Scott, I've been going over transformer hookups, calcs for ocp, delta/wye voltage and current relationships but one thing I can't get a handle on is the reduction of transformer capacity using an open delta. One situation calls out when using two single-phase transformers with a reduction at 87%(86.6), another when three single-phase transformers are used and one is removed for a reduction of 58%(57.7) and finally when sizing for load, equivalent to three transformers two transformers should be ratied up to 15.5%. Please try to clarify Thanks
Just for the sake of redundancy, I'll add some "Simple Stuff":
The Open Delta bank is able to deliver 86.6% of the KVA into 3 Phase loads.
Example: If one pot is 6 KVA and the other is 4 KVA, the output KVA is 8.66 KVA - not 10 KVA. See Bjarney's descriptions and formulas above.
If an existing Closed Delta network (three Pots connected together in a parallel / delta scheme), and one pot fails, the remaining two pots can continue to deliver 3 phase 3 wire AC - but the maximum output KVA is only 58% of the original KVA rating.
Example: 3 - 10 KVA pots connected in a delta scheme has an output power of 30 KVA max. One pot fails, due to a secondary coil becoming barbequed. Network can continue to deliver 3 phase 3 wire power, with a maximum of 17.4 KVA apparent power output - as opposed to the original 30 KVA of apparent power available.
The normal rating of a 3 Phase 4 Wire Closed Delta will have a center pot which is at least 200% larger than a single "Outer" pot.
Example: a 45 KVA 3 Phase 4 Wire Closed Delta would have a center pot rated at 25 KVA, and the two "Outer" pots rated at 10 KVA each.
Open Deltas are similar for 4 wire applications, but the "Single Phase" pot might be as large as 400% of the "Kicker" pot. All depends on the calculated loads to be driven.
For 3 Phase 3 Wire settings, the pots are typically sized equally - on both Open and Closed Delta networks.
P.S. The term "Pot" is referring to a Single Transformer - or more precisely a "Stand-Alone" single phase Transformer.
Bjarney's examples and formulas are much better than my stuff! I'll need to add this to the tech reference section!
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!