Overall impedance of a bank of three single phase oil filled transformers ?

[bdn2004]

There are three single phase oil filled transformers in a switchyard, each rated 833kVA, 12.47kV and 6.22%, 6.28%, 6.25% impedances individually.
They are connected Y - Y. When showing this as three phase transformer to analyze for short circuits what impedance is used?

 

[HamburgerHelper]

What makes you think anyone cares about a few hundredths of a PU?

------------------------------------------------------------------------------------------
If you can't explain it to a six year old, you don't understand it yourself.

 

[davidbeach]

About 6.25% on 2.5 MVA.

 

[bdn2004]

Is not that I cared about the few hundredths of a %. I just wanted to make sure you don't have to put them all three in parallel or something to reduce to a single impedance value.

 

[jghrist]

Impedances are per phase, if your analysis program does not allow entering individual impedances, then enter the lowest of the three if you want conservatively high values of fault current.

 

[davidbeach]

And the highest of the three if you want conservative load flow results. Average is generally good enough. The important part is getting the base MVA right.

 

[wbd]

also the highest of the three for arc flash but typically I use an average of the 3 as they are usually very close in impedance so small difference (6.28 to 6.25) does not make a difference

 

[davidbeach]

For arc flash it would depend. If you have unit protection, differential or the like, where time to trip isn't dependent on fault current then the lowest impedance would give the worst case arc flash hazard.

 

[HamburgerHelper]

I don't think anyone can say which impedance is worst case for arc flash without doing the study. If the fault current is low, more energy may be release due to the fault persisting longer than if you had a high current fault that cleared quickly.

------------------------------------------------------------------------------------------
If you can't explain it to a six year old, you don't understand it yourself.

 

[wbd]

True but I think the impedance differences are so small it really doesn't matter in the real world. Is there a difference between 3.7 cal/cm2 or 4 cal/cm2? 12.2 cal/cm2 vs 15 cal/cm2? The PPE will be the same in those ranges.

 

[HamburgerHelper]

I was stating that as transformer impedance in general. No one should be splitting hairs on what arc flash gear should be worn because no one is informed of small changes in available fault current from the utility.

------------------------------------------------------------------------------------------
If you can't explain it to a six year old, you don't understand it yourself.