What is the equiv Z of 3 single phase XFMR connected in 3 phase

[majesus]

When connecting single phase XFMR to make a 3 phase system, the impedance of single phase XFMR should be the same to prevent imbalance. However in this scenario I have
XFMR1=5.6%Z
XFMR2=5.76%Z
XFMR1=5.8%Z

What would the equivalent be?
Do you average them out?

Also one other question that I have been wondering for a while: suppose you have two 3 phase XFMRs. Can you connect them in parallel to increase the power capacity. Ie if you have two 1000kVA D-Y 3phase XFMR at 5.5%Z. Can you connect them in parallel to make it an equivalent 2000kVA D-Y XFMR?

 

[stevenal]

There is no equivalent. Three phase Z is only useful for analysis of balanced systems. If each phase has a different impedance you no longer have a balanced system and symetrical components can be used for analysis. If you decide the balanced assumption will give you good enough results, use the average, highest, or lowest depending on what you are looking for.

 

[rbulsara]

majesus:

Short answer to both of your questions is yes.

For all practical purposes (neglecting imbalance), you can use average Z% of the 1ph units for a 3 phase bank. The %Z of a 3 phase machine is a single phase value as all 3 phase analysis is essentially done on per phase basis.

You can parallel the two trasnformers of equal or unequal impedances and/or sizes, just have to be careful of circulatiing currents and not overloading one of the units.

Although it is very helpful if the % impedances are closely matched to minimize the nuisance of circulating currents and proper load sharing.

 

[majesus]

Thanks guys for the info...