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 Post subject: Generator modelingPosted: Thu Jun 02, 2011 11:23 am

Joined: Fri Jan 30, 2009 10:04 am
Posts: 10
Location: Toronto
2 of 600V 750kw generators paralelly supply a 2500A MCC without utility power connected. Each generator has its breaker (1200A frame set at 80%) for protection, which deternins the clearing time. To do arc flash for the MCC, how should I model the generators? What I can think about:
1. Generator subtransient reactance, no matter how the generator breakers operate.
2. Find generator feeder arcing current (arcing current flowing the breaker) using subtransient reactance. If the arcing current is within the inst. of the breaker, model the generator with subtransient reactance. If not, model generators with transient reactance (less current, longer clearing time).
3. Energy accumulation according to subtransient and transient reactance till breaker operate. ( too much work, I do not prefer)

Any thoughts?
Thanks

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 Post subject: Posted: Thu Jun 02, 2011 3:18 pm
 Plasma Level

Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1637
Location: Scottsdale, Arizona
This is another question where there is not a good (or maybe the word is simple) answer.

Option 3 is the best, it really needs the decrement curve. The problem again however is that you can get precise with this but the IEEE equations are not exact.

Perhaps assume the current is based on Xd'' but then evaluate the device operation based on the decrement. This sould give you the maximum current (even though it will be collapsing) and maximum clearing time. Just an idea, perhaps others have different methods.

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Jim Phillips, P.E.
Brainfiller.com

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 Post subject: Posted: Fri Jun 03, 2011 10:59 am

Joined: Mon Jan 26, 2009 5:00 pm
Posts: 46
Location: Midwest
Modeling the generator decrement into arc flash confuses me a bit.

The decrement curves are developed with a bolted fault in mind. Intuition tells me that the decrement will be less severe with a higher impedence fault.

The decrement also varies according to the method of excitation. A self excited machine responds differently than a PMG one. Both probably react differently to a high impedance fault than the bolted fault that the decrement curve was developped around.

If that wasn't complicated enough, the response 2 or more generators in parallel probably differs from how an individual machine would respond.

At any rate, the MCC with 2 generators in parallel should consider the possibility of having 1 or both generators in service. Don't forget the motors either!

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 Post subject: Posted: Fri Jun 03, 2011 6:32 pm
 Plasma Level

Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1637
Location: Scottsdale, Arizona
Ex twidget wrote:
Modeling the generator decrement into arc flash confuses me a bit.

The decrement curves are developed with a bolted fault in mind. Intuition tells me that the decrement will be less severe with a higher impedence fault.

I was thinking more along the lines of maximum short circuit current from Xdāā and maximum clearing time from using the decrement curve - the worst case for both.

However, you are quite correct, constructing a decrement curve based on arcing current would be quite an interesting exercise and give something totally different.

I was also considering how this affects overcurrent protection. If there is a 51V, 87G, etc. that also changes the game.

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Jim Phillips, P.E.
Brainfiller.com

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 Post subject: Posted: Sun Jun 12, 2011 5:15 pm

Joined: Fri Jan 30, 2009 10:04 am
Posts: 10
Location: Toronto
[quote="brainfiller"]This is another question where there is not a good (or maybe the word is simple) answer.

Option 3 is the best, it really needs the decrement curve. The problem again however is that you can get precise with this but the IEEE equations are not exact.

What would be the effective time of Xd"? Table 9-1 of IEEE 551 (from C37.010) recommends using Xd" for 0-1 cycle (momentary caculation), and 1.5-5 cycles (interrupting caculation). Does it make sense to carry out the energy accumulation by using Xd" up to 5 cycles, and Xd' for the rest of time. Assume clearing time is longer than 5 cycles.

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 Post subject: Posted: Sun Jun 12, 2011 6:01 pm

Joined: Fri Jan 30, 2009 10:04 am
Posts: 10
Location: Toronto
Ex twidget wrote:
Modeling the generator decrement into arc flash confuses me a bit.

The decrement curves are developed with a bolted fault in mind. Intuition tells me that the decrement will be less severe with a higher impedence fault.

The decrement also varies according to the method of excitation. A self excited machine responds differently than a PMG one. Both probably react differently to a high impedance fault than the bolted fault that the decrement curve was developped around.

If that wasn't complicated enough, the response 2 or more generators in parallel probably differs from how an individual machine would respond.

At any rate, the MCC with 2 generators in parallel should consider the possibility of having 1 or both generators in service. Don't forget the motors either!

Yes, The one generator operating scenerio study should be performed.
I usually include contributions from motors of 50HP and UP. I have not seen a guideline about this from IEEE 1584. Is there anything about motor size?

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