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 Post subject: Simple SKM Model Question
PostPosted: Tue Jul 03, 2012 8:17 am 

Joined: Tue Jun 19, 2012 4:45 pm
Posts: 8
Okay, I've begun modeling some 25kV switchgear. I am new to doing this kind of arc flash study, so I could use some guidance. I've done them before on the 480V equipment. But this is just a little different to me. I have run the study and the numbers I am getting for the incident energy are a bit scary. I get 117 cal/cm^2 as the result. Seems a bit high, but I don't really know. I want to make sure my methodology is okay.

I have attached a file of the simple one line. It is a utility connected to a transformer, which has a low side breaker protection (via an SEL 351S), which is the 25kV switchgear main. In case anything is unreadable, I will retype the information here:

Utility: 138kV
3PH: 27608A
LG: 18815A
X/R: 6.87

TX: 138-27kV
56MVA
Z= 9.26%
Z+ & Z0= .2901+j9.255

Breaker: Powell
27kV
2000A

Relay: SEL351S
Phas Time Overcurrent Settings:
CTR: 1500/5
Pickup: 4.75
Curve: U1
Time Dial: 2
(No inst or def time settings)

If anyone could take a look at what I am getting and see if I am doing it properly? This just doesn't set well with me. I really would be more comfortable if I had a second set of eyes here.


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PostPosted: Tue Jul 03, 2012 9:52 am 

Joined: Fri Mar 16, 2012 7:14 pm
Posts: 7
pazzjl wrote:
Okay, I've begun modeling some 25kV switchgear. I am new to doing this kind of arc flash study, so I could use some guidance. I've done them before on the 480V equipment. But this is just a little different to me. I have run the study and the numbers I am getting for the incident energy are a bit scary. I get 117 cal/cm^2 as the result. Seems a bit high, but I don't really know. I want to make sure my methodology is okay.

I have attached a file of the simple one line. It is a utility connected to a transformer, which has a low side breaker protection (via an SEL 351S), which is the 25kV switchgear main. In case anything is unreadable, I will retype the information here:

Utility: 138kV
3PH: 27608A
LG: 18815A
X/R: 6.87

TX: 138-27kV
56MVA
Z= 9.26%
Z+ & Z0= .2901+j9.255

Breaker: Powell
27kV
2000A

Relay: SEL351S
Phas Time Overcurrent Settings:
CTR: 1500/5
Pickup: 4.75
Curve: U1
Time Dial: 2
(No inst or def time settings)

If anyone could take a look at what I am getting and see if I am doing it properly? This just doesn't set well with me. I really would be more comfortable if I had a second set of eyes here.


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PostPosted: Tue Jul 03, 2012 9:58 am 

Joined: Fri Mar 16, 2012 7:14 pm
Posts: 7
Please see attached file.
BTW I changed the CB and relay positions on your oneline.
The high incident energy (IE) is due to the lack of protective device on the transformer primary.
The IE is higher at the lineside of CB.
When you connect the load the IE is likeky to get even higher because of the load contribution to sc.


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PostPosted: Tue Jul 03, 2012 10:17 am 

Joined: Tue Jun 19, 2012 4:45 pm
Posts: 8
FGuillenS wrote:
Please see attached file.
BTW I changed the CB and relay positions on your oneline.
The high incident energy (IE) is due to the lack of protective device on the transformer primary.
The IE is higher at the lineside of CB.
When you connect the load the IE is likeky to get even higher because of the load contribution to sc.


Thank you for your help. I have just now gotten the information for the hi side protective device. However, in my mind, it doesn't seem that it would really make a difference. I didn't have it at the time I did the intial calculation, but I didn't see how it would make much difference at the switchgear bus. If I am wrong in my thinking, please correct me.

The high side breaker (connected between the utility and the xfmr) is as follows:

McGraw Edison 145kV Breaker
2000-4000A
Protected by an SEL351
CTR: 300/5
Pickup: 5.2
Curve: U2
Time Dial: 2.5
No Instantaneous or Definite Time

It looks like, however, that my numbers were indeed correct.

To be a bit more clear, I am only really concerned with the numbers for the switchgear. We need to place the category ratings on the switchgear. This is the reason for the study.

Again, thanks so much for your help. It's good to see that I am on the right track. Perhaps this is a spot where our engineers may need to coordinate again and consider an instantaneous setting. That 30+ cycle trip time doesn't seem right to me.


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PostPosted: Tue Jul 03, 2012 11:07 am 
Sparks Level

Joined: Thu Jul 07, 2011 6:03 am
Posts: 64
Location: Netherlands
The high incident energy is simply the result of almost 12 kA of arcing current that takes 600 ms to clear. Adding an overcurrent stage with faster tripping will help here, but if you make it instantaneous you'll run into coordination issues downstream.

As for the upstream data, it certainly affects the line side calculation. Although technically not 'the switchgear bus', it applies to work done at the switchgear on the incoming cubicle.


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PostPosted: Thu Jul 05, 2012 5:28 am 
Arc Level

Joined: Wed Jun 04, 2008 9:17 am
Posts: 428
Location: Spartanburg, South Carolina
This might be a good application for a maintenance switch that asserts an input to the SEL-351. The input would be used to activate an instantaneous overcurrent element when maintenance is being performed.


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PostPosted: Thu Jul 05, 2012 8:00 am 
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Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
SKM also defaults to using the Lee method for calculations above 15 kV. This model is known to overstate the incident energy and the amount that it overstates it increases as the voltage and/or current increases. Note also that in your SKM results, it will indicate that the results are invalid for enclosed switchgear and apply to open line gear only.


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PostPosted: Mon Jul 16, 2012 1:28 pm 

Joined: Thu Feb 04, 2010 5:00 pm
Posts: 36
Location: Camp hill Pa
Yes 25kV Arc Faults can be very high energy. However, outside thse voltages are serviced by either utility workers or others who generally are trained to work on them with 10 foot poles or in buckets with faraday shields that a re connected to the power, for them to work on a single line at a time.



This is realm of requiring well train knowedgable operators.

The air and space muesum showed a film of a worker lifted by helicopter to a 350 kv line and then walking up the line with tools to the pole area to tighten bolts on the insulators. Don't ask me to do this,


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PostPosted: Mon Jul 16, 2012 1:46 pm 

Joined: Thu Feb 04, 2010 5:00 pm
Posts: 36
Location: Camp hill Pa
Farther to the above, most switchgear is designed so all maintence can and should be done without exposure to high voltage. There is even equiment to remotely rack in circuit breakers while the person is remote to the unit. Metering should be provided with in the switchgear with low-voltage test points for checking. Additionally circuit bereakers and PT transformers should be drawn out for maintenance.

A local utility requires a door with a protective mesh inside the outer door on the main wiring compartment of switchgear. They also require enough distance around the gear to allow use of hotsticks. Using an 8 foot for working distance lowers the energy considerably.

I have watched new gear being put on line by PPL where the tie door breaker compartment was opened and the shutters raised so voltmeters on two high stickers could be used to check for proper phasing between the two sources. Both workers wore rubber gloves, arm sleeves, heavy clothing face shield, were over 10 feet away from the actual meter. don't do this at home without training!!!!!!!!!!!!!!!


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