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 Post subject: Is it necessary to model Capacitors and Motor Contribution for a Utility Arc Flash Evaluation?
PostPosted: Wed Mar 14, 2012 9:26 am 

Joined: Wed Mar 14, 2012 7:59 am
Posts: 3
I am currently modeling an entire Municipal Utility's 12.47 kV Distribution System which involves four 12.47 kV Substations with 3 feeder circuits each. I have finished most of the modeling, however I am trying to determine if it is necessary to model capacitors and motor contribution.

1) Is it necessary to model any type of capacitor for a Utility Arc Flash Evaluation?

2) I have modeled approximately 140 selected service transformers, most of them businesses. These are transformers serviced by the utility. Many of these businesses have motors. What is not known is how many businesses do have motors and what they actually contain as far as motor loads. I have determined that I have three options.

A) Go to every business and ask them for their one lines and determine their motor loads or ask them to provide their motor loading. (Very time consuming, but if it necessary than so be it.)

B) Make an assumption on what businesses have major motor loads and assume a certain motor load value. (Doesn't seem smart to assume items when safety is at hand, but if this is acceptable than it would save time.)

C) Don't worry about motor contribution. (The easiest route, however does not seem to be the most safety conscious.)

Does anybody have experience or advice on motor contribution for a Utility arc flash?

Any advice on these matters will be greatly appreciated.

Thank You.


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PostPosted: Thu Mar 15, 2012 7:44 am 
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Joined: Wed Jun 04, 2008 9:17 am
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Location: Spartanburg, South Carolina
We have not modeled motor loads when analyzing utility 12.5 kV distribution systems. NESC analysis uses 1Ø faults and there will be no zero-sequence contribution from 3Ø motors. If the transformer is connected delta-wye, there will be no zero-sequence contribution at all from the customer. There will be some pos- and neg-sequence current contribution from the motors, but it will be small and only last for a few cycles. I looked at a study we did for a pump station with a 5 MVA grd Y - grd Y transformer and ten 2400 V 450-500 HP pumps. A 1Ø fault on the 13.2 kV transformer primary was 2660 A, with 424 A coming from the pump station. This is probably about the most contribution you could expect from any 12.5 kV customer.


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PostPosted: Thu Mar 15, 2012 8:26 am 

Joined: Wed Mar 14, 2012 7:59 am
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Thanks for the input. I am also concerned about the secondary side of the transformer. The Utility services and inspects these service transformers. Many of them are 1000kVA and 1500kVA pad-mounts. When the linemen open the doors, both the secondary and primary are exposed. Jghrist, would you consider motor contribution with the secondary exposed?


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PostPosted: Thu Mar 15, 2012 9:00 am 
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Joined: Wed Jun 04, 2008 9:17 am
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Location: Spartanburg, South Carolina
Incident energy on the secondary of large transformers, when calculated using IEEE-1584 methods can be very high, with or without considering motor contribution and is very dependent on transformer impedance and primary fusing. Tests have shown that arcs are not sustained on pad-mounted transformer secondaries for typical fuse clearing times, so the calculations are often very conservative. I would use the new Table 410-1 in NESC-2012 instead of calculated energy. Table 410-1 is based on testing and does not depend on fault current, so the question of motor contribution is moot.


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PostPosted: Thu Mar 15, 2012 10:00 am 

Joined: Wed Mar 14, 2012 7:59 am
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Jghrist, Thanks for taking the time to post. Do you worry about modeling capacitor banks on a 12.47 kV System?


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PostPosted: Thu Mar 15, 2012 11:22 am 
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Location: Spartanburg, South Carolina
Rcard wrote:
Jghrist, Thanks for taking the time to post. Do you worry about modeling capacitor banks on a 12.47 kV System?

No. Fault contribution is neglible and transient.


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