Open Delta Capacity

[lakee911]

I have an existing 120/240V 4-wire open delta system that has one 25KVA transformer and one 50KVA transformer (with a center tap). I understand that the transformers will have only 58% of their typical capacity. What I'm not sure about is if it will be (25KVA+25KVA)*0.58 or 25KVA*0.58. I would think that it would be the first of the two , but I wanted to check.

To complicate the matter the 50KVA is serving out additional customers/services besides the one service using the three phase. So, if that load is more than the remaining 25KVA then my three phase power is then reduced, right?

Thanks,
Jason

 

[jim dungar]

I have an existing 120/240V 4-wire open delta system that has one 25KVA transformer and one 50KVA transformer (with a center tap). I understand that the transformers will have only 58% of their typical capacity. What I'm not sure about is if it will be (25KVA+25KVA)*0.58 or 25KVA*0.58. I would think that it would be the first of the two , but I wanted to check.

To complicate the matter the 50KVA is serving out additional customers/services besides the one service using the three phase. So, if that load is more than the remaining 25KVA then my three phase power is then reduced, right?

Thanks,
Jason

Assuming identical sized single phase transformers.
An open delta transformer has 57.7% of the capacity of a closed delta.
An open delta transformer has 86.5% of the capacity of the sum of the individual transformers.
For example: (2) 50kVA transformers provides 86.5kVA = (3*50)*.577 = (50+50)*.865

For mixed sized transformers there is no single equivalent kVA rating.
The loading capabilities of the individual transformers are:

Center tapped KVA = .58T + S
Un-tapped kVA = .58T

Where T = the three-phase loading and S = the single-phase loading.

So in your case: you have basically 43kVA of three phase loading ability and 25kVA of single phase. However, if your single phase loading exceeds 25kVA then your three phase loading ability is reduced.

 

[lakee911]

Thanks. That's exactly what I needed.

Jason

 

[gt]

Single Phase 240 volt connected load

Single Phase 240 volt connected load

On an open delta bank, three phase 120/240 volt, I assume the 240 volt single phase load be conected across any of the phases i.e.: a-b, b-c, c-a? I would assume an attempt would be made to balance the three phases with this load. The single phase 240 volt load across the phatom leg does this load split 58 % to each of the transformers?

 

[Smart $]

On an open delta bank, three phase 120/240 volt, I assume the 240 volt single phase load be conected across any of the phases i.e.: a-b, b-c, c-a? I would assume an attempt would be made to balance the three phases with this load. The single phase 240 volt load across the phatom leg does this load split 58 % to each of the transformers?

On balancing 1? L-L loads, that would depend on the size and total load on the two xfmrs. In many, if not most instances, the "lighter" xfmr is larger to handle 1? loads while the "kicker" xfmr is smaller and works with the "lighter" just for powering 3? loads. As such, there would be substantially less current through the Line connected to the open end of the kicker when the system is balanced.

A 1? load across the open end of the transformers would get split 50-50 power-wise among the two transformers. The current, however, is out of phase with the voltage across their windings, thus entering power factor into the power equation. This uses up the xfmrs potential output because the kVA required is greater than the power consumed.