Technical question regarding string inverters and watts per string

[MARKSDCA]

So I have 26 panels on a Fronius 6000 watt inverter. My max typical wattage is 5600-5800 at mid-day.

I have three strings of 9-9-8 panels.

I have a leftover panel identical to the 26 panels. I also have a place to mount it that would not require ANY new racking as it would just fit into an empty space on the existing setup....I just need the clamps.

The question is.....does it matter where I add it? Can my final config be 10-9-8 or does it have to be 9-9-9? I would kind of like to just add it to the string that is physically closest to where it would be mounted. I think that would make it 10-9-8. Otherwise I think there would need to be a bunch of wire run to tie it into the string of 8 which isn't close by but doable I suppose.

I don't need to do this right away...I was thinking that since I already have the spare panel and the system will lose efficiency over the next few years...I could add it then and compensate for the "aging" of the panels. Thoughts?? TIA

[solarix]

Fronius inverters just have a single input so all the strings are wired in parallel and ought to be the same length. Interesting that the installed design is one panel short on one of the strings. The added panel should really be added to the string of 8 so that they will all be the same. I also wonder why they did not originally wire it as 2 strings of 13? You could go find the "Fronius online string sizer" and play with different combinations.

[MARKSDCA]

Fronius inverters just have a single input so all the strings are wired in parallel and ought to be the same length. Interesting that the installed design is one panel short on one of the strings. The added panel should really be added to the string of 8 so that they will all be the same. I also wonder why they did not originally wire it as 2 strings of 13? You could go find the "Fronius online string sizer" and play with different combinations.

I believe there are 6 DC input terminals in my IG Plus A 6.0

[solarix]

Yes, but they are not independent. Those terminals are where the strings get paralleled (or combined) and should have fuses for each string.
If it makes you feel any better, there are no current inverters (not counting the kind with micro inverters or optimizers) that have three independent inputs (each with its own MPP tracking). ABB and SMA brands have dual MPPT inputs though.

[MARKSDCA]

So would 10-9-8 be counter productive? Is 9-9-9 the only way to go? Is this as simple as running a wire from the new panel to the first panel in the string of 8?? All of the panels on my roof are on the same plane and completely shadeless so except for before 8am or after 5pm (trees near the horizon) they are in uniform full sun.

[ButchDeal]

So would 10-9-8 be counter productive? Is 9-9-9 the only way to go? Is this as simple as running a wire from the new panel to the first panel in the string of 8?? All of the panels on my roof are on the same plane and completely shadeless so except for before 8am or after 5pm (trees near the horizon) they are in uniform full sun.

parallel strings need to be equal. You are hurting yourself with the 9-9-8 now, a 10-9-8 would be little more effective than an 8-8-8
Note: we are talking about the electrical wiring not the physical layout of the modules.

[ButchDeal]

Yes, but they are not independent. Those terminals are where the strings get paralleled (or combined) and should have fuses for each string.
If it makes you feel any better, there are no current inverters (not counting the kind with micro inverters or optimizers) that have three independent inputs (each with its own MPP tracking). ABB and SMA brands have dual MPPT inputs though.

The new Fronius Symo systems do have dual MPPT as well.

http://www.fronius.com/cps/rde/xchg/SID-6B43002C-36462F75/fronius_international/hs.xsl/83_30208_ENG_HTML.htm#.VgigBo_BzRY

Though, it doesn't sound like the OP has one here.

[MARKSDCA]

parallel strings need to be equal. You are hurting yourself with the 9-9-8 now, a 10-9-8 would be little more effective than an 8-8-8
Note: we are talking about the electrical wiring not the physical layout of the modules.

Hmmm...so maybe I should be adding that spare panel NOW to the string of 8 so I have 9-9-9? How much am I "hurting" myself having 9-9-8 presently? It sounds like I could be making even more and increase the yield of the whole system.

I'm currently making MORE than twice as much energy as I am consuming so no urgency here. I want to buy an EV but I should still be making more that I need after that.

[bcroe]

Hmmm...so maybe I should be adding that spare panel NOW to the string of 8 so I have 9-9-9? How much am I "hurting" myself having 9-9-8 presently? It sounds like I could be making even more and increase the yield of the whole system.

I'm currently making MORE than twice as much energy as I am consuming so no urgency here. I want to buy an EV but I should still be making more that I need after that.

I am guessing shading isn't generally a problem with these panels. If you put a clamp on ammeter on
each of the 3 strings in good sun, you would probably find the 9s are putting out WAY MORE current than
the 8 panel string. 3 strings of 9 would work well if the addition didn't see different shade than the rest
of that string.

I would not be surprised that 3 strings of 8 would put out more than 9-9-8, because 24 panels would be
working hard instead of 18. Or, the mppt may be dragging the voltage down to that of 3 strings of 8.

With temperatures hitting 25 deg F below zero here, I would not wire 13
panels of 60 cels in series, but your situation may be different. Bruce Roe

[inetdog]

Hmmm...so maybe I should be adding that spare panel NOW to the string of 8 so I have 9-9-9? How much am I "hurting" myself having 9-9-8 presently? It sounds like I could be making even more and increase the yield of the whole system.

I'm currently making MORE than twice as much energy as I am consuming so no urgency here. I want to buy an EV but I should still be making more that I need after that.

Right now the 9-9-8 configuration will cause the MPPT tracker algorithm in the inverter to either:
1. operate all three strings at the voltage appropriate for an 8 panel string, giving you in effect only 24 panels (8-8-8).
2. operate all three strings at the voltage appropriate to an 9 panel string, with the 8 panel string not contributing at all. Effectively only 18 panels (9-9-0)
3. somewhere in between in voltage so that neither the 8 nor the 9 panel string are putting out their optimum output.

In any case, the result will not be noticeably better than 8-8-8.

If you went to 10-9-8, the results would be even worse, and you might get only the power appropriate to a total of 10 panels or 18 panels.

[MARKSDCA]

Right now the 9-9-8 configuration will cause the MPPT tracker algorithm in the inverter to either:
1. operate all three strings at the voltage appropriate for an 8 panel string, giving you in effect only 24 panels (8-8-8).
2. operate all three strings at the voltage appropriate to an 9 panel string, with the 8 panel string not contributing at all. Effectively only 18 panels (9-9-0)
3. somewhere in between in voltage so that neither the 8 nor the 9 panel string are putting out their optimum output.

In any case, the result will not be noticeably better than 8-8-8.

If you went to 10-9-8, the results would be even worse, and you might get only the power appropriate to a total of 10 panels or 18 panels.

According to Fronius Tech Support your first theory is correct. My system is not recognizing the 9th panel in those 2 strings.

So now the question is: Do I add my last spare panel for 9-9-9 or do I remove 2 panels and sell all 3 to somebody that can use 3 matched Solarworld 270 panels for some kind of project? Or do I just leave it as is. It kills me that if I add just one more panel I will get the benefit of adding 3 more...it will drive me nuts if I don't do it.

Fronius said having 7290 watts of panels on a 6000 watt inverter is no problem....so no issue adding the last panel for the 9-9-9 config.

[solarix]

OP needs to figure out how exactly the array was wired and add the additional panel onto the string of 8.

[MARKSDCA]

OP needs to figure out how exactly the array was wired and add the additional panel onto the string of 8.

Shouldn't this be as easy (yes I know nothing is as easy as it sounds) as disconnecting a pair of connected panels anywhere in the series of 8 from each other and running jumper cables to add in the 9th panel in between them? This 9th panel would be on the same roof plane/angle/direction, 25 feet away from the array of 8, and there is no shading whatsoever on this roof.

[inetdog]

Shouldn't this be as easy (yes I know nothing is as easy as it sounds) as disconnecting a pair of connected panels anywhere in the series of 8 from each other and running jumper cables to add in the 9th panel in between them? This 9th panel would be on the same roof plane/angle/direction, 25 feet away from the array of 8, and there is no shading whatsoever on this roof.

Sounds good to me. Just use the right type of wire (PV wire is the most likely for sun and water exposure and even without conduit, but USE-2 will also work without conduit and may be more available locally. If you use conduit, then THWN-2 is a good type. XHWW-2 may allow a higher ampacity on the roof if you are under the 2014 NEC.) and run it where it is protected from physical damage.

[Naptown]

Sounds good to me. Just use the right type of wire (PV wire is the most likely for sun and water exposure and even without conduit, but USE-2 will also work without conduit and may be more available locally. If you use conduit, then THWN-2 is a good type. XHWW-2 may allow a higher ampacity on the roof if you are under the 2014 NEC.) and run it where it is protected from physical damage.

Wire type depends on the type of inverter used.
If transformerless then PV wire must be used.
USE2 is fine for an inverter with a transformer ( galvanic isolation.)
What you use in conduit is up to the AHJ
OH and while we are on the subject of transformerless inverters if you have more than 2 steings feeding an input you need a combiner with fuses on both positive and negative legs of the source circuit.