Xr ratio infinite bus method

[hhsting]

When doing infinite bus method calculation engineer gave me percent x/r ratio instead of transformer impedance %z. He then did the following:

(Xfmr kva)/(kv voltage* x/r ratio*-.732)= infinite xfmr short circuit fault current at xfmr secondary



Can one use percent x/r ratio when doing infinite bus method? Is percent x/r same as %z?

 

[main197]

X/R ratio is different from Z. X = Sin θ * Z, R = Cos θ * Z. So, I don't think he is correct.

 

[David Castor]

No, this should be the transformer nameplate impedance, not the X/R ratio. The %Z will be on the nameplate, the X/R ratio will not be. Something got lost in translation, I think.

 

[LarryFine]

(Xfmr kva)/(kv voltage* x/r ratio*-.732)= infinite xfmr short circuit fault current at xfmr secondary

I've done short-circuit calculations, too. I don't understand how the bold part can occur, or be used.

 

[electrofelon]

I've done short-circuit calculations, too. I don't understand how the bold part can occur, or be used.

I think/assume that is just sloppy wording and he forgot the word "primary" after "infinite"?

 

[hhsting]

I've done short-circuit calculations, too. I don't understand how the bold part can occur, or be used.

Well thats the fault current at the transformer secondary since the voltage kV in the equation is secondary voltage.

So lets say I have 25kVA 13.2kV to 208/120V utility xfmr with %z of 2%. So then infinite bus method at xfmr secondary fault currents would be
25/(.208*1.732*0.02)=3.469kA

 

[LarryFine]

So he wasn't claiming to have an infinite secondary fault current?

 

[hhsting]

So he wasn't claiming to have an infinite secondary fault current?

Engineer is claiming to use infinite bus method to arrive at the secondary side fault current but using percent x/r ratio not %z impedance

No Engineer does not claim fault current is infinite at secondary of xfmr

 

[hhsting]

I think/assume that is just sloppy wording and he forgot the word "primary" after "infinite"?

Correct! Primary side has infinite current and forgot to write it not secondary side.

Typing on iphone is tough! Small letter pressed together


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