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 Post subject: Substation Switching
PostPosted: Mon Feb 20, 2012 8:01 am 
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Location: Cedar Rapids, IA
So glad I stumbled on this site - it's excellent! I hope someone can weigh in with some good guidance. I'm trying to gain perspective on the relative hazards associated with switching in substations or even line switching (open-air switching, if you will). Logically, it doesn't seem that it would be as hazardous as inside a room.

I have some arc flash study results that suggest a potential for burn injury at distances as close as 8.3' for some substation switching operations. Our employees and supervisors are insisting that even at that distance and closer, substation and line switching is not a significant safety concern.

Thoughts?


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PostPosted: Mon Feb 20, 2012 9:30 am 
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Rex Butler wrote:
So glad I stumbled on this site - it's excellent! I hope someone can weigh in with some good guidance. I'm trying to gain perspective on the relative hazards associated with switching in substations or even line switching (open-air switching, if you will). Logically, it doesn't seem that it would be as hazardous as inside a room.

I have some arc flash study results that suggest a potential for burn injury at distances as close as 8.3' for some substation switching operations. Our employees and supervisors are insisting that even at that distance and closer, substation and line switching is not a significant safety concern.

Thoughts?

If you are an IEEE member check IEEE Explore for articles written by Erling Hesla and G. Parise for the Industrial Applications Society (IAS). I believe they have an article on this subject.

I do not have practical experience on this subject, however, one way to look at the problem may be to create a table for yourself of all the potential conditions you may encounter (state table, can find in Wikipedia), including failure modes for device operation, sensing devices, communications, wiring, etc. A failure of your switching procedure, I presume, would have serious consequences even if you mitigate the safety aspects to an acceptable level. So an FMEA (can find in Wikipedia) of the procedure may be a way to create a systematic way to look at the potential failuresd, potential severity of the consequences, probability of identified failures, etc. Then look at how to mitigate at least the more serious ones.


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PostPosted: Mon Feb 20, 2012 10:22 am 
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Arc hazards at utility substations and lines is covered by the NESC. Check out the NESC section of this forum. The NESC used ARCPRO, from Kinetrics, to calculate the MV and HV table values for arc hazards. The calculations are based on single-phase faults and are less conservative than what you would get using Lee's theoretical formulas. As for any arc hazard analyses, the energy depends on the fault clearing time. If there is bus differential protection that clears faults quickly, then the arc incident energy in substations is low. If overcurrent relays are used that have to be coordinated with feeder overcurrent relays, then clearing times, and incident energy, can get high.


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PostPosted: Mon Feb 20, 2012 8:01 pm 
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Lee method is commonly implemented in most software packages because it is publicly available. However, it only gives so-so results up to 600 V. In this range the one advantage of it is that very little information is needed to use the calculation. As the voltage and/or current climbs however, it does not come anywhere near matching actual test data, and overestimates the actual hazard by a great deal.

The ArcPro model is not published so it's hard to tell much of anything about it except to say that it is the de facto standard among utilities and is used by NESC above 15 kV. Below 15 kV, IEEE 1584, warts and all, is the de facto standard. Neal and Doughty seems to have fallen out of favor except that it does a better job with fuses operating in their current limiting region. Practically every software package that claims to use IEEE 1584 tweaks it a bit by including motor transient data and by doing actual fuse melting curve calculations and feeding this to IEEE 1584 rather than using the model as published (at least that's what ETAP and SKM do).

There is no test data above 1 kV by the way for arc-in-a-box either that I'm aware of.

Now the reality is that as long as you use the open air values, arc flash hazards are significantly lower than they are for arc in a box cases. Second, you MUST put in the proper working distances when considering substation work. In my case we used roughly the same distances and the numbers came out very low. Don't forget too that the goal is to model the incident energy at the face/chest area which is the reason that even on panel boards, the working distance assumption is 18".

There are also some problems with the test data as well. Ferraz-Shawmut (II forget the new name) has a technical article where they tested the equivalent of metal clad gear with phase barriers. The results were very different since the fault never went 3 phase. Similar results happen in overhead lines...for all practical purposes almost all faults are L-G, with the occasional very rare L-L. I haven't ever seen any experimental data for L-L-L, though I have recently seen an Autojet LBS which clearly had a L-G-L fault at 23 kV going on across it judging by the damage.

Another thing to remember is that you MUST model line impedance. For instance at our main sub (we're a large 80 MW industrial site), fault currents are around 15-20 kA. But move even a short distance (a mile) away, and the maximum fault current decreases dramatically. This happens in the low voltage world too. Fault currents (and incident energies) right at a transformer secondary are vastly higher than they are away from the first bus.

The rest of the comments are pretty accurate. If your fault clearing times are really long, the incident energy calculation will be very high.

Keep in mind too that there are still a lot of problems with open line modelling. It doesn't account for circuit geometry. Plus at a certain point the thermal energy problem is far less than the plasma issue. EPRI has done some recent test work on this.

[media=youtube]fZP47mlELSc[/media]


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PostPosted: Tue Feb 21, 2012 7:00 am 
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Great post Paul


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PostPosted: Mon Feb 27, 2012 10:54 am 
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This information is very helpful and the video is an excellent resource, as well. Thanks to everyone who has offered guidance - especially you, Paul - thanks for taking the time to include such a thorough and detailed response.

Since I submitted this question, it appears that the switch handles fixed on the structures in our substations are either outside of MAD or we can engineer the hazard out through isolation.

We still have a bit to go, but it looks like we'll be able to establish adequate policies for employee protection in substations and on our lines.

Thanks again!


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