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 Post subject: Labeling Tie Breakers
PostPosted: Thu Dec 17, 2009 4:16 pm 

Joined: Thu Jun 11, 2009 12:38 pm
Posts: 33
Location: Kansas city
Hi
I need your opinion on finding the right approach to labeling a tie breaker cubicle. I have a 4160V switchgear fed from a 69kV/4160V transformer. This switchgear is also tied to another bus that is fed from a bus tie transformer (i.e. a spare transformer). In the normal operating scheme in the plant, the tie breaker is normally open.
Now that we are looking to label the tie breaker (located inside the switchgear), what all scenarios should you consider to calculate the worst case incident energy? Do you just calculate the incident energy on either side of the tie breaker based on the switching schemes where either the main transformer is feeding or the bus tie transformer is feeding? Is it necessary to parallel the switchgear to obtain the worst case result? (Paralleling gives more than 100 Cal/cm2 in IE with large boundaries!!)
The switchgear has 30 sections. Each section takes one entire column feeding electrical equipment. The arc flash label on these sections are based on worst case result obtained by considering the normal scenario & bus tie scenario. I am just confused about labeling the tie breaker section.

Any input will help.

Thank you for your time.
Best Regards.


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PostPosted: Thu Dec 17, 2009 7:21 pm 
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Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
If you look carefully at your tie arrangement and consider where the fault might be, you will notice that the tie breaker will only see the fault from one source to interrupt. The tie situation will only need to be considered on the downstream breakers with the increased fault duty.

The only other issue I can think of would be motor contribution from one of the busses to the fault if the tie has to clear it as well.


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PostPosted: Fri Dec 18, 2009 7:36 am 
Sparks Level

Joined: Thu Jul 19, 2007 7:54 am
Posts: 201
Location: St. Louis, MO
amohammed wrote:
In the normal operating scheme in the plant, the tie breaker is normally open.
Now that we are looking to label the tie breaker (located inside the switchgear), what all scenarios should you consider to calculate the worst case incident energy? Do you just calculate the incident energy on either side of the tie breaker based on the switching schemes where either the main transformer is feeding or the bus tie transformer is feeding? Is it necessary to parallel the switchgear to obtain the worst case result? (Paralleling gives more than 100 Cal/cm2 in IE with large boundaries!!)


Do you have procedure for paralleling the transformers? If so, look at them and change them so that the tie breaker is not the last to be operated, and put a note as to the reason why.

Without procedures, you may have to label for the tie configuration. Without good training, doing otherwise could lead to some very bad results!


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PostPosted: Fri Dec 18, 2009 8:33 am 

Joined: Thu Jun 11, 2009 12:38 pm
Posts: 33
Location: Kansas city
The switchgear is not intended to operate in parallel. I figured the tie breaker is like a switch with power available from both ends i.e. the spare transformer is energized and is floating. If someone's to parallel it and then create a fault then it would contribute to the worst possible IE you can imagine.

So far I get the impression that I label the tie breaker based on fault currents available from any one source. In my case, it will be through the bus tie transformer.


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PostPosted: Fri Dec 18, 2009 9:30 am 
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Joined: Tue Jan 13, 2009 5:00 pm
Posts: 532
Not sure I agree with the consensus here. Generally you look at the clearing time for the upstream device from the location you are working, not the local device. The tie breaker may have two upstream devices, one on each source. Consider drawing out a tie breaker that had failed to open a pole.


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PostPosted: Tue Apr 27, 2010 4:57 pm 
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Joined: Mon Dec 14, 2009 4:24 pm
Posts: 61
This is a main-tie-main setup? The problem that immediately springs to my mind (because it is one of my problems) is racking out the tie. In that case you have the potential for paralleled sources. If I am understanding you correctly it would be a good idea to create a switching order that one side or the other be de-energized before doing maintenance on the tie.


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PostPosted: Tue Nov 09, 2010 6:07 pm 

Joined: Tue Nov 09, 2010 5:35 pm
Posts: 1
The problem described is that when the Tie breaker is open, it is hot from separate sources on both sides. 1 source (Call it "A" bus) supplies voltage to 1 side of the tie; another source (Call it "B" bus) supplies voltge to the other side of the tie. In case of a fault, say during racking, on EITHER of these sides, ionization can/will likely cause ionization and fault BOTH sides - and BOTH sources contribute to the fault at that location. Note that this is much different from all the other feeders on the "A" or "B" buses - because the tie has 2 sources! So - how to label a tie breaker? Provided the sources are independant, best to calculate the cal/cm2 on "A" bus and "B" bus independantly and add the result. In fact, using some industry software packages, if you close both mains and the tie, and then calculate the fault on a bus (which is really what's going on), the result of the program is just as I described - a sum of the 2 buses independantly calculated.


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