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#128260 - 01/02/03 10:39 AM Open Deltas
ronko Offline
Junior Member
Registered: 01/02/03
Posts: 1
Loc: Clark County, Nevada
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
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#128261 - 01/02/03 03:56 PM Re: Open Deltas
Bjarney Offline
Registered: 04/10/02
Posts: 2527
Loc: West-Southern Inner-Northeast ...
Examples: In the first case, If there were a pair 10-kVA transformers, their capacity is 17.3kVA, and not 20kVA

30,000 VA total nameplate for three pots
0.5774 = 1/sqrt3
17,321 VA rating using only two 10kVA pots

In the second case, if there was one transformer removed from a three 10kVA pots, there capacity is reduced to 17.3kVA

20,000 total nameplate VA for two pots
0.8660 = sqrt3/2
17,321 VA actual capacity for two pots

In the third case, for two transformers serving a 20kVA load, the total nameplate capacity needed is 23.1kVA

20,000 VA load - served by two pots
1.1547 = 2/sqrt3
23,094 VA total nameplate transformer capacity for the two pots; id est, two 10kVA pots cannot adequately serve a 20kVA load.

Cooper Power bulletin R201-90-2 may be a useful reference.

[This message has been edited by Bjarney (edited 01-02-2003).]
#128262 - 01/05/03 04:57 AM Re: Open Deltas
Scott35 Offline

Broom Pusher and
Registered: 10/19/00
Posts: 2707
Loc: Anaheim, CA. USA
Bjarney has covered it very well!

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.

Scott s.e.t.

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!

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