Delta-delta versus Delta-Wye
Delta-delta versus Delta-Wye
(OP)
I apologize in advance if this has been answered before. I searched but did not find anything similar. I have a customer with a very old 1500kVA transformer (13.2kV-480V) delta-delta, with a true 3-wire delta service to their manufacturing facility. The transformer is being changed, but the main distribution panel is fine and remaining.
I recommended a delta-delta, but of course the contractor has a deal on a delta-wye; which he wants to hook up without the neutral connection. I know this will work and have a 30 degree phase shift.
Are there any negative issues anyone can suggest for applying a delta-wye in this application???
I recommended a delta-delta, but of course the contractor has a deal on a delta-wye; which he wants to hook up without the neutral connection. I know this will work and have a 30 degree phase shift.
Are there any negative issues anyone can suggest for applying a delta-wye in this application???






RE: Delta-delta versus Delta-Wye
respectfully
RE: Delta-delta versus Delta-Wye
RE: Delta-delta versus Delta-Wye
The real question is what are the negative impacts of using a delta-(ungrounded) wye transformer to supply a 480V three-wire service (2000A)?
RE: Delta-delta versus Delta-Wye
RE: Delta-delta versus Delta-Wye
It would also make that system unnecessary.
RE: Delta-delta versus Delta-Wye
Bear in mind - in that size, with grounded, ground fault should be added to the main and possibly to downstream breakers - 1000 amps and over this is a code issue. The equipment could be prone to arcing burndown without it.
RE: Delta-delta versus Delta-Wye
http
for starts.
Assuming past success (which as a good stock broker would say, does not foretell the future) tells you to not worry about the overvoltage issues, some thoughts that come to mind:
I would think your old delta ground fault detection system will work OK; it will not know the neutral is floating, or that it even exists. Maybe you can connect your ground fault sensor to the neutral. High voltage at the neutral = ground fault. However, you will see third harmonic voltages in the neutral that you will need to ignore. In a delta-delta the third harmonic current required to excite the transformer circultates entirely in the two delta windings. In a delta-wye, the third harmonic now circulates in one winding, and creates a third harmonic voltage in the neutral.
Of course, the phase shift means there could be an issue of whether you will do a closed/open transition of the system voltages to a secondary source with another xfmr that has a different phase shift.
In a delta-delta xfmr, it is (remotely) feasible that the failure of any one leg will allow limping along with a two phase open delta. In a delta-wye ungrounded xfmr, the failure of any one leg means you loose too much to limp along.
The neutral in a wye winding may have reduced insulation levels near the neutral. You need to find out of the neutral insulation is rated for ungrounded operation.
In ungrounded systems, you are not required to have the ground fault overcurrent described in NEC 215.10. See 230.95 and 250.36. The 2005 NEC Handbook has some discussions in these two sections on the matter.
RE: Delta-delta versus Delta-Wye
I agree. The existing lights should work just fine if you design the new system grounding correctly.
I generally agree with the other above posts that you should give system grounding another look.
RE: Delta-delta versus Delta-Wye
1) if you DO NOT ground the neutral point of the xmfr, then the secondary components must be rated for the FULL LINE-TO-LINE Voltage of the system.
2) No matter what, ensure that the xmfr is case grounded!
RE: Delta-delta versus Delta-Wye
" THINK BIG "
RE: Delta-delta versus Delta-Wye