Available Short Circuit Calculations

[AlWired]

When calculating the available short circuit at the secondary of the utility transformer, what is a good number to use for transformer impedance if this value is unknown? In sample calculations, the smallest value of transformer impedance I've seen is 2%. Is that a safe number to use?

Thanks in advance.

 

[MJJBEE]

The standard at the utility I work for is 1.7% on small distribution transformers like the ones you see on houses ect. on large ones is can be as much as 10% impedance so I would say anywhere between the 2 is a possible impedance for your transformer depending on size voltage ratings ect. 2% is a good nominal numbers for a small transformer but not safe for all applications.

 

[jim dungar]

When calculating the available short circuit at the secondary of the utility transformer, what is a good number to use for transformer impedance if this value is unknown? In sample calculations, the smallest value of transformer impedance I've seen is 2%. Is that a safe number to use?

Thanks in advance.

There is no single number. Each range of kVA and each construction have their own %Z.

 

[mivey]

impedance examples

impedance examples

There are many sources for this info. Here are some examples of the sources that you can get for yourself:

How about this from the Westinghouse T&D manual:

or this from GE's Short-Circuit Calculations booklet:

or this from Bussman's Short-circuit Current Calculations:

 

[mivey]

When calculating the available short circuit at the secondary of the utility transformer

There are also tables available that list the available short-circuit amps but I can't put my hands on one right this minute.

I like getting the %Z directly off of the transformer. If it is not know, I have also used a sample set of transformers from the utility to come up with a the lowest Z that sounds reasonable for the given size and voltage. You can get these off of the nameplate of transformers in the field or go to the transformer yard and get some samples.

Make sure any "rule-of-thumbs" take into consideration the new low-loss transformers as they will have lower numbers.

For an AIC calc, I usually use an infinite buss for anything above the utility transformer and that gives you some added safety.

 

[zog]

I like getting the %Z directly off of the transformer.

What a concept! This always drives me nuts for existing systems, actually looking at the transformer us about the last thing anyone thinks of.

 

[mayanees]

The ultimate accuracy with Z Numbers...

The ultimate accuracy with Z Numbers...

I've done several small Power Studies for an Air Force Base, and their requirement was that the actual factory test numbers for the given transformer be used in the analysis.
Equipped with a transformer s/n, I would call Cooper (for example) and give them the serial number of the transformer. They would connect me with the group that tested the transformer. From there, I was provided with the measured impedance and the measured X/R ratio for that specific transformer.
If the unit was not available when the study was completed, I would have to go back and redo the numbers based on the actual data after the unit was received.
It was silly because the transformers were built to a given standard +/- a small percentage.
Eventually, if the transformer data wasn't available, I would complete the study with 2 sets of results, one at the mimimum impedance and the other at the maximum impedance - which included Short-Circuit and Load FLow Analyses.
Typical size was on the order of 75 to 300 kVA.

JM

edited for sp

 

[mivey]

I was provided with the measured impedance and the measured X/R ratio for that specific transformer...

...Typical size was on the order of 75 to 300 kVA.

That seems like a lot of work for these size transformers. Seems like some assumptions would provide info that was "close enough for government work". This expression usually does not fit because the gov'ment can cadillac the stew out of something and spend more money than is really needed. Of course, the gov'ment has deep pockets...OURS! :grin:

A nameplate has what is needed most of the time. I have only used the gory manufacturer details on substation transformers. One use was when checking to see if the transformer met the bid specs and if there were any $ adjuments to be made. With arc fault analysis, coordination studies, loss analysis, etc. sometimes you need the "good stuff".

 

[mivey]

What a concept! This always drives me nuts for existing systems, actually looking at the transformer us about the last thing anyone thinks of.

On a pole-mount, you can usually read the nameplate with a spotting scope or just get on up there. Look on the mounting bracket. For padmounts, of course, the nameplate is locked inside.

[edit: spelling]