Emergency Power For Delta System

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BAHTAH

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I have a customer with an existing generator that is Wye connected to provide 240/139 3ph4w or 480/277 3ph4w. The existing service which includes a manual transfer switch is 240/120 3ph4w. What I propose I have never seen done before so I thought I would see if anyone has had any experience with this situation. What I would like to do is install a 75KVA 480-240/120 3ph4w transformer to be supplied from the 480v of the generator. The 3-phase load for this batch plant consist of only (3) 7-1/2hp 240V motors and some 120V loads associated with scales and some lighting. Under normal conditions I would keep the three-phase and single-phase separated and it is after the manual transfer switch. I do not want to install three separate transformers as this is only for emergency power that will be seldom used and then only for short periods of time. My thought is, that by over sizing the transformer I am basically treating it as a single-phase transformer with respect to the 120V loads and allowing a reduced three-phase load so as not to over-load the single-phase winding. I estimate the motors will total about 9300va per phase and that would leave a additional 7200va per single-phase winding or 14,400 @240V 1ph which is allowing for as much as 60amps of 120V loads. Since the three-phase loads are known and the single-phase loads are estimated to be not over 40amps this seems to allow some safety factor in the loading. I plan on protecting the secondary with a 150Amp breaker which is 30amps under the max available line-amps. Looking this over I figure the three-phase coil amps to be about 38.75 and the singl-phase coil amps to be about 98.75 which is below the max coil amps of 104. I would be interested on anyone input and especially hearing from anyone who has had experience with such an installation.
 
There is no commercially available single core three phase transformer for 240/120 3Ph 4w that will allow more than 5% single phase loading. This has to do with circulating currents in the delta windings.

I have found it is easier to use two different sized single phase transformers connected in an open delta arrangement.

If T = your total three phase load in kVA and S = the total single phase load in kVA then the small transformer kVA = .58*T and the larger center tapped unit kVA = (.58*T) + S.

Edit: I am not sure of the formula. I will post the correct one on Monday 11/20.
 
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Emergency Power For Delta Power System

Emergency Power For Delta Power System

Jim, I realize that sometimes the basic formulas do not always tell the entire story however using the basic relationship between Line-Current and Coil Current and allowing a very small three-phase load (fixed), appears to allow for more single-phase load on the single-phase coil. Is it the imbalance that somehow causes the problem? I know we could do the open-delta but we have a real space issue. I realize this is not the most efficient use of a transformer but it does solve or space problem.

Thanks for your comments,
Grant
 
My formulas (from a 1982 publication) were correct for sizing two single phase transformers to be connected in open delta with unity power factor:
T = total balanced 240V 3ph load and S = total balanced 120/240V 1ph load

Transformer A = .58T + S
Tranformer B = .58T

Without going into detail the preferred sizing of a single three phase transformer is 2.5S + T. So for your 9.3 kVA 3PH loading, your 75kVA transformer would seem to have room for 32kVA balanced single phase load. But, circulating currents in the closed delta are present whenever there is a voltage unbalance between phases of the delta. Among other factors, voltage differences can come from imbalanced loading and load power factor.

%i = (100*%E)/(3*%Z)
where %i = circulating current, %E = voltage imbalance, and %Z = transformer impedance

So with if your single phase load causes only a 2% voltage unbalance on a standard 3PH dry type transformer with 5.75 %Z your circulating current would be about 12% of the transformer full load.
 
Emergency Power For Delta System

Jim, Thanks for you comments. Now I see why I have never seen this done before. The calculations give a whole new meaning to balanced loading.
Thanks again.
Grant
 
Emergency Power for Deltal System

Emergency Power for Deltal System

Jim, I contacted Sola/Heavy-Duty Transformers with my application question and like you said there are limitations on the the center-tap capacity. On Sola's transformer they say to take the Transformer KVA and deduct the total three-phase KVA load. The remaining KVA is then divided by a factor of 6 and that is what is available for single-phase when divided on each of the single-phase coils. 75KVA-27.4 (3-motors)=47.6KVA/6=7.9KVA available for single-phase. 7.9/240=33 amps. I assume that the 1/6th of the KVA balance being the limitation on the center-tap is taking some of those factors you pointed out into account.

Thanks again for your comments
Grant
 
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