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 Post subject: Utility data - to much?
PostPosted: Tue Sep 11, 2018 11:42 am 

Joined: Fri Jul 10, 2009 10:14 am
Posts: 27
Location: West Central, OH
We typically receive a single set of data from the utility company. For incident energy, then 100% and 85% are used. DP&L provided nominal, maximum and minimum fault current values. So do we use the published values, max and min for the scenerios? Or stay with what we have done in the past using nominal?


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 Post subject: Re: Utility data - to much?
PostPosted: Wed Sep 12, 2018 10:45 am 
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willcoc wrote:
We typically receive a single set of data from the utility company. For incident energy, then 100% and 85% are used. DP&L provided nominal, maximum and minimum fault current values. So do we use the published values, max and min for the scenerios? Or stay with what we have done in the past using nominal?


Tricky, this is kind of like whether or not you use the various combinations when dealing with main-tie-main systems or backup generator systems.

What does it do to you incident energy calculations with the three conditions? And where do they get this range? Based on which ties are closed?


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 Post subject: Re: Utility data - to much?
PostPosted: Mon Sep 17, 2018 10:50 am 
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We typically receive a single set of data from the utility company. For incident energy, then 100% and 85% are used. DP&L provided nominal, maximum and minimum fault current values. So do we use the published values, max and min for the scenerios? Or stay with what we have done in the past using nominal?

If the utility gave you the three values, you'd be wise to use all three in the arc flash portion and then pick worst case and label with the worst case. Then include all this information in any reports you produce. Here's why, using maximum fault currents often produces low arc flash energy because over current devices trip very quickly if the fault current gets into the instantaneous range. However, with lower fault currents, the OCDs sometimes do not trip on instantaneous and will get into the short or even long time ranges. That produces a lower level of energy, but for a lot longer time so the arc flash calories are much higher. It's counter to what you'd expect, but that's how it works. Use all three values and select the worst case. Once you are past the service itself, the lower fault current will probably not have much effect on the calories, but that's not guaranteed.


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 Post subject: Re: Utility data - to much?
PostPosted: Tue Sep 18, 2018 8:28 am 
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Suggest you seek clarity from the utility. The levels could be from various system configurations as PaulEngr said, or they may be using various fault resistances. Applying your 85% to a figure that was already reduced by fault resistance would not make sense, but it would make sense to apply in to a system configuration.


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 Post subject: Re: Utility data - to much?
PostPosted: Tue Sep 18, 2018 11:12 am 
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Location: North Carolina
The inherent problem with utility systems compared to power system models typical of industrial systems is that utility systems are networked. There are multiple paths from any point to any other point as well as multiple sources. Taking for instance the town I live in which is a city of about 50,000-100,000 population, not "small" but also not large by any means, there are no less than 3 primary substations connected in a ring to transmission lines from two different utilities so that loss of any given substation or feeder does not shut the system down. This redundancy and extra interconnects proceeds down into the distribution level as well. It's not a matter of just a radial or loop feed but loops within loops. So the analysis gets more complicated because the impedance depends on what the network configuration is at the time. Often utilities model these things more probabilistically and that's the data you were supplied. Obviously one scenario for them might be "all ties closed" and another might be radial-only through the worst impedance for protection purposes while the "typical" case might be a realistic scenario of what the normal configuration is since the others are all but theoretical and would never exist in reality. So do you model based on the worst case of some obviously contrived values provided purely for protection purposes, or do you model based on realistic expectations, and are two of the three numbers provided purely hypothetical or did they do a probabilistic style analysis of the various network configurations and supply you with say the 20%, 50%, and 80% likelihood cases? From an arc flash point of view if it's the former (completely contrived upper/lower cases), you can probably safely ignore them because then you're just propagating errors. But if they are realistic at all (as in 20/50/80 percentiles or maybe based on realistic expected operating conditions) then you'd be best looking at worst case.


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 Post subject: Re: Utility data - to much?
PostPosted: Tue Sep 18, 2018 4:11 pm 
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Location: Maple Valley, WA.
I too agree with wilhendrix, run all three scenarios. For the maximum, include the motor contribution. For the minimum, turn off the motors. Then take the maximum arc flash energy for each location for the three scenarios.

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Robert Fuhr, P.E.; P.Eng.
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 Post subject: Re: Utility data - to much?
PostPosted: Tue Sep 18, 2018 7:31 pm 

Joined: Fri Jul 10, 2009 10:14 am
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Location: West Central, OH
Thanks for,the input. We did run the numbers and as expected have much higher incident energy than expected but it is what it is with the protective devices. Seems like getting this many values has not been seen by to many folks yet.


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 Post subject: Re: Utility data - to much?
PostPosted: Tue Sep 18, 2018 7:36 pm 

Joined: Fri Jul 10, 2009 10:14 am
Posts: 27
Location: West Central, OH
PaulEngr wrote:
The inherent problem with utility systems compared to power system models typical of industrial systems is that utility systems are networked. There are multiple paths from any point to any other point as well as multiple sources. Taking for instance the town I live in which is a city of about 50,000-100,000 population, not "small" but also not large by any means, there are no less than 3 primary substations connected in a ring to transmission lines from two different utilities so that loss of any given substation or feeder does not shut the system down. This redundancy and extra interconnects proceeds down into the distribution level as well. It's not a matter of just a radial or loop feed but loops within loops. So the analysis gets more complicated because the impedance depends on what the network configuration is at the time. Often utilities model these things more probabilistically and that's the data you were supplied. Obviously one scenario for them might be "all ties closed" and another might be radial-only through the worst impedance for protection purposes while the "typical" case might be a realistic scenario of what the normal configuration is since the others are all but theoretical and would never exist in reality. So do you model based on the worst case of some obviously contrived values provided purely for protection purposes, or do you model based on realistic expectations, and are two of the three numbers provided purely hypothetical or did they do a probabilistic style analysis of the various network configurations and supply you with say the 20%, 50%, and 80% likelihood cases? From an arc flash point of view if it's the former (completely contrived upper/lower cases), you can probably safely ignore them because then you're just propagating errors. But if they are realistic at all (as in 20/50/80 percentiles or maybe based on realistic expected operating conditions) then you'd be best looking at worst case.
. For this facility, they have a dedicated feeder from the substation and a second feed that is shared with other customer. The second feed has only been used a couple of times in the past 10 years. Would you consider that a probable scenario for incident energy? Similar to your comments above, not enough probability to consider it as valid?


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 Post subject: Re: Utility data - to much?
PostPosted: Thu Sep 20, 2018 10:08 pm 
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willcoc wrote:
PaulEngr wrote:
The inherent problem with utility systems compared to power system models typical of industrial systems is that utility systems are networked. There are multiple paths from any point to any other point as well as multiple sources. Taking for instance the town I live in which is a city of about 50,000-100,000 population, not "small" but also not large by any means, there are no less than 3 primary substations connected in a ring to transmission lines from two different utilities so that loss of any given substation or feeder does not shut the system down. This redundancy and extra interconnects proceeds down into the distribution level as well. It's not a matter of just a radial or loop feed but loops within loops. So the analysis gets more complicated because the impedance depends on what the network configuration is at the time. Often utilities model these things more probabilistically and that's the data you were supplied. Obviously one scenario for them might be "all ties closed" and another might be radial-only through the worst impedance for protection purposes while the "typical" case might be a realistic scenario of what the normal configuration is since the others are all but theoretical and would never exist in reality. So do you model based on the worst case of some obviously contrived values provided purely for protection purposes, or do you model based on realistic expectations, and are two of the three numbers provided purely hypothetical or did they do a probabilistic style analysis of the various network configurations and supply you with say the 20%, 50%, and 80% likelihood cases? From an arc flash point of view if it's the former (completely contrived upper/lower cases), you can probably safely ignore them because then you're just propagating errors. But if they are realistic at all (as in 20/50/80 percentiles or maybe based on realistic expected operating conditions) then you'd be best looking at worst case.
. For this facility, they have a dedicated feeder from the substation and a second feed that is shared with other customer. The second feed has only been used a couple of times in the past 10 years. Would you consider that a probable scenario for incident energy? Similar to your comments above, not enough probability to consider it as valid?


To play devil's advocate people often get the fire fighter complex (be the hero) and take lots of unnecessary risks during "emergency" conditions, which usually occurs when you are running in unusual/abnormal conditions such as thinking one worse, the utility loses a line and closes both of your plants ties to backfeed the secondary line. If you label for the "ridiculous" result you could grossly overkill the PPE requirement but it is so much simpler to administer since the report works for all conditions, causing all kinds of operational issues as a result. If you label for the typical, expected result, you have to somehow in documentation, training, etc, get employees to seek out updated arc flash data when conditions warrant it but this allows "nirmal" PPE to be used.


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 Post subject: Re: Utility data - to much?
PostPosted: Fri Sep 21, 2018 8:45 am 
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willcoc wrote:
For this facility, they have a dedicated feeder from the substation and a second feed that is shared with other customer. The second feed has only been used a couple of times in the past 10 years. Would you consider that a probable scenario for incident energy? Similar to your comments above, not enough probability to consider it as valid?


A resounding yes! If the alternate feed has only been used twice in ten years, then the primary feed is very likely overdue for maintenance. You could be transferred to the alternate for a lengthy period for maintenance on the primary feed. I know of no utility that will routinely inform customers of such a no-outage scenario. The transfer is made, IE rises, and no one knows your labeling is incorrect.


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 Post subject: Re: Utility data - to much?
PostPosted: Sat Sep 22, 2018 5:35 pm 

Joined: Fri Jul 10, 2009 10:14 am
Posts: 27
Location: West Central, OH
stevenal wrote:

A resounding yes! If the alternate feed has only been used twice in ten years, then the primary feed is very likely overdue for maintenance. You could be transferred to the alternate for a lengthy period for maintenance on the primary feed. I know of no utility that will routinely inform customers of such a no-outage scenario. The transfer is made, IE rises, and no one knows your labeling is incorrect.


The times it was used was for utility breaker maintenance. The first time also included protective relay upgrades. They also must notify the site as they must close the breaker in our equipment. Probably not a normal arrangement but what this site has.


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