fire on pv systems

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GoldDigger

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The article is not clear about the exact root cause of the failures, but just as with AC the AFCI will detect some series or parallel arcs, but will never detect a glowing connection. In the case of a parallel arc in a grounded system, opening the circuit at an AFCI may not stop the arc, since the AFCI is on the load end of the panel wiring.
 
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An AFCI should detect an arcring fault anywhere in the array, open the circuit and shut down the inverter. This absolutely does not guarantee that this will prevent a fire but should alert the user that there's a problem. I've heard of rooftop fires having the fault condition for days before the fire breaks out.
 

GoldDigger

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An AFCI should detect an arcring fault anywhere in the array, open the circuit and shut down the inverter. This absolutely does not guarantee that this will prevent a fire but should alert the user that there's a problem. I've heard of rooftop fires having the fault condition for days before the fire breaks out.
A good point. Just no guarantee, though.
I would not be confident enough in it to be at ease with known defective panels that have not blown yet, but I might be less anxious. :)
 

ggunn

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An AFCI should detect an arcring fault anywhere in the array, open the circuit and shut down the inverter. This absolutely does not guarantee that this will prevent a fire but should alert the user that there's a problem. I've heard of rooftop fires having the fault condition for days before the fire breaks out.

I talked with an SMA rep at SPI a couple of years ago when they were bringing their AFCI inverters on line, and he told me that they could detect a series arc but not a parallel arc. Also, an inverter shutting down, especially if it's a large distributed system (lots of smaller inverters) may not get attended to for quite a while. Someone has to be paying attention. I know of such a system where an undertorqued connection in an inverter caused a arcing fault which destroyed the inverter, and no one knew about it for months.
 
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SolarPro

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The types of failures that are happening in these BP modules are exactly the failure modes that the new arc-fault requirements are intended to mitigate. Bad internal connections. Failing bypass diodes. Junction box detachment. These are all characterized by series arc-faults.

Part of the problem with the legacy BP modules is that the bypass diodes were apparently not properly rated for the application. There may be other quality control issues as well, but the bypass diode issue is documented. BP had recalls circa 2006?2007.

Here's info from BP's website:

http://www.bp.com/content/dam/bp-alternate-energy/en/documents/bp-solar-product-advisory-2012.pdf
 
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GoldDigger

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Bad internal connections. Failing bypass diodes. Junction box detachment. These are all characterized by series arc-faults.

Part of the problem with the legacy BP modules is that the bypass diodes were apparently not properly rated for the application. There may be other quality control issues as well, but the bypass diode issue is documented. BP had recalls circa 2006?2007.

Here's info from BP's website:

http://www.bp.com/content/dam/bp-alternate-energy/en/documents/bp-solar-product-advisory-2012.pdf
I will have to read the paper more thoroughly, but bad connections, failed bypass diodes and even cracked cells are IMO much more likely to produce heat by a glowing connection than by a series arc fault.
The probability of a sustained series arc with DC is higher than on a comparable voltage AC system though.
 

Zee

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The types of failures that are happening in these BP modules are exactly the failure modes that the new arc-fault requirements are intended to mitigate.

Yes, good to know.
I have seen the module j boxes burnt, and the problem is in that area. I am first to admit i was a big fan of BP solar for a while.....i have a couple systems being replaced.

Goldd and GGunn,
But wouldn't the glowing LIKELY not cause a fire (panel up on rail)
whereas
the glowing eventually leads to arcing (when the glowing burns thru and creates a gap),
at which point it is a fire hazard and also extinguished by the AFCI?
 

SolarPro

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bad connections, failed bypass diodes and even cracked cells are IMO much more likely to produce heat by a glowing connection than by a series arc fault.

Arcing is the logical outcome of high resistance connections. Say you have a hot spot internal to a module and the heat causes internal connections to degrade. If the tabbing or stringing opens up, you have a dc arc. The same type of signature will accompany a poor mechanical connection in the module wiring, whether in a module-to-module connection or a connection at the junction box. In other words, glowing leads to arcing. Failure analysis indicates that arcing faults are the primary cause of PV initiated "thermal events" in the US and in Germany.

I am first to admit i was a big fan of BP solar for a while

We installed a lot of BP systems as well. I know companies that were basically exclusive BP dealers. I'm sure they got burnt by that. (Pun intended.)
 
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I talked with an SMA rep at SPI a couple of years ago when they were bringing their AFCI inverters on line, and he told me that they could detect a series arc but not a parallel arc. Also, an inverter shutting down, especially if it's a large distributed system (lots of smaller inverters) may not get attended to for quite a while. Someone has to be paying attention. I know of such a system where an undertorqued connection in an inverter caused a arcing fault which destroyed the inverter, and no one knew about it for months.

Interesting, I suppose it is very difficult to detect parallel arcs reliably with any means. I'm looking at a Santon arc detection unit that also says it is meant to detect series arcs and nothing else. A positive to negative arc (or solid fault) would be very difficult to detect and may not stop until modules are disconnected.
 

ggunn

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Location
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Electrical Engineer - Photovoltaic Systems
Interesting, I suppose it is very difficult to detect parallel arcs reliably with any means. I'm looking at a Santon arc detection unit that also says it is meant to detect series arcs and nothing else. A positive to negative arc (or solid fault) would be very difficult to detect and may not stop until modules are disconnected.

Even if an inverter could detect a parallel fault and shut down, it wouldn't help with a parallel fault. It seems to me that the inverter shutting down might even make the arc worse by removing a parallel path for the current.
 

GoldDigger

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Even if an inverter could detect a parallel fault and shut down, it wouldn't help with a parallel fault. It seems to me that the inverter shutting down might even make the arc worse by removing a parallel path for the current.

Well, it would certainly raise the available voltage to maintain the arc from ~Vmp to ~Voc. I don't know if the current split will matter much, although if a lot of current goes into the arc the MPP sweep of the inverter might pull the voltage low enough to quench the arc. :)
 

jaggedben

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Well, it would certainly raise the available voltage to maintain the arc from ~Vmp to ~Voc. I don't know if the current split will matter much, although if a lot of current goes into the arc the MPP sweep of the inverter might pull the voltage low enough to quench the arc. :)

It seems to me that the inverter reached the bottom of it's MPP range it would stop there, but votage would still be a high enough voltage to sustain an arc since MPP ranges usually bottom out at hundreds of volts..
 

GoldDigger

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It seems to me that the inverter reached the bottom of it's MPP range it would stop there, but votage would still be a high enough voltage to sustain an arc since MPP ranges usually bottom out at hundreds of volts..
Depending on the geometry, the bottom of the MPPT range might be below the arc sustaining voltage for the gap. Probably unlikely though...
 
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That would be the best possible thing that could be done from the inverter end for s series fault. But it would be probably the worst possible thing to do for a series fault.

Agreed that it would be best case for parallel (I assume you meant that) and worst for series. An intentional short circuit would probably never fly with the UL or ETL. All of the components in the DC system will be able to handle the short circuit current though.

With these cases where the inverter is blind to the problem then helpless to do anything about it, I think the best approach is to not have an arc fault in the first place.
 

Zee

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Location
CA
On a solaredge system with dc optimizers strung together in a string as opposed to the modules......
where is the AFCI: optimizer or Inverter?

If in the optimizer, couldn't that be a great way to detect or extinguish parallel and series arc faults?
 
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