Transformer Short Circuit - Primary Side

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Defenestrator

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Denver, CO
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Electrical Engineer
When calculating the 3phase fault short circuit of a transformer one would use the formula I = secondary full load amps/percent impedance.


In order to calculate the fault current on the primary side of a transformer would this equation still hold up? Using the full load amps of the primary side of course.
 
I

Ingenieur

Senior Member
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you sound like you post test questions but never respond when answered

are you an EE? you list 'designer'
you say you are studying for your PE
did you take the FE?
a little background info would help us understand your needs better
you are asking basic questions that one would know with a degree and the experience required to take the PE

are a xfmrs prim and sec i related?
and how?

what does the pu Z represent?
how do you convert it to an Ohmic value?
WHY does i rated/pu Z give i fault? i/Z is usually voltage
these are basics that will help you

imho it is better to teach yourself than just ask for an answer
 
D

Defenestrator

Member
Location
Denver, CO
Occupation
Electrical Engineer
I have my FE. Im not posting a reply because reading the original poster respond to everyone's messages is annoying to me. Thanks for your help though.
 
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Ingenieur

Senior Member
Location
Earth
TheSmoothestCriminal said:
I have my FE. Im not posting a reply because reading the original poster respond to everyone's messages is annoying to me. Thanks for your help though.
Click to expand...

feedback is appreciated
it answered your question?
need clarification?
thanks

but to just post question for such elementary stuff and not close the loop seems lazy


are you an EE?
or testing under the experience exception?

i sec = i prim x n
n = v prim / v sec
this applies for any i magnitude within the sope of this discussion

do you know what the pu z is and how it is derived?
do you know this math? R + jX = Z or Z/a deg ?
an engineer understands the equations and their derivation that he is plugging numbers into

if taking the PE power you should takeva few courses in ctks and power systems
unlikely to pass without them
 
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I

Ingenieur

Senior Member
Location
Earth
it would help if you are asking about a sample test question to post it

it's been awhile for me but this similar to what you will see

find ph-ph fauilt i at xfmr sec
follow up, what is xfmr prim and sec v during a fault
fault is at the xfmr sec lugs

system v 13200
system fault S 12 mva
xfmr: 1 mva, 13200:480, 6% pu z

hint
find sys z
find xfmr z
 
mivey

mivey

Senior Member
TheSmoothestCriminal said:
When calculating the 3phase fault short circuit of a transformer one would use the formula I = secondary full load amps/percent impedance.


In order to calculate the fault current on the primary side of a transformer would this equation still hold up? Using the full load amps of the primary side of course.
Click to expand...
Yes.
 
C

Carultch

Senior Member
Location
Massachusetts
Here is a walkthrough of how to calculate short circuit currents across a transformer:
http://www.cooperindustries.com/con...rary/BUS_Ele_Tech_Lib_Electrical_Formulas.pdf

The formula ultimately is:
Isc_secondary = V_primary/V_secondary * M*Isc_primary

Where M is the multiplier, defined as M=1/(1+f). This is a placeholder to "blackbox" the above formula.
And f, also a placeholder variable, is a factor, which is defined as f = Isc_primary*V_primary*sqrt(3)*%Z/100000/KVA

If you combine these formulas together, you will notice a trend that it takes off to a cruising altitude as Isc_primary*V_primary approaches infinity.
 
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electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
TheSmoothestCriminal said:
When calculating the 3phase fault short circuit of a transformer one would use the formula I = secondary full load amps/percent impedance.


In order to calculate the fault current on the primary side of a transformer would this equation still hold up? Using the full load amps of the primary side of course.
Click to expand...

I am confused by the question - maybe I am misunderstanding? The transformer doesn't have any effect on a fault on the primary side (unless possibly you are talking motor contribution back feeding through the transformer).
 
C

Carultch

Senior Member
Location
Massachusetts
To continue my thought, this is the "infinite bus" assumption. It assumes that either the primary voltage is "large", or the primary fault current is "large", or both, such that we don't really need to know either. We simply assume worst case scenario that fault current could be infinitely large (it really isn't), and calculate the limiting value for what we need to do on the secondary side. This allows us to not necessarily need a detailed study on the utility side of the service, and it also allows utility upgrades to have no effect on the customer-owned equipment sized for the worst case fault current.

If you don't make the "infinite bus" assumption, you'd need to know the primary side fault current. The primary side fault current depends on the utility's network of transformers, feeders and transmission lines. As electrofelon said, it doesn't depend on your transformer, unless you have sources of fault current on your side (motors, backfeeding generators, etc).
 
mivey

mivey

Senior Member
electrofelon said:
I am confused by the question - maybe I am misunderstanding? The transformer doesn't have any effect on a fault on the primary side (unless possibly you are talking motor contribution back feeding through the transformer).
Click to expand...
He was talking about primary current for secondary faults.
 
mivey

mivey

Senior Member
Ingenieur said:
the equation relating i, r and v :?
Click to expand...
In general v = i * r for a resistive circuit.

What i and what v and what r are you asking about? There are several we could discuss but I'm sure you have something specific in mind so be specific.
 
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Ingenieur

Senior Member
Location
Earth
mivey said:
In general v = i * r for a resistive circuit.

What i and what v and what r are you asking about? There are several we could discuss but I'm sure you have something specific in mind so be specific.
Click to expand...

iirc it is called "Ohm's Law"?

i (conjugate)?

humor is lost on the internet :)
 
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