Open Delta Transformer Load Across B-C

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fgreco2

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Hi there,

I am engineer at a large utility. I assist folks with Overhead Open Delta transfornmer sizing, and we've developed a process to take customer panel schedules and size transformers. Our program is set up to look at load splits across each widning. Our practice is to place the power pot (transformer) across A-B and the liter pot (transformer) across A-C. For power pot sizing we take the load across A-B at 100% + three phase load at 58%. Additionally, for liter pot sizing we take the larger of the two (A-N or C-N) + the A-C load at 100% + three phase load at 58%.

The program works well, but has a few holes however. We see an occasional customer place two pole load across B-C. In the past I've asked folks to move to A-C or A-B, but I wanted to verify from some of the industry experts what would be the load split accross each transformer for load across B-C. Can I assume this is three phase load and convert it to 3-pole load? E.g. 30A/2P breaker across B-C with 2400VA per phase (at 120/240V). (2400VA X 2)/3 = 1600VA. If not how should I handle this load? It has to go someplace, granted too much load across B-C would possibly collapse the Delta. Question is how much is too much? We limit 3-phase peak demand to 50kVA and largest single motor to 25HP for these applications. Please advise.

Appreciate any help.

Regards,

Frank G.
 
A single phase load across the open delta amounts to both power and lighter transformers being subjected to the current of that load.

Using your example of a 30A B-C load on a 240/120 open delta, each comprising transformer will be subjected to the same 30A.

kVA equivalent: 30A ? 240V ? 1,000VA/kVA = 7.2kVA each transformer

system equivalent total 2 ? 7.2kVA = 14.4kVA

It is usually less with other loads combined, but to make that determination you'll have to do the vector math.
 
The current for any B-C load must actually flow through the windings of both of the pots in series. There is no other possible path if no other loads are connected.
So the current when only the B-C load is on is easy to calculate.
The problem is that it will not be in phase with the current caused by either lighting loads or three phase balanced loads.
But you can get an upper bound on the winding currents by treating it as if it were in phase with the normal current for each pot.
The big disadvantage for the customer for wiring up such a load is that the voltage drop will be that of the two windings in series and so will be worse (potentially more than twice) the VD for correct wiring.
From your point of view, if you do not worry about VD, the only problem is the effect on your pot sizing calculation. The worst mismatch will come when the power pot is small compared to the lighting pot and your first calculation assumed that all single phase loads would be on the lighting pot, as you normally do.

PS: What Smarty said. :)
 
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Open Delta Transformer Load Across B-C

Thanks folks ...

Appreciate the assistance. This will help us when we run into load across B-C.

Take care,

Frank G.
 
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