A common problem for us is that the design of the ungrounded wye-delta three phase transformer bank requires the high side neutral not be bonded to the neutral and "floated".
For several reasons over the years we will get these delta secondary banks built and the high side neutral will be bonded. On an open phase or primary fault on the line, that bonded bank will try to feed the open primary phase and result in melted transformer fuses. Sometimes only one fuse, sometimes all three fuses will melt.
This is so common that when asked to look into what went wrong, I almost always find the transformer bank has a bonded high side neutral. Rebuild the bank to a floated high side and no more problems with that bank during an open or faulted primary phase. That is just the way a delta operates.
Today the industry has moved from our old dependable ungrounded wye-closed delta to the more popular grounded wye-grounded wye transformer bank.
Yesterday I worked a large grounded wye-grounded wye transformer serving motor loads at 480/277VAC. This occurred after an OCPD had operated and reset, single phase trip. One of the high side fuses seemed to have melted and that phase was open serving that transformer. There are several three phase wye-wye banks in the area on the same mainline serving large motor loads and none blew or melted fuses. This is the first time I can say I remember seeing this on a wye-wye. If that transformer had been a delta secondary and bonded, there would be no question as to what happened.
Do you believe that a three phase wye-wye will back feed into an open or faulted primary phase?
With any luck the way I wrote this is clear enough to those of you dealing with transformers and three phase motor loads to offer an opinion.
For several reasons over the years we will get these delta secondary banks built and the high side neutral will be bonded. On an open phase or primary fault on the line, that bonded bank will try to feed the open primary phase and result in melted transformer fuses. Sometimes only one fuse, sometimes all three fuses will melt.
This is so common that when asked to look into what went wrong, I almost always find the transformer bank has a bonded high side neutral. Rebuild the bank to a floated high side and no more problems with that bank during an open or faulted primary phase. That is just the way a delta operates.
Today the industry has moved from our old dependable ungrounded wye-closed delta to the more popular grounded wye-grounded wye transformer bank.
Yesterday I worked a large grounded wye-grounded wye transformer serving motor loads at 480/277VAC. This occurred after an OCPD had operated and reset, single phase trip. One of the high side fuses seemed to have melted and that phase was open serving that transformer. There are several three phase wye-wye banks in the area on the same mainline serving large motor loads and none blew or melted fuses. This is the first time I can say I remember seeing this on a wye-wye. If that transformer had been a delta secondary and bonded, there would be no question as to what happened.
Do you believe that a three phase wye-wye will back feed into an open or faulted primary phase?
With any luck the way I wrote this is clear enough to those of you dealing with transformers and three phase motor loads to offer an opinion.
