Safety in photovoltaic roofs feeding the grid ?

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Externet
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Safety in photovoltaic roofs feeding the grid ?

Post by Externet » Sun Oct 30, 2011 2:49 pm

Hi.
Imagine your house roof has a bunch of solar panels connected to an inverter and you are happily selling power to the grid while you are at work. No mysteries there.

A tree branch falls and brings down the 7KV high voltage line. A power company crew rushes to do the repairs. After disconnecting the high voltage feed, the lineman starts the job.

The AC your inverter is putting out is transformed at the pole back to 7KV. How is safety for the poor fried lineman implemented ?

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jwax
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Re: Safety in photovoltaic roofs feeding the grid ?

Post by jwax » Sun Oct 30, 2011 3:09 pm

I hope we hear from a lineman, or fireman on this issue.
Last I heard, neither will come close to a house with PV panels on it until the owner gives the OK that the power is off and is disconnected.
Some solar arrays may be running up to 1000 VDC!
I'm sure there are automatic disconnects in place in a "built to code" installation, but the panels themselves are also a source of electrical danger.

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Re: Safety in photovoltaic roofs feeding the grid ?

Post by Smoke_Maker » Sun Oct 30, 2011 3:28 pm

I'm not a lineman and I'm not sure this is true, but I have been told that the power company approved DC to AC inverter looks at line voltage several times a second to make sure the line has not gone dead (tree branch), if sync and line voltage is OK it continues to output to the grid.

This sounds like the simplest and safest way to keep linemen safe, stop output for a microsecond and see how far the voltage drops.
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Re: Safety in photovoltaic roofs feeding the grid ?

Post by Robert Reed » Sun Oct 30, 2011 6:06 pm

"if sync and line voltage is OK it continues to output to the grid."
That would be a great feature, but if you were a line man, would you put your life on the line for some homeowners gadget that is supposed to work? Add to that-Made in China!
Line men do carry and are required to use "Noisy Sticks", a voltage sensing device on a fiberglass pole that emits a loud racket in the presence of voltage near it. Also many times, A solid earth ground is attached to the section of line being worked on especially in higher voltage sub station feeds.
Its been a while since I have worked for our local utility company and I suspect many new rules have been put in place in regards to this post.

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Re: Safety in photovoltaic roofs feeding the grid ?

Post by gerty » Mon Oct 31, 2011 6:55 am

We toured 2 installations last week that had basically what you describe. On both installations the inverters are synced (sp?) with TVA. In the event of a power loss the inverters shut down, preventing what you describe. I asked about power failures, would you have power?
The owners both replied that thier power would be down untill the inverters "saw" power coming in from TVA (and it had to be on for a few minutes ). One of them wasn't real happy about it," 144 solar panels,144 inverters, and no power when I need it ". Neither system had battery banks connected, so it was straight solar-> inverter-> TVA. The largest system we looked at was capable of 112A @220v.

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Re: Safety in photovoltaic roofs feeding the grid ?

Post by dacflyer » Mon Oct 31, 2011 3:56 pm

i have grid tied solar on my house. (2.4KW ) and yes in the event of a down line,the inverters shut down as fast as the grid does... so safety is there.. also it is a requirement ( law )
your inverters must be compliant or they will not be allowed to pass electrical inspection.
mine if it even sees so much as a hiccup in line voltage or sync, it will shot down for a few min, and then re-analyze the line condition before it reconnects

read the features for SUNNYBOY inverters.. mine is a model 3000 ( 3kw )
and yes. with most grid tie inverters if power goes down.. so does your lights..
unless you have a grid tie system with battery back up.. and even with this system, it will not allow power to be fed back into the grid.. ( power interlocks etc ) also a very much more $$$$ system.

i am sure all grid tie inverters are safety compliant.

also most linemen treat all power lines as if they were hot.. where i work we have a motto.
" if its not grounded, its not dead "

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Re: Safety in photovoltaic roofs feeding the grid ?

Post by haklesup » Mon Oct 31, 2011 4:39 pm

Its a similar situation with generators.

Any solar installation if done to code will have a transfer switch or an inverter that will shut down in a power fail situation as described previously. One of its jobs is to disconnect the house from the grid if the grid goes down. The reason is precisely what you described, so that the grid is not energized by a home when the utility expects it to be off.

As for the HV step up, stepping the voltage up to 17KV (141x higher) will result in current that is proportionally (141x) smaller. Unless your soalr panels have exceptional power output, the lineman may get a shock but it shouldn't be as lethal as usual if its just one residental PV unit powering it.

Good point DAC, a grid tied PV system may or may not have a battery and the capability to operate in the absence of the grid.

The bottom line is, it should not happen if the installation is done properly to code.

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Re: Safety in photovoltaic roofs feeding the grid ?

Post by dacflyer » Tue Nov 01, 2011 8:28 pm

right...
battery / grid systems are good for a while.. depending on your battery bank size.
still proper systems transfer relays - interlocks and such are a must,,

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