Single phase loading of a three transformer 1

[JBD]

There is a discussion, going on in a different forum (so far it has gone on for a total of 15 pages) concerning the loading of a wye connected transformer.

The only important fact is the transformer in question would have a 208Y/120 connected secondary. With a single 208V 10A resistive load connected to terminals X1 and X2 of the transformer (X3 is not connected).

What the arguement seems to be about is that "the apparent power in the load" is different than "the apparent power delivered by the transformers".

Quote (Rattus)...
In a 120/208V wye, with a single 208V, 10A load, what is the apparent power delivered by the two transformers supplying that load?

Most would say 2.08KVA.

However, if one considers the transformers individually, one sees that each transformer delivers 120Vx10A = 1.2KVA for a total loading of 2.4KVA.

...quote

Is this a trick with numbers or a valid method for analysing transformers?

 

[stevenal]

Assuming a lossless transformer, of course.

 

[JBD]

jghrist

Do your numbers change if it is a 10A 208V load without a neutral (they shouldn't)?

And based on this, you are proving the transformer (as a unit) is actually seeing a 2080W load rather than an apparent load of 2400W.

 

[bjenks]

If it truely is two transformers tied together at each grounded phase of 120V then the neutral would not carry any current. However since the two transformers are 120 degrees out of phase then the neutral is not cancelled out and is carring the delta difference of current. This is another way of explaining the lower kVA for the system versus phase.

 

[jghrist]

If one end of the secondary windings were not connected together, then there wouldn't be any current when you put a resistor across the other ends. There isn't any net neutral current (all current flowing out of one winding flows into the other), so it doesn't matter if there is an external neutral connection or if the connection is grounded.

Yes, this proves that the transformer as a unit sees a load of 2080W instead of an apparent 2400VA. As stevenal noted, both the real and reactive power are conserved quantities.