Should I use an Input DC BUS for my FlexMax 80 Charge controllers or keep their input separate

richardimorse
richardimorse Registered Users Posts: 98 ✭✭
in Solar Beginners Corner #1
Hi, I was thinking about the need to use a 4 pole GRDi for the 140V DC input to my 3 FlexMax 80 charge controllers and noticed that the GRDi has separate DC +VE connections one for each FlexMax 80 controller, then I was thinking about using SPD's to protect the controller input (i.e. the other end of the combiner cables).

If I bus the 140V inputs i.e. add the Current, then distribute to my 3 charge controllers via the GRDi then I only need one SPD instead of 3 SPDs which would be required without the bus, the Flexmax has the 80A overload protection setting to ensure each controller doesn't overload.

Then again, if I bus the inputs, if the SPD doesn't work I will for sure blow up all three charge controllers and be left with generator power only.

So maybe it is better to either:

1) Rely on the combiner SPD and GFDi for protection keeping controller inputs separate
2) Install 6 SPDs in total at both ends of the power cable 3 in the combiners and 3 for the charge controllers (sounds expensive)
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #2
    Putting the inputs to the controllers on a common bus doesn't seem like a good idea to me.  Among other things, (assuming CCs are not really underpanelled) you could end up with a single controller having to handle 3x overpanelling.  They can generally limit overpanel current to a point, but that may really be pushing it.

    I'd just do the SPDs near the building entrance, presumably at the controllers. 
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,639 admin
    edited December 2018 #3
    MPPT controllers do not like to share a single solar array... The MPPT function assumes that a single controller has complete control of a single array. The controller "finds" Pmax-power by varying the current it draws from the solar panels while monitoring panel voltage... Then "solves" the equation for Pmp=Vmp*Imp. And then adjusts current drawn from panels to hold Voltage at ~Vmp. (newer MPPT controllers don't "sweep the array" every 5-15 minutes like older controllers--But this explanation is close enough for now).

    When you have multiple MPPT controllers on a single array, they will confuse each other as each one tries to "sweep" the array.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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