Strings

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booboo
booboo Solar Expert Posts: 39
I was told that if I have one string of panels and want to add another one to the same inverter, I have to add the exact same brand, size and number of panels to the second string. Is there away around this? Like a box that combindes the power of unlike or uneven strings or even wind generators then feed them into the inverter?

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  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
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    Re: Strings

    search for postings by Crewzer ... he has several very good detailed postings that deal with this very question, both adding new strings and mix/match panels

    Eric
     
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  • boB
    boB Solar Expert Posts: 1,030 ✭✭✭✭
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    Re: Strings
    booboo wrote: »
    I was told that if I have one string of panels and want to add another one to the same inverter, I have to add the exact same brand, size and number of panels to the second string. Is there away around this? Like a box that combindes the power of unlike or uneven strings or even wind generators then feed them into the inverter?


    Nahhh... Well, NahhhT necessarily, anyway ;)

    The thing that you should pay attention to, though, is to match, as well as possible, each string's Voc and Max Power Point Voltage. If they're a wee bit off from each other, like a few percent, not usually a problem. Usually, similar "voltage" rated modules will be very close to the same voltages as another manufacturer.

    For instance, you could combine one string containing 4 X 12 Volt modules in series and the other string with 2 X 24 Volt modules in series and parallel those two strings together.

    Those two types of module strings don't even have to be the same rated current and power as the opposite string. Just the Voltages.

    However, putting in ~series~ two different power/current rating of modules is not always a good idea.

    boB
    :D
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: Strings
    booboo wrote: »
    I was told that if I have one string of panels and want to add another one to the same inverter, I have to add the exact same brand, size and number of panels to the second string. Is there away around this? Like a box that combindes the power of unlike or uneven strings or even wind generators then feed them into the inverter?

    In general (we are talking about Grid Tied Solar inverters--correct?), when adding panels to your system, adding identical panels (voltage/current ratings) is the "most efficient"... However, you can mix and match--assuming you do not go too far off the specifications.

    For example, if you are adding parallel strings--you don't care about the current in each string (say one is 4 amps, and the other is 5 amps--they both add together for 9 amps)...

    What you do care about for series panels is how far of the Vmp (Voltage maximum power) are off from each other... Ideally, you would probably want them within 10% or so... If they are far apart, you run the risk of the MPPT algorithm finding either the lowest panel Vmp (and get reduced voltage * current) or the highest panel voltage (and only get voltage times the current from the high voltage string)...

    With current in series matching--pretty similar... Solar panels are more or less current sources... So if put two panels in series with large current differences, then the smaller panel will tend to limit the current flow through the parallel string. Again, if you can match them to within 10% or so--you should be OK.

    In any case, the farther the "mismatch" the more power you are going to loose vs if everything was identical.

    And when you have wildly differing devices (like solar PV and wind turbine)--you will need a separate Inverter (or charge controller in the case of batteries) because the needs are so different (solar panels, just open a switch if you don't need the power, opening a solar panel connect with a switch will not damage anything--with wind turbines you short them out if you don't need the power to prevent them freewheeling and self destructing in high winds).

    Most (all?) Grid Tied inverters are MPPT type devices (Maximum Power Point Tracking) and will work to maximize the equation P=V*I (Power=Voltage*Current). If you paralleled Solar PV and Wind PV, the Pmax point (V,I) will almost certainly be different between the two types of devices. And whatever the MPPT controller chooses, at best it can only maximize the output of one of those attached devices (possibly even causing the other to add nothing to the overall power being generated).

    The above would also apply to MPPT type Solar Charge controllers (like Outback and Xantrex, et. al.). Each class/size/output rating of device will need its own dedicated controller to maximize harvesting of power. And, possibly different configuration/programming (solar PV--just open a switch to stop charging; Wind--turn on a diversion/shunt load to prevent the turbine from over-speeding and the batteries from overcharging).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: Strings

    In case the link from Jim/Crewzer goes bad in a few minutes/hours/days...

    Use the SEARCH/ADVANCED SEARCH and use the following parameters:

    Search: Keyword(s): series,, string,, parallel ; Posts Made By: crewzer

    I think that the search links like:
    http://forum.solar-electric.com/search.php?searchid=123970
    

    Are static (won't show new matches) and eventually are cleaned up by the BBS software.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
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    Re: Strings

    Thanks, Bill... it already "went bad". :cry:

    Regards,
    Jim / crewzer
  • booboo
    booboo Solar Expert Posts: 39
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    Re: Strings
    For example, if you are adding parallel strings--you don't care about the current in each string (say one is 4 amps, and the other is 5 amps--they both add together for 9 amps)...

    This is confusing to me. Everyone is saying that the parallel string have to be indentical.
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
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    Re: Strings

    PARALLEL:
    The VOLTAGE needs to be very close, the amps can be whatever.


    In a SERIES string, AMPS have to be very close, and voltage can be whatever your controller can take
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  • rplarry
    rplarry Solar Expert Posts: 203 ✭✭
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    Re: Strings

    booboo,
    One thing you can do if the panels that you want to add to your system do not match your existing panels closely enough, is to add the new panels and run them to their own charge controller and then to the battery bank. That way the new panels will not affect the output of the old ones or vice versa. In effect you will have 2 seperate PV systems supplying power to your battery bank. If this system is grid tied then this may not help you and you can ignore this post.
    Larry