Utility Short Circuit Information

[cuhead]

How do you obtain accurate maximum and minimum shout circuit values from the utility companies? A lot of the smaller utilities in Colorado laugh with I ask. What is the best reference for oil filled transformer impedances?

 

[dpc]

What size transformers?

If you're on the secondary side of a transformer (480 V or lower), there will be very little difference between minimum and maximum. It will be basically a function of the transformer impedance.

The value that should be used will also depend on what you are doing with the data. For equipment ratings, I would assume an infinite bus on the primary and use the transformer impedance.

 

[Marmite]

Where I live there is a legal requirement for the utility to provide this information. You need it to ensure your installation is designed properly and to work out downstream protection settings.

Regards
Marmite

 

[tem1234]

Like Marmite said, utility have to give you their short circuit value, at least where i live.

For the transformer impedance, the best is to have it on the nameplate. There's a table with standard impedance in the IEEE buff book (1986, i think 1993 don't have the table anymore), but it can vary a lot. Transformer will have a great influence on the short circuit value, so i think it's important to have the accurate impedance.

Little question, how do you determine the minimum short circuit? The minimum will come from a 2 phase short circuit, but do you put a short circuit impedance too, like 10 ohms?

thanks

 

[magoo2]

One consideration is what replacement transformer would be used. At our utility, the designers sometimes look at what the present transformer kVA size and consider what happens if the next size happens to be used in an emergency changeout situation. Obviously, this results in higher fault current.

More typically though, in the situation where the existing kVA size can be found, there are cases where the available designs have a significant difference in impedance. The numbers we give out are always based on the lower impedance situation.

 

[AusLee]

Design for the worst case. Take 500MVA at the primary side of the network.

All transfoerm manufacturers publish the %sc.

 

[julesm]

Sometimes it's a matter of finding the correct person to ask at a small or even large utiltiy...
Someone knows! For "maximum" you can always use the infinite bus as a conservative back-up.
"Minimum" is trickier. I use it for arc flash calcs. I have had a more difficult time obtaining this value.
Good Luck.

 

[ELEP]

The Utility short circuit level goes on changing every year due to lots of extension work, enhancement of transformer capacity or increase in generation capacity.
These values are constantly monitored by System studies groups in every Utility. Try to contact appropriate person.

 

[thanglam]

If you working in a utility, your protection department should have this data.

 

[burnt2x]

cuhead,

Sorry experience you had!

The persons you have asked could be the wrong people! Suggest to take Auslee's advice, take a large short-circuit MVA (shall I say "infinite", meaning the utility is capable of delivering a large short-circuit contribution to your system during faults on your system).

glad to be of help.

 

[cuhead]

Thanks to all, you’ve been a great help. I will be more aggressive next time.

Again, Thank you.

cuhead

 

[ppaya]

Cuhead:
min SC contribution could be significantly lower then max due to system configuration change in the upstream transformer stations.
Utility will provide you with "bolted" fault values, no resistance in the fault, and X/R ratio.

 

[fjkhiqv]

please what are the otherwise considerations except the primary rating, and the equation?