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 Post subject: Arc Flash and arrays during a power outage
PostPosted: Mon Feb 15, 2016 2:37 pm 
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Joined: Wed Nov 11, 2015 9:18 am
Posts: 6
I have a question regarding a solar array on a building the possible arc flash during a power outage.

There is a 25 kw system that consists of (4) arrays. Each array is tied into its own inverter and they all tie into a single accumulator panel. The accumulator panel is tied into the main switchboard for the building.

There is a small emergency generator at this facility, so I was running it in the software to just look at the arc-flash involved at a panel that it serves in an emergency condition.

I noticed that with the main open, and just the solar connected to the main switchboard, the arc flash at a few other panels (not served by the generator) were in excess of what the hazard was with the utility connected. Most were on the load side of transformers.

Anyone run across this situation? Is this to be believed?


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 Post subject: Re: Arc Flash and arrays during a power outage
PostPosted: Tue Feb 16, 2016 9:53 am 
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Joined: Tue Jan 13, 2009 5:00 pm
Posts: 442
No, it is not to be believed. Inverters cannot provide current much beyond nameplate, and only for a very short time. How does your software model the inverters?


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 Post subject: Re: Arc Flash and arrays during a power outage
PostPosted: Thu Feb 18, 2016 2:18 pm 
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Joined: Tue Oct 26, 2010 9:08 am
Posts: 1880
Location: North Carolina
It's a fairly large array this is easily believable and frequently happens with standby generators. As the fault current decreases, your opening time increases on even the most aggressive (very inverse) time-current curves. The decrease in current is not enough to overcome the drastic increase in trip time. Thus the net effect is an increase in incident energy.

If it's smaller then I agree that it may not be modeled correctly. Speaking to the design of the inverter itself, if the information is not given, IGBT's are rated in terms of RMS currents and usually that's what the name plate reads. If you dead shorted the IGBT you'd get the square root of two increase in current or somewhat less than 200% at most but it's a semiconductor made of very thin layers of silicon. Thus it can do this for only a very short period of time (milliseconds) before the transistors explode and provide a very fast fusing effect if you don't have semiconductor fuses. Hence the reason that semiconductor fuse opening times are typically a few milliseconds at most. Often just modelling the semiconductor fuse solves the issue by itself, it it has them. If it doesn't then you can estimate assuming the IGBTs are a fuse.


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 Post subject: Re: Arc Flash and arrays during a power outage
PostPosted: Mon Feb 22, 2016 10:40 am 
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Joined: Mon Apr 01, 2013 2:21 pm
Posts: 9
FYI:
2014 NEC Section 690.12 requires a rapid shutdown capabilities for all PV circuits on a building


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 Post subject: Re: Arc Flash and arrays during a power outage
PostPosted: Mon Feb 22, 2016 4:54 pm 
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Joined: Mon Jun 07, 2010 9:40 am
Posts: 119
Also, Most inverters are designed with anti islanding protection and will not run "off line".


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