Overall Bank Rating for Three Single Phase Transformers wired Delta-Wye 2

[BlackJackJacques]

We have three 37.5 kVA 480/120 single-phase transformers wired Delta-Wye to derive 208/120 at the secondary. At 37.5 kVA, the single-phase Voltage and Current at the primary is 480V x 78.125A. Configured in Delta, the Primary Line current is 1.732 x 78.125A = 135.3A - indicating a new kVA rating of 112.5 kVA (480V x 1.732 x 135.3A) or three times the kVA rating for one transformer. Is this reasoning correct....that the system VA rating for Delta Wye transformers 3-phase are always three times the rating of single phase transformer?

Thanks

 

[waross]

For similar transformers, yes, three times.
Easier to visualize, consider the secondary;
Three times 120 Volt circuitsant three times the KVA.

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Ohm's law
Not just a good idea;
It's the LAW!

 

[BlackJackJacques]

Gotcha - understood - Thanks!!

 

[che12345]

I have the following opinion, for you attention.
1. You are connecting three pieces of single-phase transformers, each 480/120V to form a three-phase Dyn 480V/208V. It is possible and your understanding is in order.
2. What I wish to point out is to observe the phase notation (i.e. the phase relationship between the primary and the secondary). You are connecting three primaries into a delta formation. Three secondaries into a Star formation. Be aware of the phase relationship. The primary phase rotation say A,B,C. Check the secondar phase relationship. Establish the secondary say a,b,c with synscope. Attention: check the secondary voltages, any Line-to-Line=208V and any Line-to-Neutral= 120V. A DMM reads the voltage but does NOT identify the phase sequence.
Che Kuan Yau (Singapore)

 

[BlackJackJacques]

Thanks Che - understood and concur!