PV string voltage differences

[jon_1]

Hi,

I have a question that I could do with an opinion on from some of the solar specialists on the forum before I go back to the installer to query the situation.

System installed was installed in Nov 2022 and has 2 strings each with 9 400 W panels – a southerly string and westerly string. The solar inverter is a Solis S6 6kW unit.

The panel spec says roughly 35 to 37v Voc depending on light levels.

With overcast conditions giving what I would judge to be roughly equal light levels on each string there is a significant discrepancy in the string voltage levels as reported by the inverter – typically the westerly string will be roughly 80 volts lower than the southerly one.

The westerly string has 7 panels on one roof and the other 2 panels on a separate roof area with DC cabling linking the 2 groups of panels together through the loft whereas the southerly string is all on one roof.

I have only really noticed the different string voltages in the last 3 to 4 months which could indicate panel faults but I am suspicious as the string voltage difference is roughly the 2 x Voc.

Both strings are working but given the consistent and significant voltage difference between the strings I am wondering if the 2 separate westerly string panels may have been connected with reverse polarity to other 7 panels?

Grateful for any thoughts?

 

[binky]

General rule if thumb, if it's working there's nothing wrong!

Your south array will always outperform your west array, especially in this gloomy weather we have been having recently. The MPPTs adjust voltages down from Voc to increase ampage and maximise power from the arrays. A simple test you could try is to turn off the AC effectively stalling the inverter, that should then stall the DC so you can see what the stalled voltages are. Ideally you would turn off the DC and test string voltages, but messing with DC is not recommended unless you know what your doing - it's dangerous stuff .

I'm not sure what would happen if 2 panels had been cross connected, but I suspect that string would be down by the equivalent of 4 panels, or just not work.

 

[michael8554]

At the moment in the UK, sunrise is at NE, and sunset at SW.

So output should be more from the south string.

 

[jon_1]

Thanks for the thoughts.
I expect the southerly string to be the best performing in overal terms.
I will take a look at Binky's suggestions re turning off the inverter AC output to see how that effects the inverter DC readings.