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 Post subject: Complete Single line or broken up
PostPosted: Wed Nov 07, 2018 11:27 am 

Joined: Thu Apr 12, 2018 7:40 pm
Posts: 7
Hello

I am relatively new to performing arc flash analysis. I've completed multiple studies from substations down to different bus ducts and loads in industrial facilities but not for an entire facilities. I've recently changed jobs and one of the reasons I was hired was to finish an arc flash study that was started in one of our buildings then move onto the others. We have one HV service to the property for all buildings. The previous study was modeled using multiple SLD's (1 for the main, 1 the main switchgear, 1 for each bus duct etc, etc, etc).

I am wondering what is the best practice and purpose of modeling the system as a whole on one SLD vs breaking down into multiple one line diagrams. (Note using SKM)

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 Post subject: Re: Complete Single line or broken up
PostPosted: Wed Nov 07, 2018 2:00 pm 
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Joined: Fri Jul 08, 2016 10:01 am
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Location: Indiana
You can do one model if you bought enough busses with your license and use separate drawings and link them.

Or do them separately but upstream changes will be more cumbersome to handle if they affect the downstream.

Or just make one big model on a single drawing. Your call.

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 Post subject: Re: Complete Single line or broken up
PostPosted: Wed Nov 07, 2018 2:42 pm 

Joined: Thu Apr 12, 2018 7:40 pm
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bbaumer wrote:
You can do one model if you bought enough busses with your license and use separate drawings and link them.

Or do them separately but upstream changes will be more cumbersome to handle if they affect the downstream.

Or just make one big model on a single drawing. Your call.


So there is no particular reason other than personal work flow and organization preferences except for the reasons you listed. The existing model is separate drawings with links. My personal preference is one big model. It's likely not worth the effort to redraw everything to create one big model. The base project has 207/1000 buses used and I have 3 other buildings to do but smaller in scale. If I was running short on buses I suspect I might be able save on that with one big model.

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 Post subject: Re: Complete Single line or broken up
PostPosted: Wed Nov 07, 2018 6:55 pm 
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Location: North Carolina
Let's keep in mind "drawings" vs. database. Speaking from experience working with a model with nearly 2500 buses, to things come to mind. First when you have to do a complete calculation run, it takes several minutes to complete. So most of the time we don't do much of that. We just highlight a part of the model and do a windowed calculation...look in the instructions on how to do this. The big reason for a total recalculation is that as you work with SKM, the bugs just keep making it more and more unstable and it will drive you up the wall. When you start getting what are obvious calculation errors coming out and before it just plain crashes again, that's a hint that it's time to do a complete recalculation run and not just windowed calculations.

Second drawings are really just for readability. The way to visualize this is that when you view the model data, it shows the "links" in the database view but it is really hard to visually see any kind of structure this way and it's hard to "link" things together by manually editing the links. Drawings are really just visual views or windows onto your underlying database. So if you can get used to this concept then drawings are very malleable and flexible and worrying about the number of drawings and pages is purely for organization and convenience. There is no computational benefit or anything like that.

The actual calculation is a giant sparse matrix and the model is actually basically a big database. Drawings are linked to the database. You can dynamically "expand" or remove things on the drawing as needed but the database is still there in the background, even if you don't physically see an item on the screen. This brings up a usability issue. I'd say 99% of the time what we were doing is working in one area of a plant, often a building or a part of a building, or we were working with the internal distribution system that supplied the various parts of the plant spread across about 70 some miles of power lines and almost 2 dozen main substations. So we'd generally have drawings broken down by plant area and then by system voltage. At times you need to look "upstream" and it's trivially easy to expand the details on a given drawing up or down. Usually these were all temporary changes we didn't save. And it was easy to do it with a couple clicks while leaving details that aren't needed off of a given drawing most of the time. The goal here was to make it easier to find and maintain things or look up information.

This was the prebuilt ones. When we were working on particular project, normally I'd take one of the existing drawings and delete the unneeded details, or start a new one and just start linking and expanding from the existing data, depending on needs. I'm not really sure how to explain this except to say that if you routinely use SKM as a tool and don't just do all your drawings in a "once and done" fashion then you will find yourself routinely manipulating the information on drawings until it becomes second nature. At that point how to organize things is sort of obvious. Mostly you tend to keep drawings around with end users in mind because for your personal use you can add/remove items trivially.

In the same operation, this was the main plant. There was also a separate conference facility across the street that was fed by a different utility. Even though it was physically quite close obviously from an electrical perspective it was separate so I just set it up with it's own model from scratch. Granted it was so small that this one was a one-drawing page model.

In another plant that was far smaller (just a couple hundred buses), we could print out everything on one big D-size drawing that was easy to hang up in the electrician shops for easy reference.

So basically it's sort of a personal preference thing as to what makes the most sense. Unless you hit a bus limit I'd discourage having more than one model unless as I described above they are electrically not really closely connected but as far as the drawings go, make as many as you want as often as you want.


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 Post subject: Re: Complete Single line or broken up
PostPosted: Thu Nov 08, 2018 7:06 am 

Joined: Thu Apr 12, 2018 7:40 pm
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Thanks PaulEngr!!


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 Post subject: Re: Complete Single line or broken up
PostPosted: Thu Nov 08, 2018 8:54 am 
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Joined: Fri Jul 08, 2016 10:01 am
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Location: Indiana
My personal preference is not to use links within individual buildings. I like to see the entire building distribution system on one drawing. I do though have a separate model for our medium voltage distribution system. It was originally done in ETAP then converted to to SKM and I had a consultant clean it up after the conversion before I took it over. Actually 2 separate models. 2 utility feeds at 34.5kV to our north substation down to the pad mounted transformer level for each building and the same for our south substation. I haven't counted the number of buildings on each of the two models but it is 50+ all together. To do an individual building model I take the the available fault current from proper the medium voltage model and x/r etc. date and use that as my starting point for a building model as the utility.

This is on a college campus.

Run times for the calcs for me haven't been bad, at all, but I don't close to Paul's 2500 busses. I had some SKM crashing issues like Paul in the past but it has gotten better over time, again, not near the 2500 busses though. I have a 1000 bus limit have never exceeded it on an individual model.

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