

[rcw retired EE]

Steve - thank you. I remember now that we could not get the bus rotated on the secondary to swap rotation.

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.

 

[stevenal]

I prefer not say I'm changing a Dy11 to a Dy1. Note that the un-modified phasor diagrams did not show the system connections or the connected phase sequence. Also the modified drawings still indicate Dy1 and Dy11. I believe that the Dy? designation only shows internal wiring and how the bushings connect to the windings, and that only after you connect the system can the phase shift be determined. Stephenson advocated a 90 degree phase shift connection on an ANSI standard (Dy1) style transformer. Works better for me to keep the two issues separate.

By the way, I'm expecting high ratings for my FAQ. If you cannot rate it a 9 or 10, please send me your comments first. I promise to consider them. Thanks.