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wye-wye vs delta-wye transformer

wye-wye vs delta-wye transformer

As part of an electrical service upgrade and emergency generator installation, an existing 750 KVA 12.47KV wye ~ 480/277V wye pad-mount transformer was replaced with a 1000 KVA delta-wye transformer.  What problems could changing the primary from wye to delta introduce into the transformer and downstream distribution system?

RE: wye-wye vs delta-wye transformer

Should be no problems downstream, in fact probably fewer problems than the wye-wye.  You will have more fault current available for ground faults on the 480V side due to the delta primary, but this is not generally a problem, since the three-phase fault duty is the same.  

Wye-wye transformer have become popular with utilities because they are cheaper and also reduce the risk of ferro-resonance on the primary side.  

As a customer, you're better off with the delta-wye.  

RE: wye-wye vs delta-wye transformer

Make sure that none of the existing delta primary is grounded before changing the transformers.

RE: wye-wye vs delta-wye transformer

disregard my last comment about grounds . I did not read the question properly.

RE: wye-wye vs delta-wye transformer

Suggestion: The delta primary and grounded wye secondary transformer is considered the industry standard. The primary delta winding will cancel triplen harmonics that presently are propagating to the secondary winding within the wye primary wye secondary transformer. Wye primary - wye secondary are normally applied in the distribution network as "network transformer" just to complete the power distribution network without much concern about the system grounding, harmonics, zero sequence current flow, etc.

RE: wye-wye vs delta-wye transformer

The change from y-y to D-Y will improve the system performance in the secondary as well in the primary side from the harmonic point of view.

The secondary and primary side on the Y-Y connection is in phase. However, the new D-Y configuration will introduce a phase shift of 30 degree on the secondary side. This phase shifting will not create any concern and the system will run well without noticing the difference.

The larger transformer capacity will increase the SC in an order of magnitude from 16 kA to approx. 22 kA (assuming standard impedance). Consider double-checking your protective devices interrupting ratings on the existing facility specially those close to the transformer.

RE: wye-wye vs delta-wye transformer

One problem that was eluded to by cuky was the phase shift of the old system to new system.  This is fine if it is a stand alone system, but if you have any ties to other systems you will now be out of phase.  There would be a fault waiting to happen as soon as the tie breaker is closed.  If the situation exists I would suggest mechanical interlocks to prevent paralleling and warning signs.

RE: wye-wye vs delta-wye transformer

Some utilities require primary wye windings on transformers for voltage stability, among other things.

RE: wye-wye vs delta-wye transformer

A wye-wye transformer passes zero sequence current through to the utility with the result that the zero sequence impedance seen at the secondary includes the transformer impedance and the upstream utility zero sequence impedance.

A delta primary effectively removes the utility zero sequence impedance from the circuit, thereby possibly increasing the ground fault current at the secondary.

As metioned by dpc it is probably not a problem for fault interrupting duty (as long as the new transformer impedance is not lower), but it may lead to mis-coordination of overcurrent protection at the higher fault currents not previously available.

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