SolarEdge string fusing

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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
According to this tech note

http://www.solaredge.com/files/pdfs/string_fusing_requirements.pdf

you don't need to fuse PV strings when SolarEdge optimizers are used and you combine in parallel more than two strings.

I am designing a PV system with three strings per inverter, and eliminating fused combiners would be a significant savings. My AHJ wants to see something more than a tech note from Solar Edge, though.

What do you think?
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
According to this tech note

http://www.solaredge.com/files/pdfs/string_fusing_requirements.pdf

you don't need to fuse PV strings when SolarEdge optimizers are used and you combine in parallel more than two strings.

I am designing a PV system with three strings per inverter, and eliminating fused combiners would be a significant savings. My AHJ wants to see something more than a tech note from Solar Edge, though.

What do you think?
Since the purpose of the string fuse is not to protect the string against a short circuit but rather to protect a damaged string against reverse current driven by the other strings, I can see that the optimizers may not have the same vulnerabilty to reverse current that unprotected panels would have.
From the point of view of the panels themselves they do not know that they are in a string and they are not subject to reverse voltage from any source.

But that is just physics and logic, which cannot be relied on when interpreting code. I would have to hang my argument on the fact that there are, electrically, no parallel strings of panels in the resulting system, just parallel strings of optimizers.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Since the purpose of the string fuse is not to protect the string against a short circuit but rather to protect a damaged string against reverse current driven by the other strings, I can see that the optimizers may not have the same vulnerabilty to reverse current that unprotected panels would have.
From the point of view of the panels themselves they do not know that they are in a string and they are not subject to reverse voltage from any source.

But that is just physics and logic, which cannot be relied on when interpreting code. I would have to hang my argument on the fact that there are, electrically, no parallel strings of panels in the resulting system, just parallel strings of optimizers.
I agree but the AHJ wants to hear it from UL or someone like that. I'll probably just have to fuse them.
 

TommyO

Member
Location
Sunnyvale, CA
I agree but the AHJ wants to hear it from UL or someone like that. I'll probably just have to fuse them.

You can see if Solaredge will say that without fuses is the recommended installation method. If they can say their UL listing includes installing 3 strings in parallel with no fuses, perhaps that's enough for the AHJ.
 

Zee

Senior Member
Location
CA
what they said.

Curious tho...how many panels, on what size inverter?
Given that up to 25 @ P300 optimizers (and hence panels) can be strung together in one string,....why not combine into just two strings?
2 strings at 25 panels each = at least 50 panels @ 250 W = 12.5 kw.
Is your system bigger?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
what they said.

Curious tho...how many panels, on what size inverter?
Given that up to 25 @ P300 optimizers (and hence panels) can be strung together in one string,....why not combine into just two strings?
2 strings at 25 panels each = at least 50 panels @ 250 W = 12.5 kw.
Is your system bigger?
Yes.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
what they said.

Curious tho...how many panels, on what size inverter?
Given that up to 25 @ P300 optimizers (and hence panels) can be strung together in one string,....why not combine into just two strings?
2 strings at 25 panels each = at least 50 panels @ 250 W = 12.5 kw.
Is your system bigger?

Note that there is also a power limit on string size, not just an optimizer count limit.
For the P300 on single phase its 5250W. So say if you have 260W panels, that's only 20 panels.
 

Carultch

Senior Member
Location
Massachusetts
what they said.

Curious tho...how many panels, on what size inverter?
Given that up to 25 @ P300 optimizers (and hence panels) can be strung together in one string,....why not combine into just two strings?
2 strings at 25 panels each = at least 50 panels @ 250 W = 12.5 kw.
Is your system bigger?

You are limited to 15A per optimizer, and the optimizers' operating voltages add up to a nominal voltage. 350V, 400V, and 800V are three examples, depending on which inverter and grid voltage system you have.

The power per string can be no greater than 15A * inverter input nominal operating voltage.

If the system is on a building, and you want to meet the rapid shutdown rule with this system, then you are also limited to 30 optimizers in a string for the 1 V per optimizer open circuit SafeDC condition. So in commercial applications where you use the 800Vdc input at the inverters, not only do you GET TO gang up two modules on the larger optimizers, in some cases you HAVE TO.
 

shortcircuit2

Senior Member
Location
South of Bawstin
According to this tech note

http://www.solaredge.com/files/pdfs/string_fusing_requirements.pdf

you don't need to fuse PV strings when SolarEdge optimizers are used and you combine in parallel more than two strings.

I am designing a PV system with three strings per inverter, and eliminating fused combiners would be a significant savings. My AHJ wants to see something more than a tech note from Solar Edge, though.

What do you think?


This application note shows fusing...

http://www.solaredge.us/files/pdfs/application_notes_string_combiners_na.pdf

These instructions on page 41 have a note regarding fusing on more than 2 strings...

http://www.solaredge.com/files/pdfs...e-and-three-phase-inverter-user-manual-na.pdf


And in section 240.21, conductors need protection at the source.

So is fusing required in DC optimizer circuits under 240.21?
 

SolarPro

Senior Member
Location
Austin, TX
SolarEdge claims:

PV Modules
The power optimizer backfeed limitation of 0A prevents any backfeed current from reaching the modules, regardless of module fuse rating.

If this is supported by NRTL testing/certification, then Exception B under 690.9(A) would seem to apply:

Exception: An overcurrent device shall not be required for PV modules or PV source circuit conductors sized in accordance with 690.8(B) where one of the following applies:

(a) There are no external sources such as parallel-connected source circuits, batteries, or backfeed from inverters.

(b) The short-circuit currents from all sources do not exceed the ampacity of the conductors and the maximum overcurrent protective device size rating specified on the PV module nameplate.

Regardless of whether an AHJ or plan checker agrees with this logic, you can connect 25 SolarEdge power optimizers per source circuit in single-phase applications and 50 in 3-phase applications. So your number of series strings is greatly reduced. In most cases you won't have more than one or two source circuits per inverter.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
SolarEdge claims: [three strings in their technology do not need fusing]
If this is supported by NRTL testing/certification, then Exception B under 690.9(A) would seem to apply.
As far as I have been able to ascertain, it hasn't, and that's what my local AHJ wants to see to sign off on the exemption from fusing on three strings into SolarEdge. A white paper issued by Solar Edge is not enough. If anyone knows of any NRTL documentation supporting SolarEdge in their assertion, please post a link.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
This application note shows fusing...

http://www.solaredge.us/files/pdfs/application_notes_string_combiners_na.pdf

These instructions on page 41 have a note regarding fusing on more than 2 strings...

http://www.solaredge.com/files/pdfs...e-and-three-phase-inverter-user-manual-na.pdf


And in section 240.21, conductors need protection at the source.

So is fusing required in DC optimizer circuits under 240.21?

It would be nice if all those documents had dates so you could see for sure that the link ggunn originally posted post-dates the app note that you found. Hmph.

240.21 is somewhat irrelevant considering that 690.9 modifies the requirements.

FWIW, I ran three strings on a recent system with no fusing per the application note ggunn posted, and the AHJ didn't challenge it. But if they had I probably would have just opted for a combiner.
 

SolarPro

Senior Member
Location
Austin, TX
Content in Chapters 5–7 supplement or modify requirements in Chapters 1–4.

So the OCP requirements in 240.21 apply generally to electrical systems, and the OCP requirements in 690.9 apply specifically to PV systems.

FWIW: NEC 2017 will introduce the concepts of dc-to-dc source and output circuits, which will help with these issues.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
What part of 690.9 modifies 240.21 as it relates to DC optimizer circuit wiring?

All I'm saying is that you can pretty much focus on 690.9 and forget about 240.21. Everything in 240.21 except the first paragraph is irrelevant because we are not talking about branch circuits, feeders, feeder taps, or transformer secondaries. The only relevant requirement in 240.21 is 'at the point where conductors receive their supply' which is pretty much reiterated by 690.9's 'at the source'. (And nevermind that that we all ignore whatever this means and put the OCPDs at the point of parallel connection for very, very good engineering reasons that should be codified.)

As far as DC-to-DC optimizers go, I believe the current code is extremely vague on that. I don't believe that optimizer circuits are PV source circuits. They may be PV output circuits and I do believe they are inverter input circuits. How the rules in 690.9 apply to them is really entirely open to interpretation, but I don't believe that going back to 240 is going to help you figure that out either.
 

shortcircuit2

Senior Member
Location
South of Bawstin
OK then...until 2017 addresses it specifically(if it does)...this is what I have.

The ungrounded conductors of the output of a DC-to-DC converter are not specifically addressed in article 690 with regard to overcurrent protection.

So we must fall back to article 240 for guidance for overcurrent protection of these ungrounded conductors. Section 240.15(A) requires a fuse or circuit breaker in each ungrounded conductor and 240.21 requires it to be at where the conductor receives its supply.

So the DC-to-DC converter output needs overcurrent protection as required in article 240 where ungrounded conductors receive their supply.
 
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