Fault current question

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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Consulting Electrical Engineer - Photovoltaic Systems
I am helping a client gather information for a form they need to fill out for a commercial PV system, and a couple of the fields that need to be addressed have to do with the "UTILITY SHORT CIRCUIT CONTRIBUTION"; the fields are "Line to Line SC @ X/R =" and "Line to Ground SC @ X/R =". I assume that line to ground is available fault current at the transformer secondary, which I know how to calculate, but how do I get line to line SC? Also, I figure X is reactance and R is resistance, but I'm not sure what is meant by "@ X/R". Can someone demystify this for me?
 
ggunn said:
I am helping a client gather information for a form they need to fill out for a commercial PV system, and a couple of the fields that need to be addressed have to do with the "UTILITY SHORT CIRCUIT CONTRIBUTION"; the fields are "Line to Line SC @ X/R =" and "Line to Ground SC @ X/R =". I assume that line to ground is available fault current at the transformer secondary, which I know how to calculate, but how do I get line to line SC? Also, I figure X is reactance and R is resistance, but I'm not sure what is meant by "@ X/R". Can someone demystify this for me?
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Anyone?
 
As you probably know, X/R ratio is a spec sometimes given on transformers from which you can calculate voltage drop, fault current etc. I don't really see why they would need the XR ratio, just the available fault current. Did they specify at which point they want the available fault current?
 
electrofelon said:
As you probably know, X/R ratio is a spec sometimes given on transformers from which you can calculate voltage drop, fault current etc. I don't really see why they would need the XR ratio, just the available fault current. Did they specify at which point they want the available fault current?
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Is X/R the same as %Z?
 
You likely know how to calculate the 3 phase bolted fault current, which I would put into the L-L spot. That originates from the utility.
The X and R are reactance and resistance as you mentioned. X/R is just the ratio (X divided by R). Some calculation software will give you the combined X/R and not the separate X & R. This originates from the utility but is augmented by impedance (such as a transformer)
 
ggunn said:
Is X/R the same as %Z?
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No. I'll try and take a stab at it. X/R ratio allows a more accurate assessment of the resistive and reactive components of impedance since you consider the Angles and magnitudes of each component. %Z is simpler but doesn't allow as accurate and assessment. X/R data is hard to find, it is usually not even marked on the transformer data plate. Perhaps you could ask for clarification what they actually want and where?
 
ggunn said:
I am helping a client gather information for a form they need to fill out for a commercial PV system, and a couple of the fields that need to be addressed have to do with the "UTILITY SHORT CIRCUIT CONTRIBUTION"; the fields are "Line to Line SC @ X/R =" and "Line to Ground SC @ X/R =". I assume that line to ground is available fault current at the transformer secondary, which I know how to calculate, but how do I get line to line SC? Also, I figure X is reactance and R is resistance, but I'm not sure what is meant by "@ X/R". Can someone demystify this for me?
Click to expand...
This information is relatively easy to get when using software like SKM or Etap, it is usually not provided by the simpler free packages.

Of course, you need starting values from your utility.

The fault current you get from the FLA and %Z is the Line-Line, or L-L-L, value. The single phase, or L-G, value is usually not of concern for non-utility installations or equipment that is not directly coupled to a transformer secondary.
 
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