Has anyone experienced field performance or safety problems with transformers connected wye-wye? I plan to specify an outdoor, three-phase, pad-mounted transformer rated 75kva, 25/14kv-120/208v with both neutrals bonded and grounded to a ground ring around the slab. I usually specify only delta-wye connections, but the nearby transformers are rated wye-wye so I assumed that they were common practice in that area. I've heard horror stories about wye-wye connections, but I'm not sure if they were based on fact or fiction. Excuse my slow response to any questions you may ask, but I will be out of town for a week. Thanks
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Y-Y transformer problems?
- Thread starter cosmos
- Start date
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- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: Y-Y transformer problems?
At least one of our local utilities will not allow padmount wye-wye connected transformers on their grid unless they are built with "tri-plex" cores.
At least one of our local utilities will not allow padmount wye-wye connected transformers on their grid unless they are built with "tri-plex" cores.
jtester
Senior Member
- Location
- Las Cruces N.M.
Re: Y-Y transformer problems?
Y-Y padmounted transformers are very common, particularly at higher voltages, 25 kv class. I worked for a utility for years that had both a 15kv and a 25 kv system. We had delta-y at 15 kv and y-y at 25. There is no problem with a Y-Y if all things are considered.
1. You usually want a 5 legged core as Jim said, this avoids tank heating and other problems with circulating currents in a 3 legged core.
2. The bank will transmit harmonics from one side to the other, whereas a delta-wye won't.
3. Ground fault current levels will usually be less do to the ground fault interaction with the high side system, when compared to a delta-wye connection.
I wouldn't be at all concerned about Y-Y padmounts with the proper precautions.
Jim T
Y-Y padmounted transformers are very common, particularly at higher voltages, 25 kv class. I worked for a utility for years that had both a 15kv and a 25 kv system. We had delta-y at 15 kv and y-y at 25. There is no problem with a Y-Y if all things are considered.
1. You usually want a 5 legged core as Jim said, this avoids tank heating and other problems with circulating currents in a 3 legged core.
2. The bank will transmit harmonics from one side to the other, whereas a delta-wye won't.
3. Ground fault current levels will usually be less do to the ground fault interaction with the high side system, when compared to a delta-wye connection.
I wouldn't be at all concerned about Y-Y padmounts with the proper precautions.
Jim T
jtester
Senior Member
- Location
- Las Cruces N.M.
Re: Y-Y transformer problems?
post by cosmos
Also, 15 kv insulation is more forgiving than 25 kv insulation. The y insulation has less stress compared to its BIL level, 15kv/95kv, than does the delta which comes out 25kv/125kv.
Jim T
post by cosmos
The delta connection would pose no problem to a utility relay, fuse, or other protection scheme. In my experience, at 25 kv the y-y is less expensive than a delta-y. Delta-y's are available, I have used them, but they are costlier.The distribution circuits use concentric neutral cables, so I was afraid that a delta connection may create a problem for the utilities upstream relay protection scheme
Also, 15 kv insulation is more forgiving than 25 kv insulation. The y insulation has less stress compared to its BIL level, 15kv/95kv, than does the delta which comes out 25kv/125kv.
Jim T
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