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 Post subject: Fault Clearing Times
PostPosted: Thu Jun 17, 2010 5:52 am 
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Joined: Tue May 18, 2010 7:03 am
Posts: 53
The utility is providing fault currents and X/R ratios at the 12 kv pole on the primary side of their transformer for various studies I'm doing. So, I'm having to model their equipment. The problem I'm having is getting the proper protective device info. In one instance the fuse size is so large the incident energy is 370 cal/cm2. The fuse is waaaay to the right of the transformer damage curve. I'm sure it's wrong but "that's what the documentation shows". In another instance, the area of study is close to the substation so there is no protective device on the pole, I was told it's being handled by protective relays at the substation.

The protective info I have is from calling the utility back and requesting more info. Is there another way to do this? I'm using etap. I know I need clearing times but don't think I can just enter a clearing time in etap. It really seems to need the overcurrent device specified.

Looking for suggestions on how to make this simpler and more efficient. Thanks for any help.


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PostPosted: Thu Jun 17, 2010 7:49 am 
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Joined: Tue Jan 13, 2009 5:00 pm
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Sounds like they are giving you the tap fuse information. Tell them you want the transformer fuse size.


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PostPosted: Thu Jun 17, 2010 10:20 am 
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Joined: Mon Jan 18, 2010 11:35 am
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Location: Wisconsin
The results you are describing are very typical.
It is not unusual for the utility devices to be sized to operate only as a last resort in order to save their grid, they often hope the fault will "clear" itself first.

I have performed many studies where I have ignored the 'settings' of the utility devices, although I do prefer to include them.


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PostPosted: Thu Jun 17, 2010 11:44 am 
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If the protective devices are ignored, the software won't give arcflash data on the line side of the customer's breaker. If labels are placed on the breakers based on the arc flash data on the line side, how do I label them? Once I get to the load side of the customer's breaker it's fine.

Thanks for your help.


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PostPosted: Thu Jun 17, 2010 1:55 pm 
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Joined: Mon Jan 18, 2010 11:35 am
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Location: Wisconsin
mountaineer wrote:
If the protective devices are ignored, the software won't give arcflash data on the line side of the customer's breaker.

It depends on how you model your system and what options you chose. As I said I do it often in SKM's PTW software.

Quote:
If labels are placed on the breakers based on the arc flash data on the line side, how do I label them?
It is extremely unlikely that a primary side utility protective device will ever have any thing other than a >40cal/cm² condition on the 'service entrance equipment'.


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PostPosted: Fri Jun 18, 2010 8:21 am 
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JBD wrote:
The results you are describing are very typical.
It is not unusual for the utility devices to be sized to operate only as a last resort in order to save their grid, they often hope the fault will "clear" itself first.


Sure, they love to replace transformers.

It is likely the transformer fuses don't show up in the maps because they are considered to be part of the transformer assembly. Since the crews carry a fusing schedule based on transformer kVA, there is no need to track the fuses separately. Suggest you get a hold of someone in engineering or operations at the utility.


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PostPosted: Sun Jun 20, 2010 8:07 pm 
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Location: Wisconsin
stevenal wrote:
Sure, they love to replace transformers.


I did not say they did.

I said the utility protective devices are normally sized with the hope that the fault clears itself rather than requiring them to operate.

This is from a Cooper Power Systems fuse data sheet:
"Note: Table shows minimum recommended ELSP fuse rating. Recommended ELSP backup fuse will coordinate with protecting fuse and melt only on internal transformer faults." (my emphasis).


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PostPosted: Mon Jun 21, 2010 8:22 am 
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[url="http://www.cooperpower.com/Library/pdf/24050.pdf"]http://www.cooperpower.com/Library/pdf/24050.pdf[/url]

The "protecting fuse" referred to is the expulsion fuse that is used in series with the ELSP current limiting fuse to provide full range protection.


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PostPosted: Thu Jun 24, 2010 1:23 pm 
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Joined: Wed Jun 04, 2008 9:17 am
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Location: Spartanburg, South Carolina
JBD wrote:
I did not say they did.

I said the utility protective devices are normally sized with the hope that the fault clears itself rather than requiring them to operate.

This is from a Cooper Power Systems fuse data sheet:
"Note: Table shows minimum recommended ELSP fuse rating. Recommended ELSP backup fuse will coordinate with protecting fuse and melt only on internal transformer faults." (my emphasis).


See stevenal's post. The recommendation is so that the expensive ELSP backup fuse only operates for internal transformer faults. External faults will cause the less expensive expulsion fuse to operate.


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