TO FIND %R & %X VALUE 2

[28021984]

Dear all,

I am doing the design of 220kV/66kV 100MVA substation.

short circuit MVA = 20000MVA

X/R Ratio = 34.1

short circuit current = 52.486kA

From the above i want to know how to calculate %Resistance(R) & %Reactance(X)

Kindly help me

Thanks & Regards,

LSK

 

[dpc]

Use the short circuit amps and the system voltage to calculate Z ohms, then put in per unit on whatever base you want. Then use the X/R ratio to compute %X and %R from %Z.



David Castor

http://www.cvoes.com

 

[28021984]

Thank you Mr.David Castor

I have some more doubt about the calculation, please find the below calculation:

System Voltage = 220kV

Short circuit Amps = 52.486kA

Z= (220*10^3)/(52.486*10^3)
= 4.19 ohms

Z base = (V base)^2/ MVA = 484 ohms

MVA base = 100 MVA

V base = 220 kV


Z pu = Z/(Z base) = 4.19/484 = 0.008660318

R pu = 0.008196852

by using ETAP Software I got an ans. of %R = 0.01466 for positive sequence.

From the above, please note the value of "R". i don't know why there is change in value of "R".

Kindly help me

Thanks & Regards,

LSK

 

[davidbeach]

How did you get your R pu?

 

[pwrtran]

LSK
Your Z should be 2.42ohm and Zpu=0.005, thus R%=0.01466

 

[28021984]

Dear sir

R pu = Z pu / (1+(X/R)^1/2
= 0.008660318 / (1+ .341)^1/2

where X/R = 34.1%


Also I am unable to know how did you calculate that z= 2.42 ohms

can u please forward the formula to calculate the value of "Z"

Thanks & Regards

LSK

 

[pwrtran]

You missed the root 3 that's it.

 

[davidbeach]

X/R=34.1, not 0.341. It's not a percentage, but a straight ratio.