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 Post subject: Utility Contribution for AF studies
PostPosted: Thu Nov 13, 2008 9:29 am 
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Location: Cincinnat, OH
I was wondering if anyone knows of some type of rule or regulation that would make it the responsibility of the feeding utility company to divulge incoming fault current values. Sometimes they are perfectly willing to give values for a study but other times it is like pulling teeth and I have had some even say they could not give it.

Any help would be great.


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PostPosted: Thu Nov 13, 2008 9:47 am 
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Great question. I used to run the studies group for a large public utility in a past life. We could provide the info because we had a pretty big group to handle it. However we had a big legal disclaimer that went along with it. Some utilities, especially smaller ones, might not have the manpower. What I have done (and written about) is in the absense of utility data, if you are served by a transformer, try using the infinte bus AND try using various lower values of utility short circuit data. Infinte bus might be worst case IF all short circuit values trip the instantaneous of the protective device.

higher short circuit + same clearing time = higher incident energy.

However the reason to look at lower (assumed) values is to see if a lower primary current will have a big impact on the secondary fault current. Usually the transformer impedance is the dominant factor but the source could have an impact and lower the secondary curent. Many might consider a lower short circuit current to be better but a lower current could cause the protective device to take longer to operate in which case:

lower short circuit + longer clearing time = higher incident energy.

Lower currents could lead to a greater incident energy. Your just feeling out a range of currents to see if you get thrown into a higher PPE category.

Utilities are not required to advise changes to the short circuit current. They would need a huge department to keep up with this. What they should be required to do, is provide you the current when you request it. - sometimes all they give is the infinte bus value which from what I mentioned above is not necessarily the worst case with arc flash. Let them know what it is used for.

The final thing you can do (I've done it) is let them know you are performing an arc flash study and it's results affect human life and injury and the accuracy is dependent on receiving information from the utility. You can add other language but soon many of them realize they have an obligation if they were otherwise unwilling.

My own experience is most utilities are quite helpful and good to work with. Many electric utility people visit this forum and are extremely helpful as you can see by some of the posts here.

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Jim Phillips, P.E.
Brainfiller.com


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PostPosted: Thu Nov 13, 2008 12:11 pm 
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Location: Cincinnat, OH
Thanks for the reply. I have been doing these studies for several years now and was familiar with everything you pointed out. Great Points. We actually try to get operating scenarios from the utility when we do the studies, scenarios that would take a line or substation out of service that might lower or raise the available fault current. We find by doing this our bases are covered.

You are also right that most utilities are very helpful and very willing to give this information. The question came from a company meeting we were having and it was more of general discussion. Although, we have had to do like you said use infinite bus and several other lower values for comparison.

Thanks for the help.


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PostPosted: Thu Nov 13, 2008 2:13 pm 
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In my experience it all depends on who you talk to, you have to find the right person to get the info from the utility, most have a designited person for providing this info, I have bought those guys a few lunches.


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PostPosted: Thu Nov 13, 2008 6:50 pm 
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Joined: Sun Dec 23, 2007 1:44 pm
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Location: Charlotte, NC
Utility Contribution

In many cases, as I am sure you have found, the possibilities for utility supply can be many. You can do your best to come up with a best/worst case evaluation, but they won't guarantee anything (and cannot) depending on system configuration. Most utilities have a service rep assigned to commercial/industrial customers and, since they are aware of the requirements for these studies that they are also required to do, I would expect that they would consider it feckless (love that word) not to provide the required info. I can't recall even one instance when I have requested impedance info and have not gotten it.

You just have to "endeavor to perservere" and get the info.

Alan


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PostPosted: Thu Nov 13, 2008 8:57 pm 
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I've never had a utility NOT give me the fault MVA but I can see some of the small co-ops not having the resources. If I could get the info, I would probably do this, and you can comment if you think it is flawed.

Step 1: Assume infinite bus to get possible max fault current.

Step 2: Assume conservative utility MVA. For this I use the following logic. Most medium voltage pole gear is rated at 8000 AIC. Unless I am within 2000ft of the substation, I would take 25% of the 8000AIC and assume 2000A fault current. This has a reasonably good chance of being close and probably on the low side. But I want low as I this would result in the maximum clearing time and increase IE. The above is limited to 12KV to 23KV aerial lines. If it was 169KV then I would do the same, look up in equipment catalogs the typcial AIC for that voltage class and take 25%.


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PostPosted: Tue Dec 30, 2008 6:00 pm 
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Location: NW USA
It helps to have a sense of how the utility approaches such requests. Traditionally their customers were only concerned with AIC rating so the utilities chose to be conservative and in some cases would quote the highest available including the next larger service transformer size. That would not neccessarily result in conservative Arc Flash Hazard calculations. I suppose it would behove the person doing calculations to verify the serving transformer size and impedance and have a little sense about what the available primary current would be. This makes a good case for hiring local expertise :) .

Gary B


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PostPosted: Wed Feb 18, 2009 1:05 pm 
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Primary Protective Devices

I'm having a problem right now getting information on the type of protective devices used at the substation secondary (over 2 miles away). This powers a local switchyard and then powers my building. I've got all sorts of equipment I need to put into my analysis!


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PostPosted: Wed Feb 18, 2009 2:36 pm 
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I am assuming that you are working with your local account rep. I would contact the rep by phone and make the request and then follow it up with a letter to the rep with your request to include:

1) Per unit positive, negative, and zero sequence components for the impedance at your supply point.

2) Available fault duty at the service location.

3) MVA and Voltage base for their calcs.

4) Relay curve, or curves, and settings for their upstream protective device.

Tell them that you need the info in a timely manner to complete your arc flash hazard analysis.

If the utility does not respond with the info, I would contact the Utility Commission for your area and send them a copy of your request.


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PostPosted: Wed Feb 18, 2009 3:21 pm 
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Location: Spartanburg, South Carolina
Some utilities are very cautious about providing information to be used for arc flash analyses. I have had a utility refuse to provide information on available faults and relay settings for a wholesale delivery point at a municipal utility, when the request was made for an arc hazard analysis. I was able to get the the data when I requested the same information for a relay coordination study.


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PostPosted: Wed Feb 18, 2009 4:53 pm 
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As I have said before here, I have never in 29 years had a problem getting the info, but obviously you and others posting here have encountered roadblocks. Guess some utilities view it differently. Glad I haven't had to deal with those yet. Given that the information is critical to most all facility studies, I would absolutely make the local utility commission aware of the problem!

If the first written request is not successful, I think I would make a second more urgent one, with a cc to the appropriate agency.

There is no excuse for a utility not to give up the necessary info and if they are smart they will advise you that the supply system configuration is subject to change without notice! Maybe you should lie to them and tell them you are doing a coordination study?


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PostPosted: Fri Feb 20, 2009 2:31 pm 
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No, don't lie. For coordination, I will give you the worst case for coordination:

Lowest impedance transformer allowed by spec connected to an infinite bus on the primary. Impedance is adjusted downward by IEEE tolerance while voltage is adjusted upward to ANSI limits. Some might even go to the next transformer size on the assumption that when your present one burns out the next one will be larger because you must be overloading it.

These assumptions will usually tend to make your IE calcs less conservative than they should be.


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PostPosted: Fri Feb 20, 2009 5:43 pm 
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The remark about the coordination study was of course "tongue in cheek". And if you give me infinite bus when I ask for the actual data, I will know the difference and will be back to get the real stuff. If someone is doing these kinds of studies, then they also should be able to know the difference.


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PostPosted: Wed Feb 25, 2009 4:18 am 
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In my experince anything realted to dealing with a utility is usually as slow as a turtle...

Though I have never had any problems getting the information needed, you just have to ask the right people or ask those people who to ask.

Pester them, sooner or later you get what you want.


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