Close-Transition Automatic Transfer Switch with Delta and Wye Sources

[VTer]

Assuming the same phase sequence and frequency, is it possible for Close-Transition Automatic Transfer Switch to perform close-transition transfer if one source is 480V, 3-Phase 3-wire Wye and other source is 480V, 3-phase, 3-wire Delta?

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[ScottyUK]

Possibly, depending on system grounding arrangements.


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If we learn from our mistakes I'm getting a great education!

 

[VTer]

Thanks Scotty,

Delta source is existing Delta-Delta transformer with un-grounded 480V Secondary, and the Wye Source will be from a new 480Y/277V grounded wye emergency diesel generator. Since we only need a 3-wire system, the generator neutral will only be solidly grounded at the generator location.

I am thinking that as long as the phase voltages magnitudes and frequencies are within specified limits; and phase sequence is the same, then close-transition should not be an issue. Can someone confirm?

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[rcw retired EE]

No problem with phase angles because you are paralleling a generator with transforemr. No problem with voltages unless there is a ground on the delta system.

Assume phase A on delta service gets a ground. B-G and C-G = 480V. When the ATS closes it will momentarily place the generator's B-G & C-G 277 volts to ground on all three phases.

You might solve that issue by installing a zig-zag grounding transformer or at least a set of ground detection lights off three 480/120V traanformers connected phase to ground. That would hold the delta voltages closer to the generator output.

Check with the ATS supplier about their synch check circuit and if it will get fooled by the floating ungrounded delta.

 

[waross]

A high impedance ground on either the generator or on both systems may be a better choice. You don't want your generator coming in with a solidly grounded neutral if there is a low impedance from one phase to ground on the system.


Bill
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"Why not the best?"
Jimmy Carter

 

[rcw retired EE]

Hi-res ground on the generator. That's a much better idea, Waross.

 

[waross]

Rather than a zig-zag, consider a wye delta bank. A zig-zag may be an expensive special order.
You may construct a grounding unit by connecting three small transformers in wye delta. The wye point is grounded. The delta stabilizes the wye point and need not be connected. An impedance may be added from the wye point to ground or may be added in the delta to limit the ground fault current.
Cheap, off the shelf dry type lighting transformers are suitable for high impedance grounding.

Bill
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"Why not the best?"
Jimmy Carter