Impedance ratio

Ainsley Whyte

Senior Member
Location
Jamaica
Occupation
Senior Electrical Engineer
Can someone explain impedance ratio X/R, Really i am working on replacement of a sub- station and was ask for the X/R Ratio to check short circuit current.The truth is the substation is very old from the 60 s therefore information is very difficult to get regarding to this switch gear and i need to determined the current MVA ratings with the plan to increase MVA ratings for more loads.
 
Last edited:

Russs57

Senior Member
Well, you know what X/R means.....so I'm guessing what you really want is some firm numbers to work from when factual information isn't available.

Probably about the best you can do is reference a standard like : IEEE Std. 242 – IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems.

A web page like this might help a little. It does suck to have engineering come down to "educated guesses". Just err on the side of caution.


This one has some historical info.



Doubt I helped any but good luck.
 

Ainsley Whyte

Senior Member
Location
Jamaica
Occupation
Senior Electrical Engineer
Now that I have my short circuit current and my impedance ratio, how can i calculate my MVA size for a switch gear ? of course I have the
the single line diagram.
 
Last edited by a moderator:

paulengr

Senior Member
Now that I have my short circuit current and my impedance ratio, how can i calculate my MVA size for a switch gear ? of course I have the
the single line diagram.
Wrong direction.

Impedance of the transformer is based on the size, system voltage, and %Z. Impedance is a result, not an input. Given voltage and assuming ANSI standard %Z we can calculate MVA backwards but that’s not how it’s done.

MVA is determined by a load study. You are looking at a short circuit study. Typically first we determine the required load. In an upgrade situation it is more common to measure load then estimate the new load. Short circuit studies play a role in determining AIC, as inputs to coordination studies, to stability studies, and as inputs to arc flash studies. Theoretically we can get to AIC then go back and modify %Z on the transformer spec (alter the core and copper loss ratios) but this is rarely actually done. Usually we just use ANSI standard %Z as a consequence, not a variable.
 
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