Strings of different voltages

stmbtwle
stmbtwle Registered Users Posts: 3 ✭
in Advanced Solar Electric Technical Forum #1
My array was originally 3 strings of 9, 41v panels each (369v total)  After removing and reinstalling for a re-roof, the installer connected them in two strings of 10 (430v OC) and one of 7 (299v OC).  I raised hell and won't let him turn it on because of the difference in voltage.  He claims "we do it all the time" and his boss claims it won't hurt the inverter.  I'm worried about that 130v discrepancy in the strings and
A will it damage the panels and
B what will it do to the output.  
We spent more time arguing about it than it would have taken to rewire the panels.

Panels are SPR 225, Inverter is an SPR 6000

After 2 weeks the system is still shut off and he hasn't been paid.   What to do?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,622 admin
    #2
    At best, the 2x strings of 10 will produce all the power... And the one 7x string will produce close to zero power. You can see what running one string at 100% vs 143% (10/7=1.43) of Vmp from this post:

    https://forum.solar-electric.com/discussion/comment/359660#Comment_359660

    You can see that the power output "plunges" as the output voltage is raised (very sharp knee on the right side of he curve).

    I don't "think" the 7x string will be damaged (I am not a solar panel engineer) by that .

    Note that the array is (probably) already wired up... So the 7x is seeing 143% of Vmp/Voc array rating--If there was/is damage from "over voltage"--It already would have happened even if the solar inverter was turned on.

    Anyhow--I am with you. The array needs to be properly rewired... You are losing 7/27 or 30% of your array's rated output as presently wired.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • stmbtwle
    stmbtwle Registered Users Posts: 3 ✭
    edited May 2022 #3
    Thanks for the info.  I don't THINK the panels have been damaged as the parallel connection is in the DC disconnect switch and as far as I can read the diagrams nothing is actually connected until the switch is thrown (it hasn't been).  On top of that each circuit is fused.  What really makes me mad is that this is SUPPOSED to be a solar company and their people are SUPPOSED to know what they're doing.  I can only wonder how many installations they've miswired and then BSed the homeowner when he didn't get the power he paid for.


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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    edited May 2022 #4
    Yeh, most roofers are experts in PV technology ; .

    Maximum Voc of inverter input is 600vdc but max operating Vmp is 480v. 

    If the three groups are not on separate MPPT inputs and just tied in parallel, the lowest voltage string will set the maximum voltage and you will get less power output.

    Not likely that damage will result, however when inverter is shut down, current may flow between higher voltage strings and lower voltage string if just tied in parallel and individual string breakers are not opened,

    Double check all the panel cable connectors are not damaged and snap sealed well. They can get cracked or broken when disassembled and any water intrusion into connector with the DC potential will cause electrolysis corrosion.

    After getting back to 9 panels per string, I would individually check output for each string to make sure each are performing properly.

    I would suggest you get a knowledgeable PV installer to inspect the system before turning it on.  A bad connection can create vigorous DC arcing that can start a fire.  Because you have the lower voltage string, it can allow current to flow from the higher voltage strings into lower voltage string meaning any arc fault shutdown by inverter will not shut down the current between strings. Each string should have its own arc fault breaker, not just regular breaker.
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