Two single phase transformers connected in a bank

[jacus83]

So here is an interesting one. I have a customer that has a two single-phase transformers banked together, connected to three-phase primary service.

Configuration is similar to below, except X1/X2/X3 go from left to right.

Couple questions on this configuration. Is it possible to connect this configuration to form Delta-Wye and achieve 120V to neutral on all three phases?

For some odd reason I cannot locate two single phase transformer bank configuration thats connected in a Delta-Wye configuration.

Why can't we connect X2 (neutral) on secondary of both transformers together?

The configuration above give us 240V between all three phases, but only 120V between b-n and c-n, but what about a-n?

Need some help with this one. Thanks in advance.

Jack

 

[davidbeach]

What is a-n? A simple bit of geometry, or Pythagoras if you wish, will produce that answer.

With just two transformers you can't have three branches that all meet at a wye point and have an equal phase separation.

On both triangles, erase the A-C and c-a lines, there's no transformer winding there; then some of the rest of it will become clearer.

 

[jacus83]

David, I see what you're saying. What would be the voltage between A phase and Neutral for this situation? 208V?

I don't really care about having a balanced Wye, since we wouldn't have a 3 phase 120V service connecting to anything.

What would happen if I connected both neutrals together?

 

[waross]

No you can't get 120:208 three phase from two transformers.
This connection was used to develop 120:240 Volts single phase for lighting and other single phase loads.
The addition of a "Teaser" transformer develops 240 volts three phase for three phase loads.
Typically the transformer connected for 120:240 Volts may be much larger than the second transfoemer.
A hint on the voltage at a-n
Make 120 Volts equal to one unit.
You have a right triangle, n-c-a.
n-c = 1 unit
c-a + 2 units
Solve for a-n
This connection is called an "Open Delta".
This connection is more often used with two primary phases and a neutral.
On long rural lines it saves the cost of installing insulators and conductors for the third phase.
There is a connection that I have heard called the "Winnipeg" connection which will produce 120:208 Volts three phase power from two phases and a neutral but it requires three transformers.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jacus83]

How do I connect this to 3-phase panel?
I suppose it would be okay if I only had 3 phase breakers/3 phase loads and no 1 PH (1 pole) breakers on the A phase?
How do I prevent someone plugging in a 1 PH breaker and connecting it to neutral?
Put a warning label on the panel? This seems like it would be a code violation to me.

 

[jghrist]

See NEC 408.3(F)(1), High-Leg Identification

 

[waross]

You may use two services.
One service will be 120:240 Volts single phase.
The second service will be 240 Volts three phase.
OR
You may use a three phase panel and don't connect any loads between the neutral and the "High Leg" or the "Wild Leg". (208 Volts to neutral.)
Follow up on jghrist's post.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jacus83]

Thanks everyone this was very helpful!

Jack