Calculating SUbstation X/R Ratio

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Wrascally

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Hi All,

I'm looking for a little guidance here.

I am working on a coordination study for an industrial facility. The utility has given me positive and zero sequence impedance values but did not have the X/R ratios of the substation handy. The impedance values are in the form of "z = .5 + j3.93 ohms"

Could anyone offer me guidance in arriving at the X/R ratio from these values - I have not been successful in finding a formula.

Thank You!
 
If they gave the overall impedance in that form, then it would be as easy as that. But what they gave is the positive and zero sequence impedances. The negative will be the same values as the positive, but opposite in direction. What is needed is the matrix manipulation that adds the three to give a total. I don't recall how that is set up. If i get a chance to read through the book on my desk, the one used in the course in which I learned symmetrical components, I might give it a shot. In the mean time, perhaps someone who remembers the process might chime in with the method needed here.
 
Short Circuit Calculations

Short Circuit Calculations

If X/R ratio is not available assime 12. Also the IEEE Buff Book and Red Book have some good assumptions to use based on the type and size of transformer. Caution! It is ok to use the assumed X/R but never assume the %Z!

Dan Craven
 
Thank you both for your input.

Charlie, they provided both a positive and zero sequence value in the form of
"Z= .xx + jx.xx ohms" I am completely unfamilar with showing a value in this way. I'm assuming this is a utility standard that I have not run across to date. Regardless, it would be great to understand what is actually being represented.

Ron, you are correct in your instrucitons to use simple division. I was able to get the info. needed for this project. Now i'd like to learn something from this so I can understand it the next time I run across it.

Thanks again.
 
I agree with Charlie that Ron's approach won't work until you combine the three sequence impedances into a total "Z". Then you can do the X/R calculation.

Utilities provide impedances in the sequence impedance form where you get positive and zero values in a R+jX for each. As Charlie says, negative sequence impedance is usually equal but opposite the positive value. No one of them represent the actual impedance of your system, they are portions of a model. The X/R ratios of the sequence components won't necessarily be even close to the system X/R.

You can find lots of information by googling symmetrical components.

Jim T
 
The impedance in the form of Z = R +jX for positive and zero sequence provided by the utility, gives you the X/R value used for three phase faults (positive) and ground faults (zero). Therefore, there are two X/R ratios, and you already have the information you need.

Computer programs used for determining fault currents at various points in the system use the positive and zero sequence information in the calculations. Large values of negative impedance can cause calculations to not converge, therefore negative impedances should be avoided.

When we assume a 5.75% impedance on a transformer, that is the positive sequence impedance. This is the same for many other pieces of equipment. In the case that the zero sequence is unknown, the positive sequence is used as a substitute, because in general, the 3 phase fault current will be higher then the single line to ground, although there are situations where that is not the case, albeit quite infrequent.

Coordination studies are complex evaluations that require knowledge of power system analysis and protective relaying philiosophy, much of which is gained by experience. Studies of this nature should not be attempted without oversight by a trained professional.
 
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Once again, thanks to all for your input.

kingpb said:
Coordination studies are complex evaluations that require knowledge of power system analysis and protective relaying philiosophy, much of which is gained by experience.
Click to expand...

Kingpb - I'm working on the experience side of things. I figgure any day where I can go home and say I learned something new today is a good day.
 
Give me the exact numbers for positive and zero sequence impedances. I have access to a computer analysis program that can take that information and give me back the same configuration in terms of %Z and X/R. I'll give it a try, and see what I come up with, just as a learning experience.
 
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