sizing 4 wire delta transformer 1

[mcteach]

Doesthe winding of a 4 wire delta transformer that supplies the 120 loads on a 4 wire delta come oversized to account for the 3 phase and single phase line to neutral loads? Put another way, if I order a 30 KVA transformer, does each phase only handle 10 KVA, or is one winding (A to C phase) oversized?
Thank you.

 

[davidbeach]

Maybe, maybe not. There will generally be a note in the catalog that tells you how much single phase load the transformer can handle.

 

[waross]

It gets a little complicated due to differing phase angles.
1> Some three phase transformers do not tolerate unbalanced loading well. Beware of unbalanced loading on three legged cores.
2> Consider a three transformer bank of three 10 KVA transformers with the single phase load on A phase.
B phase and C phase will may be considered as forming an open delta with a virtual transformer across the open side. This virtual transformer will have similar characteristics to the A phase transformer.
Now for the sake of single phase loading, the bank may be thought of as one real transformer on A phase in parallel with the virtual transformer formed by B phase and C phase.
The real transformer and the virtual transformer will each supply 10 KVA.
The total single phase loading on one phase of a delta transformer bank is thus 2/3 of the total rating of the transformer bank or 20 KVA in our example.
But, the load currents in B and C phase will be 60 degrees out of phase with their respective voltages. One will be leading and one will be lagging.
Three phase loads will add directly to single phase loads on A phase but will be 60 degrees out of phase with the single phase currents on B and C phases. This is at unity power factor. At less than unity power factor it gets more complicated.
This describes the loading of three single phase transformers in delta configuration. Some three phase transformers will have similar loading.
Be aware that there may be switching transients or severe circulating currents in a Wye/Delta transformer., depending on the connection of the primary wye point.
Consider an open delta connection if you need a three wire delta service.
Three phase diesel generators are commonly reconnected to feed single phase loads. the connection that I see the most often on newer sets is either the delta, or the double delta connection. This is very similar to single phase loading on a three phase transformer bank.
The total single phase load on one phase is 2/3 the three phase KVA rating.

Bill
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