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open delta replacement

open delta replacement

open delta replacement

(OP)
I have an open delta transformer arrangement (12470v-240/120 delta-delta)that I would like to replacement with a 3 phase transformer. The existing set-up is 1-100 kva single phase center-tapped to get 240/120v. The second transformer is a single phase 37.5kva which is connected to the first by a stinger. I would I like to replace it with an 12470-208/120, 150kva 3 phase delta wye. I have about 80 kva of single phase load and 60 kva of 3 phase load. Am I figuring this right by saying I would have an equivalent of 50 kva on 2 legs of the transformer and 20kva on the third.

RE: open delta replacement

80 KVA plus 60 KVA will only sum to 120 KVA if you have some unusual power factors. Possible but very unusual.
If it is working why change it?
120:208 Volt three phase is widely available and 120:240 Volt wild leg is not nearly as widely available.
120:208 Volts is nice and symmetrical but symmetry isn't everything.
It may work just fine, there are probably millions of 120:208 Volt services in north America. However, some of your 240 Volt loads may have issues with 208 Volts. Most 240 Volt loads are happy on 208 Volts but there is always the possibility of embarrassment.
 

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: open delta replacement

Refrigeration will be very un happy on 208 if it was happy on the 240.

Keith Cress
kcress - http://www.flaminsystems.com

RE: open delta replacement

(OP)
I am replacing because it's leaking. I also checked the 3 phaase load and it all rated 208 - 240

RE: open delta replacement

I have encountered numerous wild leg systems. It has been my policy to convert 240 Volt closed delta (three transformer) systems to 120/240 Volt wye systems.
Delta secondary banks (and particularly wye delta banks) create problems for both the end user and for other users on the distribution circuit.
The conversion is generally just a reconnection of the existing transformers.
The occasional voltage issues with end use equipment are less onerous than the problems that are eliminated and permanent solutions are fairly economical. Sometimes the only solution required is an adjustment of tripping elements.
I have not found the same issues with open delta systems. I leave open delta banks as is. If one transformer is faulty, just replace the faulty transformer.
If you do go to the wye system, it is best to go through the breaker panels circuit by circuit so as to balance the load between the three phases.
If you have an existing four wire service with a three phase main panel the single phase loads (120 Volt and 240 Volt) should be redistributed on what was the wild leg.
If you have separate panels and meters for single phase and three phase it starts to get more expensive.
It is unlikely that the existing single phase meter will meter even two phases of a wye system. The meter base will have to be changed to accommodate the new meter. The three phase meter may be suitable but, depending on local utility requirements this meter may need to be changed out.
I would recommend just replacing the faulty transformer with the same with the taps on the same settings.
Been there done that.

Bill
--------------------
"Why not the best?"
Jimmy Carter

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