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 Post subject: Rsys for DC Generator
PostPosted: Mon Feb 03, 2014 7:13 am 

Joined: Mon Feb 03, 2014 7:06 am
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Hi I am working on an arc flash study and the source is a DC generator. I am trying to calculate the Iarc, but I have difficulty define the Rsys. From the datasheet, there are resistances includes: Armature, armature (1-10), Main circuit (A-D), Interpole, Differential field & Compensating field. I got the circuitry for the last three resistances, however, I do not how the rest of the resistance is connected, is there any typical configuration or approach? Thanks and please inform


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PostPosted: Mon Feb 03, 2014 7:44 am 
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Joined: Fri Jan 03, 2014 6:57 am
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Location: the Netherlands
Don’t know what formula you are using to determine the arcing current but this is what I found : The NFPA 70E isn’t much help. In ANNEX D.8 they mention a formula but: state the following ‘Testing completed for Bruce Power(see reference3, which follows)has shown that this calculation is conservativelyhigh in estimating the arc flash value.

They state that the Iarc is 50% of the Ibf

In the near future I need to do an arc flash study on an DC system as well but I am not yet at that stage so I have not yet checked if the data is valid. What I did already find on the internet is this: [url='http://www.poweranalytics.com/designbase/pdf/DCSC.pdf']http://www.poweranalytics.com/designbase/pdf/DCSC.pdf[/url]

Assuming that you are having problems with determining the short circuit current of the DC system and not actually the arcing current, you could try the IEC 61660 for a better explanation.

Hope this helps a bit!

If it doesn’t just wait a few days and someone who does know what he is doing will answer your question.
EDIT:



SKM PTW refers to
  • DC Short Circuit - ANSI standard 399 and 946, calculates the initial rate of rise and peak fault current.
  • DC Short Circuit - IEC standard 61660, calculates the peak fault current, time constants, time to peak, and steady state conditions.


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PostPosted: Mon Feb 03, 2014 9:10 am 

Joined: Mon Feb 03, 2014 7:06 am
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Thanks Luc.

I use Iarc = 0.5*Ibf to start my analysis

The DC generator has variable speed and the information is not provided neither...


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PostPosted: Mon Feb 03, 2014 5:05 pm 
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Use stall current and forget about trying to get through the generator circuit. Armature current at stall under strong field conditions is what you need if you want maximum short circui current. Although the peak power point would seem attractive in some instances (flashovers) you can be at stall current and still rotating. Using the generator performance curve is much easier. The 50% factor has to do with modeling DC arcs. Maximum power transfer to an arc occurs if half of the voltage drop occurs across the arc, and half in the system. This very simple relationship does not exist in AC where the arc is constantly extinguishing and restriking.


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PostPosted: Tue Feb 04, 2014 7:31 am 

Joined: Mon Feb 03, 2014 7:06 am
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Thank you very much PaulEngr!

It is an old DC generator (D12). There is no performance curve on the datasheet. However, since armature resistance is given, is it correct to calculate stalling current = Vsys/R_armature?


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PostPosted: Tue Feb 04, 2014 8:52 am 
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Yes, V/R should give you stall current. Is there not a locked-rotor current rating on the nameplate or in the specs?


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PostPosted: Tue Feb 04, 2014 9:03 am 

Joined: Mon Feb 03, 2014 7:06 am
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Thanks Larry.

Just a dump question: since it is a DC generator, not a DC motor, is it correct to use stalling current as Isc?


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PostPosted: Tue Feb 04, 2014 9:09 am 
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Billy wrote:
Thanks Larry.

Just a dump question: since it is a DC generator, not a DC motor, is it correct to use stalling current as Isc?



The physics are the same for a DC motor and a DC generator. The only difference is on a motor you apply power to get rotation and on a generator you apply rotation to get power.


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PostPosted: Wed Feb 05, 2014 10:51 am 

Joined: Mon Feb 03, 2014 7:06 am
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Thanks Larry and others for the answer! I am new to this forum and start reading more these days, a lot of helpful knowledge!!

Got 2 more questions tho..

1) I am using etap for DC arc flash simulation, knowing AF method "Max Power" is 3 times higher than "Stoke & Oppenlander", is there any drawback using the latter?

2) The electrical compartment has access from the front and from the back, where the front is covered by a door and the back is covered by a bolted panel. Should there be a label on both sides? Not sure what should be the correct approach... Thanks


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PostPosted: Fri Feb 07, 2014 2:08 pm 
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Not familiar with exactly which arc flash models E-Tap is using. The answer is that you need to do one of two things. This is a decision you need to make for yourself and make it a defendable one. This is sometimes challenging. For instance in AC if you are above 15 kV, then IEEE 1584 no longer applies. Then what? Do you use tables in 70E? Tables in NESC? Arc Pro? Duke Heat Flux? Lee theoretical? Nobody has done actual test work at that voltage so you are pretty much free to choose whichever one you want. There are some definite arguments against Lee and 70E and NESC are "consensus safety standards" so right, wrong, or indifferent, those might be considered a little more credible. As long as you have some train of logic, it's hard to defend something that can't be proven or disproven conclusively. Same with the model in Annex D of 70E for DC arc flashes.

As to labeling...you don't need a label in a location where you will not be doing energized maintenance of any kind including voltage and current testing. Everything else gets a label. Some plants have some "understood" rules such as what to do with motor peckerheads, lighting panels, and instrument panels which cuts down a lot on small boxes. There is a very specific list in NEC on items that specifically need labels that grows with each new edition but it contains the "if doing maintenance" caveat and realistically, actual maintenance practice is a far better judge.

So if you won't be unbolting the back panel and doing energized maintenance, no label needed. If you are, then you do. And it gets more complicated because your working distances on the back sides are frequently much different from the front sides.


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