SKM Softare Expert???

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anbm

Senior Member
Any experts for SKM software here?

When I use utility block, under utility contribution window, if the power company give us the Three phase available fault current (in Amp or MVA), and the X/R value.

Are there any formulas to calculate Line to Ground fault current value (in MVA or Amp) and X/R?
 

skeshesh

Senior Member
Location
Los Angeles, Ca
I would not consider myself a SKM expert by any means but I have worked with the software on various projects. Having said that I don't really know what the question has to do with SKM.

As I understand it you're asking if it is possible to calculate the line-to-ground fault current while knowing only the three-phase fault current and a X/R ratio. The short answer is not with only that information.

You could however work out parts of the info: if you wanted to start working backwards from I(SC 3ph) = V(p-p of system) / sqrt(3) / Z(SC) you could find Z(SC) as I assume you know the p-p voltage. Still you would need the zero sequence impedance of your system to find the line-to-ground fault: I(SC L-G) = V(p-p of system) / sqrt(3) / ( Z(SC) + Z(0 seq) ).

That's my understanding of it at least, but stick around as there are many members with greater knowledge and experience than that of mine who should chime in any moment now.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
If they don't give the L-G value, I use the 3Ph value in both spots (3Ph and L-G).
This is usually safe at 480V, but be careful at MV levels. Last week we noticed a problem duplicating an existing report. The system was an ungrounded wye to ungrounded wye transformer. Setting the MV L-G equal to the 3PH value resulted in lower incident energy values.
 

anbm

Senior Member
I agreed. Talked to a supported person at SKM, he said for a quick calculation,
when knowing 3Ph fault in Amp.

L-G Fault = 3Ph Fault / 3, use same X/R value for both items.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
Not sure who you talked to, but that to me is not a good assumption. It is very possible to have the L-G fault current higher than the three phase current.

For a quick calc, simply make the L-G the same as the three phase. If you need more precise studies, then you need the L-G values.

This is not related only to SKM; ETAP and other programs require the same info.
 

mull982

Senior Member
Not sure who you talked to, but that to me is not a good assumption. It is very possible to have the L-G fault current higher than the three phase current.

For a quick calc, simply make the L-G the same as the three phase. If you need more precise studies, then you need the L-G values.

This is not related only to SKM; ETAP and other programs require the same info.

I agree with this. As I have learned especially close the the transformer secondary the L-G fault can be greater than a 3-phase fault.

What type of connection is the utility transformer. Remember if it is a Delta-Wye connection than this will not pass any zero sequence current from the Utility therefore there is no need to be concerned about or even enter a value for this in the Utility model. If it is a wye-wye connected transformer then you should enter a vaule.
 
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