aelectricalman
Senior Member
- Location
- KY
Some time ago I posted a discussion about an ungrounded delta system that has 2.4 volts phase to ground. I know that when evaluating an ungrounded delta there is no reference to ground however, can referring to ground be used to evaluate potential issues?
What happened was that an anomaly was detected on the secondary side of a 12.47kV Delta /480V ungrounded Delta transformer. Phase A on this XFMR was 2.4 volts to ground while the other two phases were 480V. This is indicative of a corner grounded Delta system. The problem was that this is not a corner grounded transformer.
A few of you said that maybe the corner ground comes from a VFD, and sometimes a reference ground is pulled off of one of the phases, to get 277V. I was a little puzzled by that so I dug a bit deeper because I didn't think the situation applied.
What I found is that there is a 480V ungrounded Delta/480V Wye Iso transformer in line between the drive and the primary transformer mentioned earlier. There are three undgrounded conductors coming into the XFMR and three undgrounded conductors coming out of the Iso XFMR. Basically no neutral or grounding (which I understand the grounds will not be derived at the XFMR). In the top of the VFD, there are three ungrounded feeders coming in. There are no rigid or metal conduits attached as the cables ride in a cable tray to the VFD then are transformed to rigid, 1 foot away ( no neutral).
So, with this being said, I do not see how the VFD could be causing this corner ground. With the Iso XFMR as a buffer, the corner ground has to be originating in between the 480v ungrounded Delta secondary and the 480 v high side of the iso transformer, since this anomaly cannot pass through the induction of the Iso transformer.
It can't be from the power company because it is taking place on the secondary side of their XFMR.
That would be a dead short to ground on one of the phases or a defect in the XFMR's (since they are designed and installed at the same time. The interesting thing is that this is not happening only once. The issue is replicated next to it in another 12.47UD/480 UD XFMR. Interesting at minimal.
What happened was that an anomaly was detected on the secondary side of a 12.47kV Delta /480V ungrounded Delta transformer. Phase A on this XFMR was 2.4 volts to ground while the other two phases were 480V. This is indicative of a corner grounded Delta system. The problem was that this is not a corner grounded transformer.
A few of you said that maybe the corner ground comes from a VFD, and sometimes a reference ground is pulled off of one of the phases, to get 277V. I was a little puzzled by that so I dug a bit deeper because I didn't think the situation applied.
What I found is that there is a 480V ungrounded Delta/480V Wye Iso transformer in line between the drive and the primary transformer mentioned earlier. There are three undgrounded conductors coming into the XFMR and three undgrounded conductors coming out of the Iso XFMR. Basically no neutral or grounding (which I understand the grounds will not be derived at the XFMR). In the top of the VFD, there are three ungrounded feeders coming in. There are no rigid or metal conduits attached as the cables ride in a cable tray to the VFD then are transformed to rigid, 1 foot away ( no neutral).
So, with this being said, I do not see how the VFD could be causing this corner ground. With the Iso XFMR as a buffer, the corner ground has to be originating in between the 480v ungrounded Delta secondary and the 480 v high side of the iso transformer, since this anomaly cannot pass through the induction of the Iso transformer.
It can't be from the power company because it is taking place on the secondary side of their XFMR.
That would be a dead short to ground on one of the phases or a defect in the XFMR's (since they are designed and installed at the same time. The interesting thing is that this is not happening only once. The issue is replicated next to it in another 12.47UD/480 UD XFMR. Interesting at minimal.
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