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 Post subject: 208 volt, 150kva
PostPosted: Wed Oct 20, 2010 8:57 am 
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Joined: Tue May 18, 2010 7:03 am
Posts: 53
Modelling a system with 3 single phase, 50 kva transformers, 2%Z each, set up as a 3 phase 150kva, 2%Z. Three phase available fault current of 2950A at 12kv. I'm getting fault currents of almost 21ka on the secondary side of the transformer. After 75 feet of aluminum quad service drop cable, the fault current is about 8.6ka. This is the first 208 system I've done so the fault currents seem high on the secondary side of the transformer and the drop in fault current seems high after the first cable.

Is there anything obvious here I'm missing?


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PostPosted: Wed Oct 20, 2010 4:22 pm 
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Joined: Tue Feb 24, 2009 7:05 am
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(Didn't ran actual calcs, just my feeling)

The %Z value is quite low (2%), so it's normal that the fault current at the secondary be on the high side. When you add the service drop cable, its impedance is probably quite high in comparison, so that's why you get such a drop in fault current after 75 feet.

Try the same calcs with a higher %Z: the fault current should be higher and closer after your 75 feet cable.
Try also with a way bigger cable: the drop should be quite smaller for the same 2% %Z.


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PostPosted: Thu Oct 21, 2010 5:18 am 
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Thank you for your comments. I dug around quite a bit in some old GE literature and found recommended impedances for studies. I think 2.4% was the number I found for the single phase pots. The 2% seems low but it is the number the utility provided. I played around quite a bit with different settings yesterday and the difference at the service entrance varied about 500 to 800 amps. I'll fuss with it some more. Thanks again.


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PostPosted: Thu Oct 21, 2010 5:29 am 
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Joined: Wed May 07, 2008 5:00 pm
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Location: Rutland, VT
I just looked at some 50 kVA distribution transformers in stock (utility company) and they have an impedance of 1.4%. I would imagine that like most utilities, we need to keep losses down and the regulators want losses as low as possible.

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PostPosted: Thu Oct 21, 2010 10:59 am 

Joined: Wed Jun 24, 2009 4:45 pm
Posts: 41
Location: WA State
mountaineer wrote:
The 2% seems low but it is the number the utility provided.


The 2% is a good number for a utility 50 kVA single phase pole type transformer 7200/12470Y-120/240V.

Our database has thousands of units all falling within the 1-3% range.


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PostPosted: Thu Oct 21, 2010 11:58 am 
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Joined: Tue May 18, 2010 7:03 am
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Thanks for all the replies - It makes me feel more confident about my results.


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