is this supposed to be?

[mbrooke]

It is. It’s a back to back T.
notice the secondary diagram. With the neutral connected up on the coil that high the resulting V-G voltages are the same.
think of an open delta missing one coil... think of this one as missing two..
the voltages of X1, X2, and X3 will be the same also.

How much copper and iron does this connection save? I see it in 90% of 15kv class 3 phase pole pigs. Also, what is the amp draw on the primary when all 3 secondary phases and evenly loaded?

 

[Hv&Lv]

How much copper and iron does this connection save? I see it in 90% of 15kv class 3 phase pole pigs. Also, what is the amp draw on the primary when all 3 secondary phases and evenly loaded?

It’s something I’ve studied some time back, but have never installed one or seen one installed on a system.
of course, I haven’t been everywhere..(nowhere close)

I have seen and used many single overhead transformers that are open deltas, like a 10/15..

I don’t ever see us using one either. Cost savings? I don’t see it either.

 

[mbrooke]

It’s something I’ve studied some time back, but have never installed one or seen one installed on a system.
of course, I haven’t been everywhere..(nowhere close)

I have seen and used many single overhead transformers that are open deltas, like a 10/15..

I don’t ever see us using one either. Cost savings? I don’t see it either.

They are supposed to be lighter and cheaper.

 

[Hv&Lv]

They are supposed to be lighter and cheaper.

Yes, but how much is the question vs reliability and additional loading later.
it’s like earlier in the discussion about the two phase 120/208. Way back when it was done for savings. We won’t build it anymore. Open deltas are different before someone brings them up...
Usually the savings are wasted when the additional loading takes you into the kVA range where you need the true three phase.

 

[wwhitney]

Can someone post the vector math for this connection?

The diagram posted shows the vector math in geometric form. Tip to tail is vector addition. Are you asking for it in algebraic form? And if so, in what coordinate, Cartesian or Polar?

Cheers, Wayne

 

[mbrooke]

Yes, but how much is the question vs reliability and additional loading later.
it’s like earlier in the discussion about the two phase 120/208. Way back when it was done for savings. We won’t build it anymore. Open deltas are different before someone brings them up...
Usually the savings are wasted when the additional loading takes you into the kVA range where you need the true three phase.

I agree for open delta, but 3 phase T connected pole pigs are exceptionally common.