Delta-Delta Transformer connected to existing 208-Wye System

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Tank11

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Hello,
I've been brought into an existing project where a 208-wye system was upgraded to 480-volts to incorporate new motor loads. An electrician connected a dry-type, delta-delta transformer to the existing 208-volt system and then things got wild. I'm not sure how the neutral was connected but I assume it was center tapped creating a wild leg on the 208-volt system. In electrical theory terms, what happened when this system was energized?

Any help is appreciated.
 
The only issues are the line to neutral loads on the high leg and that the line to line voltage has increased from 208 to 240. For many line to line loads that is not a problem.
 
If they connected 208 delta to primary and got 480 delta out of the secondary they should have never had to touch the neutral. I dont understand what you are saying has happened.
 
You cannot easily get from 208Y to 480V using a standard delta-delta transformer.

If you actually have a centered tapped 240V delta winding configuration, the voltages will be 240V L-L and 208V L-N on one leg, and 120V L-N on the other two legs. Typically these are not found on transformer primaries, so you are probably using a transformer wired in reverse.
The reverse wired 240:480V step-up will result in 208V in and 416V out, because the taps will be located on the secondary side they will not function very well.
If a 208Y/120V neutral point was connected to a center tapped delta winding, you could expect all sorts of improper voltages and current flows.
 
Tank11 said:
Hello,
I've been brought into an existing project where a 208-wye system was upgraded to 480-volts to incorporate new motor loads. An electrician connected a dry-type, delta-delta transformer to the existing 208-volt system and then things got wild. I'm not sure how the neutral was connected but I assume it was center tapped creating a wild leg on the 208-volt system. In electrical theory terms, what happened when this system was energized?

Any help is appreciated.
Click to expand...
I think we need some clarification on what was done. Are you saying the old 208 Y system was refed with a 120/240 center tapped transformer? If that is the case then all the line to neutral loads on one leg of the system saw 208 volts and let the magic smoke out.
 
texie said:
I think we need some clarification on what was done. Are you saying the old 208 Y system was refed with a 120/240 center tapped transformer? If that is the case then all the line to neutral loads on one leg of the system saw 208 volts and let the magic smoke out.
Click to expand...
Yes, that is how I read the original post too.
 
Tank11 said:
Hello,
I've been brought into an existing project where a 208-wye system was upgraded to 480-volts to incorporate new motor loads. An electrician connected a dry-type, delta-delta transformer to the existing 208-volt system and then things got wild. I'm not sure how the neutral was connected but I assume it was center tapped creating a wild leg on the 208-volt system.
Click to expand...
There should be no neutral involved in any way with a delta-delta transformer. Motors certainly don't need a neutral. Are you sure it is one, with no H-0 or X-0 terminal?

Or, after reading your post and the responses again, it sounds like you fed two phases with 120v to the neutral, and the third with 208v to the neutral.

If so, the wrong transformer was installed. It should have been a 480v delta to 208Y/120v unit.
 
An existing 120/208-volt building was re-fed with a 500 kVA delta-delta transformer 480:240. Service was upgraded to 480 and then the existing portion was fed with this transformer. I agree that it's the wrong transformer and it needs to be delta/wye. My question is what physics are involved when this was connected? I'm told the neutral was connected on a center tap between two phases. I believe this creates a "wild leg" and any 120-volt load would see a lot more voltage than 120.
Photo.PNG
 
Tank11 said:
An existing 120/208-volt building was re-fed with a 500 kVA delta-delta transformer 480:240. Service was upgraded to 480 and then the existing portion was fed with this transformer. I agree that it's the wrong transformer and it needs to be delta/wye. My question is what physics are involved when this was connected? I'm told the neutral was connected on a center tap between two phases. I believe this creates a "wild leg" and any 120-volt load would see a lot more voltage than 120.
View attachment 2558819
Click to expand...
You are correct. The secondary of this transformer is 120/240 4 wire delta which creates a high leg of 208 to neutral on one leg. Any line to neutral loads connected to the high leg will see 208 volts.
Based on your description of the situation, this is the kind of thing that is so basic that all involved should be run off the job and reported as incompetent to the electrical board. This should be basic level stuff for any EC. Did the parties involved actually energize this before discovering the incompatibility? If so, I would expect the damage to be pretty severe and costly.
 
Something to touch on would be if the existing panels and breakers were used. There may be an issue requiring Delta breakers due to the higher voltage to ground. The 120/240 rating would be violated on the high leg phase which is 208 to ground.
 
Tulsa Electrician said:
Something to touch on would be if the existing panels and breakers were used. There may be an issue requiring Delta breakers due to the higher voltage to ground. The 120/240 rating would be violated on the high leg phase which is 208 to ground.
Click to expand...
It sounds like the xfmr should have been 480 3 phase delta to 120/208 3 phase wye.
So the xfmr is what's wrong.
This is no small job where a 10 kva xfmr was possibly specified wrong and no one caught it till it was too late.
This was a 500 kva xfmr that should have never gotten as far as it did before it was discovered.
I would hate to be sitting where the buck stops on this one.....
 
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