Min and Max current for Fronius install

quique
quique Solar Expert Posts: 259 ✭✭
in Solar Beginners Corner #1
Ok I'm reading through my manual and it reads:

Nominal Input Voltage: 390 (230min-600max)
Nominal Input Current: 8.3A
Max Input Current: 14A
Max array short circuit current: 18A

My solar panel specs are:

Vmp: 18v
Imp: 11.7A
Voc: 21.5
Isc: 12.3A

So I'm thinking 10 panels will give me:

Maximum Vmp: 180V oops not enough
Maximum Voc: 215V ditto

So I need some 20 panels:

Max Vmp: 360V ok
Max Voc: 425V still safe

As for current:

Max Imp: 11.7A
Max Isc: 12.3A

This is where my question comes in:

I would still be safely under 14A which is the total system input allowed, and i would still be in the individual string maximum, right?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Min and Max current for Fronius install

    This is why almost all the inverter companies have on-line string sizing tools. I think this is Fronius's: http://www.fronius.com/froniusdownload/tool.html

    You panel Isc is 12.3, so any number of these in series will not exceed the 18 Amp maximum array short circuit current rating of the inverter. Two parallel strings would.

    The maximum input Voltage of 600 is for Voc calcs, so the string could have no more than (600 / panel Voc 21.5) 27 panels, not including allowance for Voltage increase due to cold temps.

    They want the nominal input Voltage to be around 390. If you divide that by the panel Vmp of 18 you get 22 panels in series. 20 panels would give you 360 Volts, which is actually a tad low.

    Your panels are 210 Watts. Twenty-two of them would be 4620 Watts. How many Watts is the inverter?
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  • quique
    quique Solar Expert Posts: 259 ✭✭
    #3
    Re: Min and Max current for Fronius install

    Yes I am using the online tool, but my solar panels are not listed, they were bought locally from a supplier who brings things from China.

    I understand 1 string of 10, would indeed be Isc 12.3A max. I wouldn't plan on going with two strings. I also understand 2 strings would because it would result in 12.3x2 = 24.6A which is greater than 14A which is the maximum allowed. Im just unclear as to the nominal 8.3A listed.

    The inverter is a Fronius IG Plus V 3.0 model. Recommended PV Power is 2500-3450 which is indeed constrained for the resulting 4620. This is why Im trying to figure out the best thing to do.

    I could mix and match panels. I have some 48V-2A ~ 100W panels which I would actually have to measure to make sure. They are about 7 years old and I don't have their specs anymore. But Im thinking of combining them with the 210W panels, but Im not sure how recommended that is. Is that something that could be done? I know what would happen to the Wattage and Voltage, but I have no idea what would happen to the amperage.

    Otherwise I have to buy panels that would fit my inverter which I could do, but I was just wondering about the mix/match.

    If I were to buy panels, Im being offered Renesola 245W which are, according to Forinus Config Tool:

    Voc = 37.3
    Isc = 8.73

    Vmp = 29.9V
    Imp = 8.19A

    In this case 10 panels would give me never more than 373V and never more than 8.73A.

    10 panels would also give me 10 x 245 = 2450W.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: Min and Max current for Fronius install

    No offense, but your panels have weird specs. Most GT inverters would be connected to "GT" panels with a Vmp around 30 and an Imp around 8. Yours are "true 12 Volt" panels with a Vmp of 18, and so to get the Wattage up they have parallel two panels in one giving it the very high Imp of 11.7.

    One string, no matter how many are in the string, will have the same Imp and Isc as a single panel.
    Two strings in parallel will double both current specs. So if the panels were "more typical" you might have an Isc of 8 and two strings would still be below the maximum of 18. But because the current specs are high you are stuck with a single string. You can not mix and match panels here; the specs will not work out no matter how you try it.

    Your inverter is rated for 3450 Watts? If you divide that by the 210 Watt panel rating you get 16 panels. But that drops the Voltage down to 288, very low on an inverter that needs 245 to start.

    Short answer: these panels are not suitable for the inverter. Sometimes a bargain is not a bargain.
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  • quique
    quique Solar Expert Posts: 259 ✭✭
    #5
    Re: Min and Max current for Fronius install

    Oh I haven't bought the panels. I just happen to have some 210w panels that I use for my offgrid.

    I haven't bought the panels for this one. What I was wondering was if I could mix and match those 210w(21v-11a) panels with some 100w (48v-2a) panels.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: Min and Max current for Fronius install
    quique wrote: »
    Oh I haven't bought the panels. I just happen to have some 210w panels that I use for my offgrid.

    I haven't bought the panels for this one. What I was wondering was if I could mix and match those 210w(21v-11a) panels with some 100w (48v-2a) panels.

    No, there's no practical way to mix them. If you put one of the 2A panels in series with the 11A it will restrict the current to 2A costing a large amount of the power potential from the larger panel. In parallel you would need three 48 Volt panels to eight 18 Volt panels to have both strings at 144 Volts, then double that to 288. It sounds like it would work but ... you'd have 16 of one and 6 of the other and the reverse Voltage difference would be significant so that if the lower Voltage panels were shaded they'd draw power from the higher ones. In other words it would be a mess with potential problems.

    Get more suitable panels.
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  • quique
    quique Solar Expert Posts: 259 ✭✭
    #7
    Re: Min and Max current for Fronius install

    Ok so I found specs for these panels:

    245W
    36Voc
    30V
    8.60Isc
    8.17Imp

    Now 10 panels will give me 360V! Better, right? :)
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
    Re: Min and Max current for Fronius install
    quique wrote: »
    Ok so I found specs for these panels:

    245W
    36Voc
    30V
    8.60Isc
    8.17Imp

    Now 10 panels will give me 360V! Better, right? :)

    Yes, but not quite: One 10 panel string of those would have Vmp 300 (operating Voltage) and Voc of 360 (maximum Voltage on input). You really would put twelve panels in series to get the array Vmp to 360 (at 8.17 Imp). Voc would then be 432 under normal circumstances and still below the maximum. That would be a 2941 Watt array.
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  • quique
    quique Solar Expert Posts: 259 ✭✭
    #9
    Re: Min and Max current for Fronius install

    Ok why would I need 360vmp? The inverter says the min is 260v
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #10
    Re: Min and Max current for Fronius install
    quique wrote: »
    Ok why would I need 360vmp? The inverter says the min is 260v

    You do not want the array Vmp at the minimum level to turn the inverter on, but rather in its optimum operating range which is around 350 to 400 Volts (if I'm reading the right data sheet).

    You do want the array Wattage to approach or even slightly exceed the GTI's output rating. Panels do not produce full power all the time, and it's a good idea to have your inverter putting maximum as long as possible in order to make the payback on investment happen sooner. It can take it.
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