A couple fault current questions

[electrofelon]

Here is the text of the email I received from the POCO (engineer?) RE fault current:

The table I’m looking at shows a max short circuit current of 37,600 Amps at the transformer for a 3-phase 500kVA 480/277 V transformer.

Assuming that is what he meant to say, didnt cross columns, etc, They must be assuming 1.6%Z. That seems super low. Maybe they lump together pole mounts in that chart - dont they typically have a much lower Z than padmounts? The charts in here on page 6

http://www.cooperindustries.com/con...ch_Lib_Short_Circuit_Current_Calculations.pdf

Give an even higher value of 51500 for a 500 kva. Has anyone actually seen such low Z values?

 

[topgone]

Here is the text of the email I received from the POCO (engineer?) RE fault current:



Assuming that is what he meant to say, didnt cross columns, etc, They must be assuming 1.6%Z. That seems super low. Maybe they lump together pole mounts in that chart - dont they typically have a much lower Z than padmounts? The charts in here on page 6

http://www.cooperindustries.com/con...ch_Lib_Short_Circuit_Current_Calculations.pdf

Give an even higher value of 51500 for a 500 kva. Has anyone actually seen such low Z values?

I've seen 4%Z on a 500kVA. I have also seen 1.68%Z for a 50 kVA!:lol:

 

[mivey]

Has anyone actually seen such low Z values?

The important question is has your utility seen these low values and the answer would appear to be yes. If you just look at data posted in this thread you can see your utility is not alone and some utilities see even lower. You really should go by what your utility gives you as they should be the best source for you.

 

[mivey]

Maybe they lump together pole mounts in that chart - dont they typically have a much lower Z than padmounts?

The data I have trends that way but that is not going to be a given for all utilities. Just look at some of the data posted in this thread.

 

[mivey]

It most likely has primary fusing,
Ip rated 23 A

I would check that
this should limit let through well below 37000

Normal primary fusing will not unless they also use current limiting fuses and that cuts out in about 1/4-1/2 cycle or so.

I have seen a trend where some utilities do not use them as much any more. Some think the pressure reliefs prevent tank ruptures but don't think about them only being able to vent so much at a time. With the bigger substations and stiffer supplies they should be using them, at least closer to the station. Oh well.

 

[Ingenieur]

Normal primary fusing will not unless they also use current limiting fuses and that cuts out in about 1/4-1/2 cycle or so.

I have seen a trend where some utilities do not use them as much any more. Some think the pressure reliefs prevent tank ruptures but don't think about them only being able to vent so much at a time. With the bigger substations and stiffer supplies they should be using them, at least closer to the station. Oh well.

1.6 is inline with what other utilities are providing

I would still query about fusing
can't hurt, may help
even if fused at 50 andblets go at 500 within a few cycles the sec will be limited well below 37k

they are covering THEIR butt with YOUR money lol
the fact that the actual Z is 5.2 adds insult to injury

edit
data plate shows fusing

a decision
what is the cost delta of say 17k vs 37k
weighed against the chance of future changes
the 37k is based on infinite bus, bolted fault at sec lugs, chances are they can't delivery that fault current
that's why we get paid the big bucks lol

 

[Bugman1400]

assume
V 480
Ifs 37000 3 ph
sec Z = 480/(sqrt 3 x 37000) = 0.0075 ohm

reflected to primary (12460/480)^2 x 0.0075 = 5.05 ohm
Ifp ~ 12460/(sqrt 3 x 5.05) = 1425 A

Your calc seems correct but, I think I worded the question incorrectly. I meant what would be the fault current for a 3PH fault on the highside (@12.47kV) of the xfmr? However, it seems the conclusion is that a lowside fault of 37kA is not achievable with a 500kVA xfmr w/ 5.2% impedance.

 

[mivey]

...it seems the conclusion is that a lowside fault of 37kA is not achievable with a 500kVA xfmr w/ 5.2% impedance.

Correct. Until the transformer impedance drops below 1.625%, any source impedance ahead of the transformer would drop you below 37 kA on the secondary.

 

[Ingenieur]

Your calc seems correct but, I think I worded the question incorrectly. I meant what would be the fault current for a 3PH fault on the highside (@12.47kV) of the xfmr? However, it seems the conclusion is that a lowside fault of 37kA is not achievable with a 500kVA xfmr w/ 5.2% impedance.

that 1425 is the primary or highside
3 ph and gnd fault are the same when conductors are ignored
ph-ph will be sin120 or 87% as much

Z in ohms not per unit
3 ph = V/(1.732(Zline +Zxfmr))

Gnd fault = V/(1.732(2 Zline +Zxfmr))

ph ph = V/(2(Zline +Zxfmr))

 

[mivey]

3 ph and gnd fault are the same when conductors are ignored

and if source is ignored or transformer configuration is ignored.

Gnd fault can be higher than 3 ph fault on secondary of a delta-wye transformer. Gets worse for long source feeders.

I know Ingenieur knows that but was just pointing it out for others that may not know.