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

 

[ActionDave]

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.

It's called an open delta.

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?

No. You need three transformers to make a wye system.

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

It doesn't exist.

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

You would end up with a colliding neutral.

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

You get 208V and with an open delta it is not a stable voltage.

 

[mivey]

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?

If you have a 4-bushing secondary (x1 x2 x3 x4) you can.

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

Because it would not be a common configuration so is harder to find.

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

Because they are different voltages and also both are not "neutrals" for the system.

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

It is 208 volts and has a 90 degree phase difference from the other two mentioned.

 

[mivey]

You get 208V and with an open delta it is not a stable voltage.

Define not stable and why you think it is not stable.

 

[ActionDave]

Define not stable

You should not connect anything L to N on a high leg system, especially one that is an open delta.

and why you think it is not stable.

Because my boss and his brother, who are both smarter than me, told me so.

 

[ActionDave]

If you have a 4-bushing secondary (x1 x2 x3 x4) you can.

Where do these exist, and if they do why has every three phase system using only two transformers I have ever seen in my life been an open delta.

 

[mivey]

You should not connect anything L to N on a high leg system, especially one that is an open delta.

With the correct breaker you could. It is a voltage source as well.

Because my boss and his brother, who are both smarter than me, told me so.

If overloaded, any transformer source could become "unstable". The high leg, while an odd voltage, is not going to go all crazy and what-not. It is part of a stable set of voltages under rated loading. If it goes "unstable", so do the rest.

 

[mivey]

Where do these exist, and if they do why has every three phase system using only two transformers I have ever seen in my life been an open delta.

Because it is the standard connection and most likely the only one you would see.

 

[jacus83]

Because it is the standard connection and most likely the only one you would see.

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.

 

[jacus83]

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

 

[ActionDave]

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.

Take a look at 110.15 in the NEC.

 

[ActionDave]

And 408.3 also.

 

[jacus83]

Thanks everyone this was very helpful!

Jack

 

[big john]

If you have a 4-bushing secondary (x1 x2 x3 x4) you can...

Do you have a diagram for that? My understanding has been that an open-deltapen-wye won't work as a functional bank.

 

[GoldDigger]

Do you have a diagram for that? My understanding has been that an open-deltapen-wye won't work as a functional bank.

If you put two coils from two separate phases in series you will get either a phase vector halfway between the original two or if you reverse the polarity you get a vector in the direction of the third phase. The two secondary windings in each transformer have to be totally isolated rather than having a common center point terminal.
You cannot derive the third phase from the first two by simple addition, but when you add in the possibility of phase inversion you can get there.
However, if you are talking about using two or more isolated secondaries in your two "single phase" transformers, you are really talking about using four or more transformers. They primaries are, however, indeed connected across only two input phases (using all three input phase wires), so in that limited sense you can go from an open delta to an open wye.

 

[mivey]

GoldDigger covered it. I have a diagram but can't get to it now. I can upload later if anyone interested.

 

[Haji]

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

If we connect, it would be a short circuit.
What is the magnitude of short circuit current?

 

[mivey]

If we connect, it would be a short circuit.
What is the magnitude of short circuit current?

Dropping three center-tapped 120/240 volt 15 kVA transformers in an example system:
V_AN: 199.2 V (high leg)
V_BN: 114.8 V
V_CN: 114.8 V
V_AB: 229.6 V
V_BC: 229.7 V
V_CA: 230.3 V

some sample faults were:
A->N: 1.28 kA (high leg)
B->N: 1.37 kA
C->N: 1.37 kA
A->B: 1.60 kA
B->C: 1.60 kA
A->B->C: 1.85 kA
Center_AB->Center_BC: 0.877 kA (center-tap1 to neutral)
Center_CA->Center_BC: 0.875 kA (center-tap2 to neutral)
Center_CA->Center_AB: 0.875 kA (center-tap2 to center-tap1)

 

[kwired]

If you put two coils from two separate phases in series you will get either a phase vector halfway between the original two or if you reverse the polarity you get a vector in the direction of the third phase. The two secondary windings in each transformer have to be totally isolated rather than having a common center point terminal.
You cannot derive the third phase from the first two by simple addition, but when you add in the possibility of phase inversion you can get there.
However, if you are talking about using two or more isolated secondaries in your two "single phase" transformers, you are really talking about using four or more transformers. They primaries are, however, indeed connected across only two input phases (using all three input phase wires), so in that limited sense you can go from an open delta to an open wye.

Not totally following your description, but if you do this do you still have the ability to supply a three phase load?

Do you still have 240/120 or if it is essentially an open wye with 120 to neutral then line to line is now 208?

 

[GoldDigger]

You can feed a three phase load, with an appropriate adjustment to the transformer capacities.
Yes, you could I suppose calm it an open wye, but it has all four wires.