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 Post subject: Utility & Generator Modelling
PostPosted: Thu Sep 23, 2010 8:19 am 

Joined: Wed Aug 12, 2009 11:30 pm
Posts: 5
Hello,

I have modeled a commercial building using Utility Power, is it necessary to model the building with the Generator the building has on site? This will affect the labels since at some field panels the category will differ from the use of utility or generator power.

Where can i find a document stating this?

Thank you


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PostPosted: Thu Sep 23, 2010 11:18 am 

Joined: Wed Aug 12, 2009 11:30 pm
Posts: 5
Just to clarify, I have modeled this for an arc flash study


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PostPosted: Thu Sep 23, 2010 12:21 pm 
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Joined: Wed Jan 28, 2009 2:19 pm
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Location: Georgia
Don't know the specific document, but I would run the calculations using every power supply scenario. Then post labels for the worst case at each bus. If the study is done with only utility power, the labels are dangerous and could allow someone to get hurt when generator power is supplied.


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PostPosted: Thu Sep 23, 2010 2:50 pm 
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Joined: Mon Jan 18, 2010 11:35 am
Posts: 523
Location: Wisconsin
OSHA says follow NFPA70E
NFPA70E suggests IEEE1584 for calculations
IEEE1584 says to use the available short circuit current.

No where does it say what engineering practices to follow in regards to making decisions or assumptions.

My company's policy is to perform enough different scenarios of fault current to 'feel confident' the results represent the worst case incident energy regardless of the system configuration.


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PostPosted: Fri Oct 08, 2010 4:02 pm 

Joined: Wed Jul 07, 2010 9:26 pm
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Location: Toronto, ON, Canada
Every scenario should be taken into consideration, both normal and standby, as well as the interlocks etc..., and the results of the worst case scenario shall be used for the label, typically a lower available short circuit (e.g. generator and standby operation) will produce the worst case scenario incident energy, since protective devices are generally slower in operating for lower fault values


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PostPosted: Thu Jan 27, 2011 11:49 am 

Joined: Mon Jan 24, 2011 10:49 am
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Re: System Modeling & Calculations

I usually consider all possible operating conditions in my study. Both SKM and ETAP allow you to select and print your warning labels based on any scenario or the worst case scenario.

We have had a project in the past where the Electrical Safety Policy prohibits performing energized work when the facility is on standby power, in this case, the standby scenario should be ignored.


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PostPosted: Fri Jan 28, 2011 12:51 pm 
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Joined: Thu Jan 10, 2008 8:49 pm
Posts: 498
Location: New England
Typically, when on gnerator power the IE will be very high because the fault current is low and clearing times are long. The IE values calculated when on the generator may be much higher than under utility. Calculate both ways, but if generator supply IE's are higher, than on your label put "No Live Work when on Generator Supply".

On generator is going to be for short and limited durations. Just have a policy in place where you can't do live work when on the generator so that you don't need to apply arc flash 70E.


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PostPosted: Fri Jan 28, 2011 11:04 pm 
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Joined: Wed Jun 04, 2008 9:17 am
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Location: Spartanburg, South Carolina
The IE with while on generator only is reduced significantly if you take generator fault current decrement into account.


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PostPosted: Mon Jan 31, 2011 3:20 pm 

Joined: Tue Feb 17, 2009 8:38 am
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Location: Westminster, MD
... not trying to state the obvious here, but if a generator has an automatic switchover you have to consider the generator source because someone could be working on some switchgear when the system transitions to generator backup.
As a PE I'm obligated to represent the absolute worst-case scenario so if there's a chance that the system could be powered from multiple sources I'm modeling it as one of the scenarios.
John M


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PostPosted: Tue Feb 01, 2011 2:00 pm 
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Location: New England
Two things:

1. The IE will usually go UP on the generator. Yes, you have much lower fault current values, but that corresponds to much longer clearing times. The time to clear can go way up. So IE goes up as well. Not always, but generally speaking.

2. I stick with my premise that you only need to calculate for IE on utility, if, you have training that says no live work when on generator power. In terms of the transfer, thats part of the training. The first thing that happens in the transfer is a total loss of power, followed by engine starting, followed by transfer. If electricians are working on generator supplied loads, they will see and experience the loss of power, as it takes at least 4 seconds to transfer to generator (and thats fast). Its pretty hard NOT to see a transfer to the generator. So they stop where ever they are, and come back when its on utility power. Again, very low probability of lossing power in the first place, and simple training takes care of the rest.


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PostPosted: Tue Feb 01, 2011 3:34 pm 
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Location: Spartanburg, South Carolina
haze10 wrote:
Two things:
1. The IE will usually go UP on the generator. Yes, you have much lower fault current values, but that corresponds to much longer clearing times. The time to clear can go way up. So IE goes up as well. Not always, but generally speaking.

But the current goes way down on a generator after about 10 cycles.


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PostPosted: Thu Feb 03, 2011 11:03 am 
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Location: New England
That's exactly the point. The generator has high impedance, say 20 to 25%.
Fault current is very limited. Lets look at a typical. You have a 1500kva utility transformer with 5.75% impedance supplying 480v switchgear. The switchgear has a branch breaker with: long time, short time and instantaneous setting. The instantaneous is set at 20K amps. The breaker feeds a MCC. The same MCC is also fed via a break before make transfer switch supplied a 500Kw generator with a 750Anp molded case CB main. The generator impedance is 20%.

Upon a 3 phase bolted fault (which is the assumed for arc flash) to the MCC when on utility, we get a hypothetical 24K amp fault current. The switchgear breaker trips in 2 cycles.

When the MCC is fed via the generator, we have the same identical fault.
Only now we have 3K amp fault current. The 750A breaker, seeing 3000 amp fault current takes 20 cycles to clear.

When you run the math in the Ieee formula for IE, the longer clearing time of the generator breaker will produce a higher IE compared to the utility switchgear breaker, even though the fault current is higher on the utility.

This is of course hypothetical, and I did not take the time to analize the curve of each breaker for exact data, but the theory is correct.

This is not to say that in real life the generator IE would actually be greater than the utility IE, as the arc may self extinguish before the 20 cycles. But arc flash studies are based on the IEEE formula.

If you tried to solve for the worse case, ie the generator, you might be traking an IE above 40 (so not capable of any work) versus an IE of 7, capable of all work in a comfortable level 2 FRC.

The need to do live work while on emergency power, is very limited. So unless you have a defined need, just prohibit live work when on generator.


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PostPosted: Thu Feb 03, 2011 1:46 pm 
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Bruno wrote:
Hello,

I have modeled a commercial building using Utility Power, is it necessary to model the building with the Generator the building has on site? This will affect the labels since at some field panels the category will differ from the use of utility or generator power.

Where can i find a document stating this?

Thank you

Without a complete model to review one can only guess, and guesses are not good in this line of work, as to which source will yield the worst-case IE. This may or may not be the source that yields the highest available fault current.

Run your arc flash analysis for all operating scenarios, because there are too many variables to consider for offering a summary answer.

And if there is a closed transition operation on the ATS where the gensets and utility will be tied together and operating in parallel for more than a minute or so, be sure to model that scenario as well.


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PostPosted: Thu Feb 03, 2011 1:52 pm 
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Bruno wrote:
Hello,

I have modeled a commercial building using Utility Power, is it necessary to model the building with the Generator the building has on site? This will affect the labels since at some field panels the category will differ from the use of utility or generator power.

Where can i find a document stating this?

Thank you


Furthermore, in a complex electrical distribution system with many buses you typically wind up with IE going up on some buses and down on the others, when analyzing two or more sources on the same electrical system.

So again, model and test every possible source and source combination.


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PostPosted: Fri Feb 04, 2011 6:02 am 
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Location: New England
If you are operating generators in parallel with the utility you are right. But emergency generators are never closed transition. They can not predict when power will fail and start before that point in time. Emergency generators only are the subject of my comments. I am not suggesting to 'guess' at IE, only stating that generators have high impedance by design, and that creates a predictable result under fault current.

In most cases with auxilary or emergency generators, if you don't have a good policy to exclude live work while they are running, you'll be dealing with some high IE's all the time.


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PostPosted: Fri Feb 04, 2011 8:49 am 
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haze10 wrote:
If you are operating generators in parallel with the utility you are right. But emergency generators are never closed transition. They can not predict when power will fail and start before that point in time. Emergency generators only are the subject of my comments. I am not suggesting to 'guess' at IE, only stating that generators have high impedance by design, and that creates a predictable result under fault current.

In most cases with auxilary or emergency generators, if you don't have a good policy to exclude live work while they are running, you'll be dealing with some high IE's all the time.


I agree on all your points. And no disrespect was intended.

I was merely pointing out that there are a lot of variables at play, in particular protective device trip characteristics, especially when multiple adjustable trip units are involved, so that the outcome may not be obvious. My point is simply that if modeling software is available then it is best to take full advantage of it and model every source scenario.


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PostPosted: Wed Mar 16, 2011 3:58 pm 

Joined: Fri Feb 25, 2011 10:05 am
Posts: 8
Bruno wrote:
Hello,

I have modeled a commercial building using Utility Power, is it necessary to model the building with the Generator the building has on site? This will affect the labels since at some field panels the category will differ from the use of utility or generator power.

Where can i find a document stating this?

Thank you

This is depending on your organization practice. In general, you may print labels based on the most severe case (maximum energy).


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PostPosted: Wed Mar 16, 2011 5:19 pm 
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Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
haze10 wrote:
But emergency generators are never closed transition.


Actually not the case. Many ATS systems are closed transition back to utility once utility power is restored this day and time, albeit for a very short duration.

Many operators will also transfer to the generator when a storm is approaching, and most times that will also be short duration closed transition.

That said....To say never is incorrect.


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