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 Post subject: Transformer Fault current
PostPosted: Wed Feb 17, 2010 12:49 pm 

Joined: Thu Apr 02, 2009 4:33 pm
Posts: 29
Location: OH
Can someone explain to me the correct way to find the fault current on a transformer. I've requested from the utility several times to give me the primary fault current for arc flash calculations, but they keep sending me the secondary fault current (where their responsiblity ends). The problem is that their supplied values far exceed my calculations using the transformer impedence and with infinite primary source. I thought you were supposed to use the nameplate impedence on the transformer? Even when using the nameplate values the utility values are far greater. Which one should be used? I've stated that the values would be used for arc flash studies and to send the primary fault currents min and max but they insist on giving me max values. I don't believe this sytem to be that strong to use those values. They supply the percent x and r values on a 100mva base and fault current on the secondary. What gives? Sorry if I sound like a broken record.


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PostPosted: Wed Feb 17, 2010 2:17 pm 
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Location: Charlotte, NC
First off, I would be surprised if they would furnish you min and max values as that would be speculative and cumbersome for them. I would expect them to furnish the actual if that is what is requested, or at least infinite source for your transformer values as some seem to be doing.

Is your transformer Y-Y, D-Y, or something else? What is the size, %Z, and sec. voltage?

Are you sure you did your calcs correctly? Have to ask.

I guess there might be some utilities that have settled on using a minimum %Z for particular transformer sizes.....don't know


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PostPosted: Wed Feb 17, 2010 5:32 pm 

Joined: Thu Apr 02, 2009 4:33 pm
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Location: OH
I should have listed more info. The transformer is 300 KVA 2.95 IMP
12470/7200 to 480/277 y to y. My calculations come up to around 13,400 amps. The utility supplied me with 37500 amps. The only way I got that answer was to use a 1% IMP. I think I am to use the name plate data.
Am I correct? If so, which info do I use for arc flash calcs? The 13,400 or the utility supplied 37500?


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PostPosted: Wed Feb 17, 2010 8:12 pm 
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Joined: Sun Dec 23, 2007 1:44 pm
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Location: Charlotte, NC
For your transformer I come up with about 12,200 amps infinite. Not a big difference but have to wonder where their values are coming from. 1% is low for most any transformer I know of.

As to what to use, I would go back to the utility and request verification of their numbers and the basis for them.

Seems someone has made an error.

Good Luck!


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PostPosted: Wed Feb 17, 2010 8:48 pm 
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Joined: Thu Jan 10, 2008 8:49 pm
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Location: New England
I come up with the same 12,200 amp for the finite bus. On most utilities, unless you are very close to a large substation, you can expect a significant reduction in fault current. 12.2KA is an infinite bus, which is never there. You finite bus calcs will be much less. Something is wrong and you should ask your utility to resolve it.


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PostPosted: Thu Feb 18, 2010 6:22 am 

Joined: Thu Apr 02, 2009 4:33 pm
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Location: OH
Thanks for all the reply's. Here is the data the utility furnished me. The transformer is 300 kva 12470/7200 to 480/277 wye to wye. Bay-o-net fuse 25 amp. The x/r is %R 402 + %jX 627 on a 100 MVA base. They gave me a fault current of 3 phase 37,269 amps on the secondary side (where their delivery point ends). When I saw this I thought it may not be correct. The information off the transformer data plate is: 300 KVA 2.95% IMP 12470 primary to 480 volt secondary.
Thanks


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PostPosted: Thu Feb 18, 2010 6:33 am 

Joined: Thu Apr 02, 2009 4:33 pm
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Location: OH
I have another question about fault current. It looks as though the utility may have errored on the above. My question is, they have given me several studies and they keep giving me maximum fault current even though I say it is for an arc flash study. When doing a study, is there a rule of thumb on using a percentage of the fault current for an arc flash study or should I use what they provide even if it looks like they are supplying me with maximum fault current? Thanks for any answers, as I am trying to sort all of this out.


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PostPosted: Thu Feb 18, 2010 7:34 am 

Joined: Thu Apr 02, 2009 4:33 pm
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Location: OH
I double checked the transformer data plate. The utility had it listed as 480/277v. That is wrong, it is 208/120 with 2.95% IMP. I calculated 28,228 amps at infinite bus. That is still off from the 37,269 that the utility supplied. Since it seems they are supplying higher fault current than I am calculating using the transformer impedance off the name plate,should I use my calculations (using trasformer impedance of the transformer data plate and infinate bus) or the higher supplied utility data? Also, since these locations are not next to a substation, should I reduce the fault current even lower for arc flash calculations?


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PostPosted: Thu Feb 18, 2010 9:08 am 
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The utility is giving you available fault current for sizing equipment, are assuming you will not want to upgrade equipment when they replace the transformer. They are giving you data based on a larger transformer or lower impedance transformer, or both. Push your call past customer service and the techs, and ask to speak with an engineer. There will likely be someone who understands since utilities are now required to use FR when needed.


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PostPosted: Thu Feb 18, 2010 9:55 am 

Joined: Thu Apr 02, 2009 4:33 pm
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Location: OH
This information was supplied from the utility engineer.


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PostPosted: Thu Feb 18, 2010 11:03 pm 
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Location: New England
Couple of things. First, you need to buy the IEEE 1584.
You are 'required' in the analysis to perform the calculation at both 100% and 85% or arcing current.

Using the utility info would be wrong at this point. High fault currents typically result in shorter clearing times for overload devices and thus lower Incident Energy levels. So using the utility numbers may calculate to 'lower' than actual IE values, even with the 85% calc.

For specifying equipment you can use the utility numbers for determining interrupting capacty (plus motor loads if significant) But for your arc flash analysis, you need to made some effort to get actual data. I would start by asking the utility to give the MVA to the primary of the transformer. Then use the transformer nameplate impedance. If they won't give you the MVA, then you have to make some guesses. Sounds like you are NOT next to the substation, so, as a guess, others can comment, use 50MVA, or, take the infinite transformer current and reduce by 25%. These are rough, but for now better than what the utility is telling you.


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PostPosted: Fri Feb 19, 2010 6:17 am 

Joined: Thu Apr 02, 2009 4:33 pm
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Location: OH
Thanks Haze 10. I have a copy of the IEEE 1584. I also have a software package. I am interseted in the bolted fault current to enter into the program.
I am just trying to get this info from the utility. The utility is giving me the bolted fault in a 100MVA on a %R + %Jx format. Their fault current is actually more than calculating using transformer impedance with infinate primary. Brainfiller helped me figure out the utility MVA and primary amps with the information the utility supplied in another problem. The problem I'm having is when I figure the total Z and try to subtact the transformer impedance to find the utility impedance, the transformer impedance is more the the total Z after the base conversions. The utility just supplies the fault current on a 100MVA base with %R + %jX format. The software requires primary fault amps, MVA, KVA or amps. I'm just trying to convert their info. The utility "only" supplies this info on the secondary side. I'm trying to find the primary side. That's why I was asking for profressional opinions on what percentage of infinate primary source could be used. You mentioned a 25% reduction. What does eveyone else think? The more I read about this arc flash stuff,it becomes more confusing with all the "what ifs". It also appears to "not be" an exact science. Thanks for all the replies.


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PostPosted: Fri Feb 19, 2010 9:24 am 
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Doesn't sound like an engineer. Utilities can operate under the industrial exemption, and throw the word about loosely. Try asking for a PE or the chief engineer. If they give you the same value used to rate equipment you are speaking to the wrong person.


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PostPosted: Fri Feb 19, 2010 9:58 am 

Joined: Thu Apr 02, 2009 4:33 pm
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Location: OH
I'll keep trying. Thanks for replying Stevenal.


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PostPosted: Fri Feb 19, 2010 7:48 pm 
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Location: New England
What voltage is the utility supplying to the primary of the transformer? How far are you from the substation that supplies the overhead supply.

50MVA and 25% reductions are just guesses but they prove reasonable.


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PostPosted: Sat Feb 20, 2010 6:11 am 

Joined: Thu Apr 02, 2009 4:33 pm
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Location: OH
Haze10, the utility is supplying 12,470 volts underground primary. I would guess about 3/4 of a mile from the substation.


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PostPosted: Sat Feb 20, 2010 6:18 pm 

Joined: Mon Feb 15, 2010 6:40 pm
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I believe you base the fault rating by the MAX available current.
I believe they add for liability honestly
we're at 22k for a 1ph 220v. 200amp. commercial.
3 phase 48ov- 35, 65k
Here in PHX the enginers phone number is on the calculations/print.
Being a ark flash study they would be liable and you too if you did not go by what they SET it as.


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PostPosted: Sat Feb 20, 2010 7:31 pm 
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Joined: Sun Dec 23, 2007 1:44 pm
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Location: Charlotte, NC
wingnut,

Your previous post makes no sense to me....would you please explain.

Alan


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PostPosted: Sat Feb 20, 2010 9:43 pm 
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Wingnut, in this case we are saying that the information the utility is supplying is 'too high' and that using it will produce incident energy levels that are 'too low'. The liability is in not applying sound judgement, and following an error blindly, knowing its an error. You can not force the utility to provide you accurate information. I guess you could try to sue them in court, but you'd probably lose anyway.


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PostPosted: Sat Feb 20, 2010 9:47 pm 
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rayman,
I would try 50MVA at the primary, then add in the xfrm impedance. Tell us what you get with that for secondary fault current. In this case going with 'lower' values would be the safer bet as it produces lower arcing current and longer clearing times. 50MVA, unless its very unusual, would be reasonably close. How long would you guess the underground feeder to be from the pole? On the street, do you see typical wood utility poles?


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