wye-wye vs delta-wye transformer
wye-wye vs delta-wye transformer
(OP)
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
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
RE: wye-wye vs delta-wye transformer
RE: wye-wye vs delta-wye transformer
RE: wye-wye vs delta-wye transformer
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
RE: wye-wye vs delta-wye transformer
RE: wye-wye vs delta-wye transformer
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.