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ekstra   ara
 Post subject: Modeling T and D lines
PostPosted: Tue Nov 11, 2008 8:21 am 

Joined: Sun Jun 01, 2008 5:00 pm
Posts: 5
I think we might be making this more difficult than it needs to be. Are people modeling the arc flash on transmission and distribution lines in segments i.e. every mile or every 5 miles to see the effect of the changing short circuit current as you move out on the line. This could become very overwhelming. Wouldn't it be easier to model it at the substation and maybe at the end of the line?


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PostPosted: Tue Nov 11, 2008 11:04 am 
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Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
Transmission and Distribution

We are taking an approach somewhat similiar to what you describe. Since the duty at the bus could be 5k to 10k+ and could be in the 1 to 2k range out the line several miles, we are modelling the lines based on the over current relay curves. Then we are picking points at 150%, 200%, 500%, etc. as well as the inst. to see how the IE models on any portion of the circuit based on the TCC curve.

We are taking this approach based on the fact that work could be done at any point on the overhead portion of the system.

Still trying to decide exactly what kind of time cutoff makes sense given the relay operating times at low current levels, and depending on whether the work is from a bucket, pole, or on a padmounted tx, transclosure, etc.

Hope it helps,
Alan


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PostPosted: Tue Nov 11, 2008 11:27 am 
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Joined: Wed Jun 04, 2008 9:17 am
Posts: 428
Location: Spartanburg, South Carolina
Generally, distribution lines have instantaneous overcurrent relays for the initial trip, with time overcurrent relays for subsequent trips after reclosing. If the instantaneous settings cover the entire line, and if operating practice is to block reclosing whenever line work is done, is it necessary to check arc flash energies with the time overcurrent relays?


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PostPosted: Tue Nov 11, 2008 12:02 pm 

Joined: Sun Jun 01, 2008 5:00 pm
Posts: 5
Transmission and Distribution Modeling

Thanks for the quick responses!

Do you use the lower of the phase and ground relay or do you just use the ground relay for the single phase arc flash, i.e. arcpro? ... or do you model three phase with phase relays and three phase short circuit current based on the IEEE 1584 programs - I think that would be overkill on some lines with the separation. I can't see the single phase escallating like it could with equipment enclosures.


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PostPosted: Tue Nov 11, 2008 12:25 pm 
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Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
Distribution Lines

I think yes it is important to look at time time over current function for the following reasons:

Since we are looking at arcing and not solid faults, hard to know what the actual current will be...so we try to be a little conservative in our approach.
Long circuits generally have areas with max fault duties under the setting of the inst attachments.
Many inst attachments are set higher than normal for coord. with a downstream device.
Many inst attachments are actually disabled these days to avoid the first blink on the circuit in an attempt to blow the fuse before the first breaker operation.

We use the faster of the phase or ground for the time to clear. I agree that it is unlikely that the fault will escalate to all phases, so we model single phase with Arcpro.

We are finding that 8 cal PPE easily covers most all of the issues for the OH primary.

Thoughts?
Alan


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PostPosted: Wed Nov 12, 2008 6:32 am 
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Location: Scottsdale, Arizona
acobb wrote:
We are finding that 8 cal PPE easily covers most all of the issues for the OH primary.


Alan, 8 cal is what I have been seeing also. Like I'm sure you have seen, it appears that with reasonable relay settings and a bit of working distance combined with the fault being in open air and single phase, the incident energy is not always as bad as people might first think.

An approach I have taken is to work backwards. Think in terms of category 2 / 8 calorie as being the answer you want. Then see if all the locations on the line, substation, scenarios etc. will be OK with cat 2. If so, cat 2 is your answer. If not, there might be a few exceptions where the moon suit is required (great on a hot day!) :( This "backward" approach seems to be catching on. Of course most people want to avoid the moon suit

To avoid the high energy areas and avoid the moon suit, maintenance settings might be an option. As more companies are switching to digital relays a.k.a. SEL that have dual set points, the second set point can be low (being careful about nusiance tripping, load pickup etc). Then when live work is conducted, enable the lower settings as maintenance settings. Of course the settings will mis-coordinate but the second set point is only enabled when someone is working on the line and in that case nuisance tripping is prefered to high incident energy and injury.

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PostPosted: Wed Nov 12, 2008 7:20 am 
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Joined: Sun Dec 23, 2007 1:44 pm
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Location: Charlotte, NC
When the alternate settings option is available, we are also looking to lower (or activate in some cases) the inst setting with an additional switch to be operated in conjunction with the reclose block in cases where the inst has been raised or disabled to coordinate with a downstream device.

Helps a bunch when looking at the 150 to 200 % of pickup levels of the relay.

Thanks,
Alan


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