Fault Current Calculation

[GiSH]

Ok, So I have
X/R Ratio
Peak fault Current
RMS fault Current
of a 33kV nominal network
How do I calculate the specific X and R values from this for a netwrok infeed, ETR 120's just confusing me...
And how do I calculate the equivalent network infeed
I think it would be
S = (V*V) / (SQRT((X*X)+(R*R)))

 

[Cooda76]

Hi,
From what I have understood,
you have a RMS fault current, take its reciprocal multiply by the base MVA and you got it.
cheers !

 

[rbulsara]

Is this a homework? Sounds like one...If so should not be supported here.

 

[GiSH]

No its not homework, I'm trying verify the results I get from a new piece of software but my basic knowledge has gone out of the window...

 

[rbulsara]

I dont think you can calculate X/R ratio with the information you have. You will have to know phase angle of the SCC, if you want to know. Or you will have to know X and R values of all the componets from sources up to the fault location. In many case the utiltiy co has provided me with Thevenin's equivalent of their network which will give you X and R values.

In most cases, the sofware asks for or assumes a default value of the X/R ratio which for a 33kV system could be 8, 10, 12 or 15 at the max. Also it depends on the purpose of you analysis also. If this is for sizing ratings of 33kV or lower equipment, above assumption would suffice.

 

[rbulsara]

By the way refer to IEEE Buff and/or the Red book for refreshing the basics.

 

[GiSH]

>>>rbulsara>>>
Cheers for that, and I'll check out red book too

Would this be acurate enough for getting X and R from the ratio?

X = Z sin ([tan-1](X/R))
R = Z cos ([tan-1](X/R))

And then sizing a slack bus according to the RMS fault contribuiton

 

[rbulsara]

The formulas are correct, resultant X and R will be as accurate as the assumption of the ratio. How will you get the Z however?

 

[GiSH]

ah.. you've got me there....
I've seen this used before
S being the rating of the Slack Bus

S=(V*V)/Z

Now if the slack is given the stated RMS fault contribution from the network we have S and V so Z is easy to get... however, how accurate do you think this would be?

 

[rbulsara]

Now I am more confused as to what you are trying to do.

Plus I do not know what a Slack bus is. May be you can explain little more or someone else can chime in..

 

[1x1x1x]

To calculate the X/R ratio from the peak and Rms current check the IEEE 242 -2001 chapter 2.4 (eq. 2-3)

 

[rbulsara]

Re-reading your post..you already have x/r ratio.

Then what Cooda76 said is correct method to find Z.

Then use the formulas you indicated for R and X in your second last post. I think you have it there..