X/R Calculation Confusion
X/R Calculation Confusion
(OP)
Hi all. I am doing PER-UNIT calculation using P.U. (R + jX) as per ANSI standards. In calculating the X/R ratio at the fault point, I used equivalent (P.U. jX)/ (P.U. R).
However, I read a book that states that calculating X/R ratio using complex form (P.U. jX)/ (P.U. R) is not conservative and not recommended for ANSI calculations. It stated that an Rseparate/Xseparate should be used.
I am bit confused as to what and how to calculate separate R and separate X. Are they different from (P.U. jX)/ (P.U. R)?
regards
However, I read a book that states that calculating X/R ratio using complex form (P.U. jX)/ (P.U. R) is not conservative and not recommended for ANSI calculations. It stated that an Rseparate/Xseparate should be used.
I am bit confused as to what and how to calculate separate R and separate X. Are they different from (P.U. jX)/ (P.U. R)?
regards






RE: X/R Calculation Confusion
RE: X/R Calculation Confusion
RE: X/R Calculation Confusion
RE: X/R Calculation Confusion
RE: X/R Calculation Confusion
The X/R at the fault point will be (equivalentP.U.R)/(equivalent P.U.X) Will this approach going to be adequate in determining the X/R at the fault point?
David, would you agree?
RE: X/R Calculation Confusion
R and X in Omic values may be combined as resistances in parallel.
To combine R and X in PU values you must also consider the base MVA that each value is based on. Something like an average weighted by the inverse MVA bases.
Bill
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"Why not the best?"
Jimmy Carter
RE: X/R Calculation Confusion
What do you mean by "To combine R and X in PU values you must also consider the base MVA that each value is based on. Something like an average weighted by the inverse MVA bases".
I understand that each P.U. R and X are based on a common and equipment MVA base, however, most of them actual impedances are also in P.U. of their own MVA rating. Converting each one of them to ohms will defeat the purpose of using per-unit for simplicity. Can you give me a simple sample?
RE: X/R Calculation Confusion
MVA_b1 = MVA Base 1
Z_1 = impedance of some actual Z at Base 1
MVA_b2 = MVA Base 2
Z_2 = impedance of the same actual Z at Base 2
Z_2 = Z_1 * ( MVA_b2 / MVA_b1 )
It isn't necessary to convert to ohms and back to a new base. Although if you do so and cancel out terms, the above equation is what you'll get.
RE: X/R Calculation Confusion
P.U. Z2 = Z1 * (MVAbase/MVAequip)*(kVequip/kVbase)^2 P.U.
For cables, since the actual R and X values in ohms are calculated:
P.U. Z2 = Z1*(MVAbase)/(kVbase)^2 P.U.
From here I calculate the P.U. Rs and P.U. jXs separately and then get the equivalent X/R at the fault point using these values.
I always came into the belief that this the way to go, till I read some article saying that this is not conservative enough. My question would be, what is then the right way to go to be conservative enough using the same P.U. values I calculated?
Your assistance is greatly appreciated.
RE: X/R Calculation Confusion
You would then calculate the thevenin equivalent circuit for both the R and X networks at the fault location. These values are now your R_separate and X_separate.
It should be pretty clear that the X/R values you would get from simplifying separate R and X (scalar) networks would be different to the values you'd get from the network reduction of complex phasors,
i.e. reducing two parallel complex impedances, say Z1 || Z2 = Z1 * Z2 / (Z1 + Z2), involves complex division, which in turn involves multiplying complex conjugates. Contrast this to network simplification with scalar quantities, which is just straightforward arithmetic. You should probably prove it for yourself with a simple example :)
RE: X/R Calculation Confusion
I'd refer to Conrad St. Pierre's book on Short Circuit calculations for an explanation of the logic behind the separate R and X reduction.
David Castor
www.cvoes.com
RE: X/R Calculation Confusion
I did a power system analysis project recently and came across this acticle with made power calculations very quick and simple.
It may not be directly linked to the above question but may be of interest.
Danny
http://s.p
RE: X/R Calculation Confusion
www.powers
Shalhoob