I hope this is the proper forum for this question. I have three single phase transformers connected in delta - delta. The primary is 2300V, the secondary is 460V. The center of one transformer secondary winding (x2 and x3) is grounded. I'm reading 470V between all 3phases. A and C phases to ground read 235V, and B to ground reads 400V. This is the first time I've seen a set up like this. Can anyone tell me if these readings are correct. Thanks
delta center tap grounded secondary question
- Thread starter Tfret
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- Wisconsin
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- PE (Retired) - Power Systems
Yes those readings look fine.
It involves trigonometry, but the high leg L-G voltage turns out to be equal to 87% of the L-L voltage
You have one transformer that is tapped in the center, this means you will read 50% of the L-L voltage, when you measure L-N on this coil (you are calling these A and C to ground)
This N point and one of the other coils creates a right angle triangle, where one leg is A-B and the other is A-N. Solving for the unknown voltage of B-N, we get .8666x the L-L voltage.
So using your numbers:
A-C = 470V,
A-N = 235V
C-N = 235V
B-N = 407V.
The difference between what is measured and what is calculated has to do with rounding in the formulas and the readings.
It involves trigonometry, but the high leg L-G voltage turns out to be equal to 87% of the L-L voltage
You have one transformer that is tapped in the center, this means you will read 50% of the L-L voltage, when you measure L-N on this coil (you are calling these A and C to ground)
This N point and one of the other coils creates a right angle triangle, where one leg is A-B and the other is A-N. Solving for the unknown voltage of B-N, we get .8666x the L-L voltage.
So using your numbers:
A-C = 470V,
A-N = 235V
C-N = 235V
B-N = 407V.
The difference between what is measured and what is calculated has to do with rounding in the formulas and the readings.
ATSman
ATSman
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- San Francisco Bay Area
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- Electrical Engineer/ Electrical Testing & Controls
Not to get picky but shouldn't the N in your post be replaced with G ?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
That is an optional connection at this voltage level.
It's OK to use N. The NEC definition of neutral point includes "the midpoint of a single-phase portion of a 3-phase delta system."
kwired
Electron manager
- Location
- NE Nebraska
You have exactly what you would have with a 240 volt delta but all voltages are doubled. The high leg is still a high leg, just like the 240 system.
I almost never see this with a three transformer supply, as the POCO would likely just use 277/480 Y in those cases, but if they are supplying you with 2 transformer open delta system this is what you get, because a wye does not work with only 2 transformers.
buddhakii
Senior Member
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- Littleton, CO
Just ran into my first one of these myself. Couldn't figure out why the engineer specified 480/240 volt equipment. Told him there is no such thing, it's gotta be 480/277. Boy did I feel stupid. Not the first time and it won't be the last.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Yea and don't land any of that 240 volt equipment on that "B" leg or you will feel more then (well I don't like the word stupid) but you will feel it, seen a few of these on farms before but that was about the only place.
My first time with a high leg delta was wiring a 120 volt explosion proof gasoline pump motor, I ran a new feed to the panel and thought WoW look at all those spare breaker spaces, well needless to say that pump wasn't cheap and my boss wasn't very happy with me, but what could I say no one warned me:ashamed:
stevebea
Senior Member
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- Southeastern PA
The boss wasn't real happy about buying a slightly used pump motor? :happyno: One of our guy's caught the stinger leg last week installing temp receptcales on job site..... the carpenters fried three Dewalt battery chargers before they realized something wasn't right. :rant:
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