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System Complexity - Double Ended Substations
https://brainfiller.com/arcflashforum/viewtopic.php?f=33&t=3740		 Page 1 of 1
Author:  Jim Phillips (brainfiller) [ Sun Jun 28, 2015 6:39 pm ]
Post subject:  System Complexity - Double Ended Substations
Power distribution systems can range from very simple to very complex. Judging from some of the comments over the years at the arc flash forum, many people have very complex systems. This can require many different scenarios when it comes to modeling the system for an arc flash study.

This week's question is quite basic. It is about packaged unit substations with a low voltage secondary (<1000 volts) sometimes referred to as a secondary unit substation. These are also referred to as "double ended substations", "secondary selective" or "main-tie-main" substations.

Do you/your client(s) have low voltage double ended substations?

Yes - 1 to 5 double ended substations
Yes - 6 to 10 double ended substations
Yes - More than 10 double ended substations
It depends - many locations/clients
No
Author:  Voltrael [ Mon Jun 29, 2015 4:09 am ]
Post subject:  Re: System Complexity - Double Ended Substations
None where I work now. The paper mill I worked in a few years ago had several double ended substations, one set of ends connected to utility power and the other ends connected to internal generation.
Author:  CarlM [ Mon Jun 29, 2015 5:05 am ]
Post subject:  Re: System Complexity - Double Ended Substations
I misunderstood the question. Actually I see a lot of these systems in my work. What I was thinking of was primary transmission loop system where the utility supplies the services from a loop that can be fed from either end. I find this more often in the cities.
Author:  airjockey42 [ Mon Jun 29, 2015 6:58 am ]
Post subject:  Re: System Complexity - Double Ended Substations
My customer's system uses foure 12.47 kV "Sub-Transmission Systems" to serve double ended substations serving 85% or more of the plant load. The way the system is arrange it makes it very difficult, if not impossible to develop a low fault current scenario. I mentioned in an earlier Post that the probable worst case would be after a total electrical failure , which would have the system energized with zero motor contribution for a short period.

While this is a worst case scenario, i feel it is totally impractical to label everything with this scenario. if you did there would be no way to get the equipment running without everyone wearing 40 cal suits, which might not be adequate, The customer uses remote operation for most majpr equipment anyway.
Author:  JKlessig [ Mon Jun 29, 2015 8:49 am ]
Post subject:  Re: System Complexity - Double Ended Substations
Quote:
While this is a worst case scenario, i feel it is totally impractical to label everything with this scenario. if you did there would be no way to get the equipment running without everyone wearing 40 cal suits, which might not be adequate,


This leaves me a little confused. This scenario that you say is "worst case", IS the scenario you are actually in, WHEN you are recovering & restarting.
For my money this is exactly why you would use that one for your labels.
As well, if you are putting people in an area where a 40Cal suit "might not be adequate",
sticking your head in the sand, and labeling it as something lower, just so you cqan start up,
would ummmmmm..... probably be considered criminal if it came to that.

[Certainly if I were going to do something in an area where 40cal might not be adequate,
I would rather be wearing it, than not, everthing else being equal.]

If this Worst case was some oddball scenario, that you have to work to set up,
then you might have a reasonable arguement. But I would say not in this case.

I am not sure I would be happy working on a system knowing the the presence
of motor fault contributions was the ONLY thing keeping my I.E. down to a reasonable level.
Author:  Gary B [ Mon Jul 20, 2015 8:03 am ]
Post subject:  Re: System Complexity - Double Ended Substations
I have worked one facility where there were more than a dozen possible distribution arrangements due to different number of generators operating, tie breakers closed etc. Most areas calculated worst case as Unworkable, but could also experience low, workable calculated exposure, in other particular arrangements.

The switchgear was labelled as "unworkable" but much extra information was provided in book format to allow required work to be performed when in other arrangements.
Author:  PaulEngr [ Tue Jul 21, 2015 8:23 am ]
Post subject:  Re: System Complexity - Double Ended Substations
JKlessig wrote:
Quote:
While this is a worst case scenario, i feel it is totally impractical to label everything with this scenario. if you did there would be no way to get the equipment running without everyone wearing 40 cal suits, which might not be adequate,


I am not sure I would be happy working on a system knowing the the presence
of motor fault contributions was the ONLY thing keeping my I.E. down to a reasonable level.


That's usually not the case. Usually motor fault contribution increases fault current although I could envision a situation where lack of motor fault contribution increases opening times.

I've seen this, too. When you have a lot of double ended substations, there are two scenarios that cause a lot of grief: The first is assuming all mains and all ties are closed. The second is assuming that all ties are closed and one main is opened. Effectively they both result in all loads on one side of one bus, and the all mains closed scenario also decreases system impedance to a minimum including paralleled transformers. Closing both ties is very unusual partly because it allows circulating currents between transformers if they are not perfectly balanced and is almost never used except when continuity of service is critical, but even then begs the question about why backup power is not being used. Running single-ended instead of splitting the bus happens frequently if for no other reason than because the whole reason for double ended equipment is so that the incoming equipment can be shut down for maintenance. But it is atypical to switch all substations over to a single bus and run the entire plant single ended intentionally if there are a lot of double ended substations (typical of petrochemical, paper, etc.). So again, this would be a rather unusual scenario.

Similar, and often even more complicated, scenarios exist when considering sites with multiple generators such as glass plants.

It's not a matter of trying to "skirt the system" just to avoid a result. Some of my results end up in the 150+ cal/cm^2 range, based on the engineering study. I'm unfortunately not making that up. It's what SKM produces for a result with a particular double ended substation fed by another double ended substation with an enormous (25 MVA+) motor contribution in which the equipment involved exceeds 15 kV so SKM defaults to Lee model. This would only happen if:
1. The sync relay allows it (not sure if circulating currents would show up or not).
2. All breakers are put in manual to bypass the break-before-make interlocks.
3. Electricians/engineers were ignoring plant practice which is always break-before-make, or didn't stop when "bad things" were already happening and didn't take a time out to assess why things are happening and get back on track.
4. All the labels that have lots of warnings saying not to do this are ignored as well. Chances are that this would have to be done with an engineer because by the time they get this far out on a limb, they're already calling and asking for support.

The above scenario is very typical of the same plants that have this arrangement of equipment.

As far as engineering ethics goes, it would be a huge violation of ethics to label everything with 150+ cal/cm^2 and basically tell someone that they can't do any task which requires arc flash PPE because none exists. It will then become virtually impossible to do maintenance on the equipment which in turn means that paradoxically, we are drastically increasing the likelihood of an arc flash occurring. We can instead look at the "lower energy" scenarios and put in procedures, labels, and training to work under those guidelines. As an engineer I'm ethically responsible for presenting options and and consequences and advantages of each one. I can of course make recommendations, and this is what it would be. Where I would be remiss in my duty is in not presenting the "worst case" option or in doing only the labelling and not presenting the case to those making the decisions so that all parties involved can make informed decisions. Just make sure to document this stuff so that if it ever does end up argued by lawyers, you protect yourself.

If the attitude is "git 'er done" then it really doesn't matter what the labels or the plant rules say because they're going to "git 'er done" anyways and end up getting someone hurt or killed sooner rather than later. I don't care how much you stamp your feet, claim "engineering opinion", etc. It will happen once the priorities of safety, quality, and production get inverted.

This is not a theoretical discussion for me. I had just gone through some deep soul searching due to a near miss in which I had made the case 5 years ago that some significant changes needed to be made to a critical safety system that was bypassed. During that time I made the remarks that with current trajectory where the main priorities (safety, quality, production, politics) had been inverted to the opposite order, someone was going to be seriously hurt or killed within the next year. Less than 2 months later and almost exactly 3 weeks ago an employee and good friend of mine that lives in the same community was burned in an arc flash. He knew better but the pressure to "git 'er done" was so great that he missed a critical step, made a critical human error mistake, and paid the price for it. I have replayed the events of the past year ot two back in my mind several times. I come to the same conclusion each time. If I stood my ground, I'd be ostracized and/or terminated. If I only presented the "worst case", they'd find someone else to do it or simply ignore it. Like it or not the ship had already sailed and even if that particular employee was not severely injured it would be someone else, still someone I know or cared about, and the results would still be the same. I'm just hoping that his injuries which were relatively minor (released from the hospital 3 days later) provide a wake up call to change directions.
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