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 Post subject: Utility Fault Current Scenarios
PostPosted: Wed Jan 05, 2011 8:27 am 
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Joined: Wed Jan 05, 2011 8:12 am
Posts: 1
Location: Kitchener, ON
Hello,

I work for a small firm in Canada, and I am working on numerous arc flash studies. Before this job, I worked for an American company for a few years performing arc flash studies as well. I am trying to determine a rule of thumb for high and low case available fault current scenarios. At my previous job in the US, I used 35 MVA as a low fault current scenario and 80 MVA as a high fault current scenario. If 3ph fault values given by the utility were in between this range, then the value was deemed to be realistic. If it was out of this range (i.e. 90 MVA), I would run an additional case (i.e. 35 MVA low case) to depict realistic values. I'm not sure what range is realistic and what range isn't. I think this is very important because if your utility fault values are unrealistic/inaccurate, then your whole study is garbage. I know there is an IEEE article written on this, but I don't have access to it. I saw a study that was done here in Ontario by one of our competitors and they used 800 MVA as an assumed fault current source, and I think that 800 MVA is way too high. They didn't run additional scenarios, only 800 MVA. Can somebody give me their opinion on a realistic range of available fault current values?

Your help is greatly appreciated


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PostPosted: Wed Jan 05, 2011 11:57 am 
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Joined: Wed Jul 07, 2010 9:26 pm
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Location: Toronto, ON, Canada
I know that in Ontario, for maximum fault currents many utilities use the maximum Ontario Energy Board fault values from the transmission system code Appendix 2 under fault levels: for 27.6kV primary 4 wire: 17KA 3P 12KA SLG, 27.6 3 Wire: 17KA 3P 0.45kA SLG, for 13.8kV primary: 21kA 3P 10kA SLG, you can also find maximum faults for 44, 115, 230, and 500kV depending on the primary voltage. However, there is no "minimum fault levels rule of thumbs" in the standards and we need to rely on the utilities, although they can be very slow in releasing the info. and sometimes inaccurate.


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PostPosted: Thu Jan 06, 2011 9:31 am 
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Joined: Tue Oct 26, 2010 5:00 pm
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Location: Maple Valley, WA.
This is one of the most difficult things about doing an Arc Flash Study. Utilities tend to want to give you the highest value of fault current. They do not understand how lower fault currents can make a protective device operate longer and drive up the arc flash energy values.

We actually ask the utility company what their fault currents are and the kVA and impedance of their transformer(s). Many times they will provide infinite available fault current for the primary side. However, if they give you the kVA and impedance and use it in your study, you'll find that it will be pretty close to real world conditions. You can vary the primary fault current quite a bit, and it has a small impact on the secondary side.

We sometimes will call the utility engineers and ask them what their "ball park" values of fault current are for the primary side. Usually, they are reluctant to put it in writing buy they will tell us over the phone. For example, knowing the primary fault current is around 13,000 for a 15 kV system is better than using infinite.

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Robert Fuhr, P.E.; P.Eng.
PowerStudies


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PostPosted: Fri Apr 15, 2011 1:03 pm 
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Joined: Tue Apr 12, 2011 6:11 am
Posts: 17
Location: Oregon, USA
Perhaps this may be naive of me, but it would seem to me that having an understanding of the 'arc flash' study needs of their customers, and how and why that differs from short cicuit analysis needs, should be a fairly significant requirement for a role so crucial as utility company electrical engineer.

Shouldn't this become a priority for utility companies to be sure their customer service related engineers are conversant with arc flash study requirements and prepared to provide such requests ?

I mean, hello, NFPA 70E is an OSHA requirement for how long now ?

But instead, it seems that some utility engineers still don't have a clue about 'arc flash', or why anyone would possibly be interested in requesting anything besides MAXIMUM available short circuit current data.


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PostPosted: Fri Apr 15, 2011 1:36 pm 
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Joined: Tue Apr 12, 2011 6:11 am
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Location: Oregon, USA
To be a little more specific, regarding Robertfuhr's comment above :

"This is one of the most difficult things about doing an Arc Flash Study. Utilities tend to want to give you the highest value of fault current. They do not understand how lower fault curren(t)s can make a protective device operate longer and drive up the arc flash energy values."

How is this possible that after all this time since the arc flash topic has been a part of OSHA's requirements, that supposedly professional EEs, and the electrical utility companies they work for, still "do not understand" such things !?

I am saying this because I recently had to deal with an utility company engineer who seemed as though he had never encountered the concept of 'arc flash'. I thought that was strange at the time. But now, according to Robert's comment above, this is how utility EE's still "tend" to be. Tres bizarre, no?

And WHY are these furtive attemtps to curry meaningful, indeed essential information unofficially over the phone from the utility EE's still necessary !!??

I seriously don't understand why at this point in time all that would be necessary would be for the EE to hear/read,

"Sir, we are performing an arc flash study for XYZ facility. May we please request you forward all currently relevent data. Thank-you."

Is that just a little too logical , , , or something ?


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PostPosted: Thu Jul 14, 2011 9:07 am 
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Joined: Thu Jul 14, 2011 6:26 am
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Location: Sterling, IL
I received, in writing from the Electric Company, a statement that the maximum available fault current at the transformer feeding our company is 8,032 amps. (Transformer is 34,400 to 4,160, fault current on the 4,160 side) That always seemed low to me, but now I question it even more. Does that seem low to anyone else?


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PostPosted: Thu Jul 14, 2011 9:07 pm 
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Joined: Wed Jun 04, 2008 9:17 am
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Location: Spartanburg, South Carolina
Not if it's a 5 MVA transformer. What is the transformer rating and impedance?


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PostPosted: Mon Jul 18, 2011 8:59 am 
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Finding a utility engineer with knowledge of arc flash should not be that difficult since the utilities themselves have had to follow the 2007 NESC arc flash requirements since 2009. You do, however, need to get past the techs that will give you the equipment sizing maximums.

As far as having a wide range? We like to keep the power on, and go to great lengths to do so. We will transfer customer load to a different feeder, or to a different substation. To keep a substation energized, we'll transfer it to a different transmission line. And the neighboring systems can likewise affect our source impedance by their actions. And a burnt out service transformer will be replaced by what's in the warehouse, which might have a different impedance.

What the industry needs is an intelligent source impedance meter. This device would sit at the service and alert to source impedances that varied significantly from normal or exceeded programmable limits. It would perform the measurement by occasionally very briefly overloading the system while monitoring the resulting change in voltage.


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