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 Post subject: Incident energy in older switchboard from directly connected generators
PostPosted: Mon Mar 04, 2013 3:28 pm 
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Joined: Sat Nov 20, 2010 10:17 am
Posts: 13
Location: Victoria British Columbia
I have a situation where the generators supplying power to a switchboard are brought up to the circuit breaker located in the switchboard with no protection between the generator and the line side of the circuit breaker. If there is a fualt on the load side of the circuit breaker, on the main bus bars or out in one of the field devices, the circuit breaker trips and limits the incident ernergy to around 7 cal/cm┬▓. If the fault is on the line side of the circuit breaker where the generator lead come into the switchboard there is no way to disconnect the generator from the switchboard and there is no differnetial protection to shut down excitation. The only way to stop the incoming power would be to manually shut down the generator thrpughthe fule shut of valve for instance. This results in an incident energy of over 136 cal/cm┬▓. So what incident energy do you put on the label or do you have more than one label? In the worst case there is no safe way to open the switchboard and test it to prove it has its power off and it is safe.


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PostPosted: Tue Mar 05, 2013 4:08 pm 
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Joined: Wed May 07, 2008 5:00 pm
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Location: Rutland, VT
A couple of thoughts:
1. Unless the incoming generator breaker is isolated from the switchgear it feeds, operation of any breakers on that switchgear would be Prohibited as the AFH is 136 cal/cm2.
2. What is the normal operation here? Are the generators performing as a stand alone system (i.e. normal configuration, no utility source), in parallel with the existing system or operate only as needed for when normal source (utility) is lost?
3. What kind of excitation system is on the generator?
4. What size generator is this? Could a arc flash relay be installed on switchgear that could shutdown excitation system and fuel supply?

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Barry Donovan, P.E.
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PostPosted: Fri Mar 08, 2013 5:04 pm 
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Joined: Tue Oct 26, 2010 9:08 am
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Location: North Carolina
In the case of generators you often cannot simplify to "one" worst case and must train your folks and label for multiple scenarios because the worst case is not typical but is reasonably possible.


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PostPosted: Mon Mar 11, 2013 2:02 pm 
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Joined: Sat Nov 20, 2010 10:17 am
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Location: Victoria British Columbia
Thanks very much for the response.

The generators are used on Navy ships they are rated for around 1363 KVA, 3 phase, 450 volt, no ground, wye connected, neutral to the AVRs only. There is no local breaker at the generator. They provide normal power to the ship when away from home. The supply cables are run from the generator to the switchboard and then connected to the generator breaker in the actual main switchboard. The system has a form of differential protection that de-excites the main excitation however they are also excited through compounding coils that provide self excitation for about half of the generator output (designed for emergency battle damage power). The differential protection currently cannot shut this off. There are many Arc Flash protection systems and other devices that we could employ to send a signal through the integrated control system to shut down the fuel rack and through the switchboard PLC to de-excite the generator.

I'm wondering how civilian systems are designed. I have seen the direct results of failure due to water ingress in the merchant Navy of this design the results of which were extremely catastrophic.

Not certain if we can apply multiple labelling in this case as the bus bars are run through the back of the switchboard and are run directly behind the control cabinets which are not entirely isolated form the bus sections.


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PostPosted: Thu Mar 28, 2013 1:47 am 
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Location: San Antonio, TX
I have been able to mitigate the AFIE at the line side of a generator breaker located in the switchboard when there is at least another parallel generator providing a sizable contribution to the line side of this breaker. Is that your case?


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