# Fronius Symo 8.2-3-M String Voltage Question

#1

I have a Fronius Symo 8.2kW inverter with 10.08kW of 315W panels arranged as:

• MPPT1 - 12 in series
• MPPT2 - 2 parallel sets of 10 in series
When I look at the string voltage and current on Fronius SolarWeb (Analysis screen) I consistently see the 12-string voltage lower than the parallel 10-string voltage. It’s not uncommon for my parallel string panels to current clip at the 16A limit of the inverter but even at currents below clipping I regularly see about half the current on the 12-string MPPT with a lower voltage.
Example:
MPPT1 - 12.79A, 307.9V, 3832W
MPPT2 - 6.38A, 292.3V, 1818W
I know the whole current/voltage/power curve is non-linear for the panels but it seems to me that MPPT2 is not really operating at the same efficiency as MPPT1. All panels have equal sun exposure so if MPPT1 can produce 30.79V per panel (10 in series) then why do I only get 24.35V per panel on MPPT2?
Is there a setting for the MPPT which might be incorrect here or something else I can do to optimise this as it seems like MPPT2 could be doing more?
A screen shot shows the data.

#2

What panels are you using?

Im not sure if i’m confused with what your saying or you confused yourself?
Does mpp1 have 10 or 12 panels it seems contradicting?

This shows mpp1 (with more panels? Showing a higher voltage…?)

#3

Are you sure your strings series numbers are correct?
If you have different sized series in strings the numbers should be different.
But it sounds like its the opposite? The one with extra panels has a lower voltage?
That either means one (or more) panels arent working or its not configured how you think?

I have 2 strings of 10 on the west and 1 string of 10 on the north. Both run at a very similar voltage 98% of the day.

#4

So 10.08kW / 0.315kW/Panel = 32 panels.

MPPT1 = 12 in series leaving ( 32 - 12 ) = 20 for MPPT2. So 2 x 10 in parallel. It seems like a curious arrangement. If all panels receive equal sun I would that thought that it was better to have two equal strings unless the inverter is hitting a voltage limit.

#5

well it seems something might not be as you think with yours strings, at 31 volts per panel at NOCT you should have something like 370 volts on the 12 string and just over 300 volts on the 10 string setup,also at NOCT perhaps 18 amps on the 10 string setup,and 9 amps on the 12 string. as you are slightly north of me and on the east coast your system should be able to generate the above with no problems, i went to look at your extended data to see if i could see anything that might give me a clue, but its hidden, Jim

#6

Looking at the screenshot that you provided is shows 3726W for MPP1 and 1773W for MPP2.

Following on from the above 3726 / 12 = 310W / panel whereas 1773 / 20 = 89W / panel! Something’s amiss.

So the MPP1 panels are performing at 98% or the rated output whereas MPP2 panels are performing at only 28% of their rated output.

#7

Hi,its possible looking at the data that as fronius inverters have multi DC inputs 2x2 a 4x1 that there is a input load fuse missing,usually only the too sets of DC inputs on the right have fuses fitted at the factory, the other one needs to be fitted when the inverter is installed, the 10 x 2 strings should be showing about 18 amps max and are only showing 6 + amps less than half what it should be,Jim

#8

Sorry for the late reply, I’ve been away. I should explain that my system current limits on the parallel strings at 16A (Fronius spec) and I’ve deliberately picked a point where there is no current limiting to discuss the voltages. I think you’ve mixed up the panel counts in your calculation but I agree there is an efficiency difference.
By my calculations, 3726/20 = 186W per panel and 1773/12 = 148W per panel. This is still not right when you consider all panels are in identical orientation.
I will go back to my installer to check the string configuration. If I work on 2*11 panels in parallel and 10 panel on their own then the numbers all come out a lot closer to even. The problem here is that I’m not getting the best output as there are now 22 panels going into current limit on one string.
Ideally (staying within open circuit voltage limits) I’d like to have 14 panels by themselves in one string and then two parallel strings of 9 panels but I’m not sure if dropping the number of series panels would impact on output during winter.

#9

Sorry, got my MPPT1 and MPPT2 wrong way around. MPPT1 has the 2 parallel strings of 10, MPPT2 has the single string of 12.

#10

My thoughts too but voltage and current limit is the problem. If it was 16 panels in series then the open circuit voltage would be about 650V, above the domestic limit. If you go 2 strings of 8 in parallel on each MPPT then both MPPT’s will hit current limit and lose even more output.

#11

Sorry I got those string arrangements the wrong way around, it should be 12 in series on MPPT2, 210 on MPPT1.
I just wonder if it’s actually 2
11 and 1*10, this actually makes all the voltages and power outputs much closer to even between MPPT’s.

#12

Can you rewrite out the configuration when confirmed by the installer and we can look at it from scratch again

#13

OK, I have re-checked with the supplier and installer and they confirm the 10+10 and 12 panel string configuration. I am fairly sure this is correct because they read out over the phone the open circuit test results at 400V (10 panels) and 480V (12 panels) and these are consistent with the panel data sheets. However they could not explain the lower voltage on the 12-panel string.
This got me thinking (because I don’t know for sure which string they assigned the open circuit voltages to) and I hope somebody can provide a possible answer. What would happen if the actual wiring was done as 10+12 and 10 panels.

• The 10 panels on their own would give 400V open circuit and would give around 300V during operation.
• What would the open circuit voltage of 12+10 panels in parallel give? Depending on the effect of any reverse current through the 10-panel string this might be still be 480V as read by the installer.
Under load the MPP voltage would probably be somewhere in between the 10 and 12 panel vales so maybe this would explain the result.
I have asked the installer to check my system, hopefully they can get out and resolve this issue. So far they have been good to deal with.

#14

well,the open circuit voltage is only used in the design of a system to make sure its not possible to exceed 600 volts DC. the voltages you see on your inverter readout are much lower,say jinko 315 watt penal 40.9 open c v and 33.4 volts at noct, a big difference, the normal cell operating temperature is what you will see most of the time and is used for system output checks. your inverter is rated at 18 amps DC input, not 16 as you say ,so 2 strings of 10 in parallel at 9 amps per string is OK, volts for this should be about 250 give or take a few depending on what make of panels you have,if you had a 10 and a 12 on the same MPPT, then the volts and amps would be limited to the out put of the 10 string, a 12 string should be seeing 9 amps output and about 310 volts, that what i get on my 12 string, reverse currant or voltage is not possible between strings useless you have a system FAULT

#15

No, definitely 16A per string according to the Fronius data sheet and I can see clipping at 16A when it occurs. Panels are 315W Trina Honey-M’s but open-circuit and MPP volts are very similar to those Jinko numbers (STC figures).
The reason I quoted open circuit voltages is because these were open circuit test results quoted by the installer. They also happened to make sense for 10 and 12-panel strings which might explain the installer not seeing a problem.
If I take the NOCT Umpp value of 30.9V then I would expect about 309V from 10 panels and about 371V from 12 panels. What I’m getting are voltages of around 308V (supposedly from 10 panels * 2 parallel strings) and 294V from supposedly 12 panels. These voltages get higher in morning and evening with low output current but the relativity is still there.
I’m not worried about a short between the two MPPT’s, I think this is a wiring error. I have now asked the installer to check but my guess on this is that instead of having:

1-2-3-4-5-6-7-8-9-10
+ |- MPPT1
1-2-3-4-5-6-7-8-9-10 /
and
1-2-3-4-5-6-7-8-9-10-11-12 ]-MPPT2

the installer has mistakenly hooked up:

1-2-3-4-5-6-7-8-9-10-11-12
+ |- MPPT1
1-2-3-4-5-6-7-8-9-10 /
and
1-2-3-4-5-6-7-8-9-10 ]-MPPT2

It would explain a lot of what I can see.

#16

MPPT1 with the two parallel string is raising the string voltage to knock the panels off their maximum power point and thereby reduce the current to the max allowable. MPPT2 with the 12 panels is driving the string voltage down to maximise the current and power.
Hippiesparx
Positronic Solar

#17

Been away for a while so sorry for the late reply. I’m not sure that this would explain the behaviour as I deliberately chose a point where the currents were well below the 16A cutoff to demonstrate the voltages. Both strings should be able to go to max power point for the available sunlight and I would expect about 20% more volts on the 12-panel string.
I finally had a discussion with system designer and he agrees that there is something not quite right so they will be sending somebody to check.