Available fault current

[stevenal]

So it happened again today when I gave an engineer an estimate of fault current at the terminals of a 120/240 V single phase transformer. He was agast. Dealt with lots of other utilities, and never heard of a 75kVA that could deliver so much current. First he had a hard time believing that half winding line to ground faults deliver about 1.5 X the full winding line to line value. He's going to check with some manufacturers on this point.

I assume infinite bus on the primary. I use a low limit impedance from our specs, not the nameplate, since transformer may be replaced at some point. I lower that value by a factor of 7.5% based on ANSI/IEEE impedance tolerances. Voltage is increased 5% also based on standards.

What's going on? Am I overly conservative? Why aren't others giving similar figures? On what basis are they ignoring the line to ground (most common type of fault)multiplier. What do you do?

 

[stevenal]

"it would have been well documented somewhere in so called standard tables that are available for all kinds of transformers and SCC they can provide."

It is in some, as DPC pointed out, and ignored in others. And all the tables are too old. The line to ground value drops quickly with cable length, and at some point the line to line will exceed it. Maybe they all assume cable length is sufficient.

"Utiltiy companies would have made big deal about it"

Seems my colleagues are ignorant of the effect.

"For one thing, if you look at the equivalent circuit of a transformer, (See Std. Elect. Engineer's Hand Book) the reactance of the transformer in ohms is a fixed value X, a result of mutual and leakage flux. The reactance of a tranformer is NOT Xs+Xp , adjusted to turn ratios. The nameplate %Z is a test result value and not a straight inductance measurement. Only conversion to turn ratio apply to Rs and Rp and load impedance Z which is zero for our discussion."

Any impedance whether real or reactive must be adjusted by the square of the turns ratio in order to find its equivalent value when viewed from the other side of the transformer. I don't have the standard handbook handy, but be aware of any equivalent circuit. They are all approximations to some degree or other. The equivalent circuit that lumps all the impedance on one side or the other is not applicable here. The one to use has an ideal transformer in the center, which indicates where to apply the N^2 adjustment.

 

[stevenal]

There's a thread started by someone trying to get a handle on this stuff at

http://groups.google.com/groups?hl=en&lr=lang_en&ie=UTF...

The fact that his name can be truncated to form my moniker is pure coincidence.

 

[rbulsara]

http://esp.apsc.com/resource/metering/esrm/800.pdf

This is a table pulished by Arizona power service, upto date for 2003.

Includes infinite bus and 20 feet of cable (a very reasonable assumption), also seem know that L-N and L-L could be different for single phase xfmrs. Yet, for 100kvA,240/120V max. Isc (equipment rating ) is 22,000A.

I cant see 37,000A for a 75 kvA tx even with 20 feet of cable rated for 80% of service size. Of course exact cable size or impedance is not given, but still indicative of the fact that no need to exaggrate our estimates.

 

[Borti]

IEEE Std 242-1986 gives typical impedance data for single phase transformers and impedance multipliers for line to neutral fault in Table 14.

 

[stevenal]

Thanks. I consulted the standard when I built my spreadsheet. Looks like they dropped the table for 2001. Note the impedance range for a 75kVA extends to 1.2%. Looks like I'm not conservative enough.

 

[jghrist]

Stevenal,

I think that if you use the minimum allowable impedance according your specs, you are conservative enough. IEEE Std 242 (thanks Borti) doesn't have a plain 1.5 fault current multiplier, however. There is a multiplier for the transformer resistance (0.75) and a separate one for reactance (0.6 for 75 kVA). The effective multiplier for fault current depends on the X/R ratio of the transformer.

If X/R is 1.8 as suggested in IEEE-242, then the fault current multiplier would be 1.566 not 1.5. Any X/R > 1.182 would give a fault current multiplier > 1.5.

As you say, the line-to-ground fault current drops off considerably with service cable length. Including some minimum cable length in the calculation would be reasonable.

I'm still curious about the effect of paralleled windings. Surely, the available fault would be higher for paralleled windings than for a single winding.

 

[alehman]

Leakage reactance of a winding depends not only on the square of the number of turns, but also is proportional to the diameter and inversely to the length of the winding.

x1=1.6*pi³*f*N1²*r/L*((1-p)*d3+d1/3)
x2=1.6*pi³*f*N2²*r/L*(p*d3+d2/3)

xeq(ref to winding 2) = x1*(N2/N1)²+x2

N= # of turns
r= mean radius of winding
L= length of winding
p= fraction of flux linking inner winding only
d3= space between windings
d2= depth of inner winding
d1= depth of outer winding

Without knowning the configuration of the secondary winding, it would be impossible to calculate the equivelant leakage reactance while utilizing half of the winding. Short of measurement, manfuacturers pubished data is the only way to go.

 

[stevenal]

Jghrist,

"If X/R is 1.8 as suggested in IEEE-242, then the fault current multiplier would be 1.566 not 1.5."

Exactly how I do it in my spreadsheet with various multipliers based on the X/R value of each size transformer. I said "about 1.5X" above. Still bothers me I'm using old data for new transformers. The IEEE committee dropped this whole section for the updated standard, so maybe they were unsure of its applicability also.

"I'm still curious about the effect of paralleled windings. Surely, the available fault would be higher for paralleled windings than for a single winding."


Going by the Westinghouse diagram, if I parallel and short the secondary windings, I get an impedance of ZHX13 looking into the high side. This is simply the percent Z refered to the high side. Same high side current, twice the low side current as for the full winding.

Trouble is, if I try the same thing with windings in series, I don't get the same impedance. I thought this was how ZH13 was defined, but a ZH12 term remains. Still missing something here, or else the Westinghouse diagram is wrong.