Inverter Startup Question

miscrms
miscrms Registered Users Posts: 15 ✭✭
in Solar Beginners Corner #1
Can anyone verify this for me?

I'm looking at a Sunny Boy SMA5000US. The MPP tracking Voltage goes down to 250V, but the startup voltage is listed as 300V. I'm assuming that the array output will be near N*Voc until it passes 300V, then the inverter will fire up and the array output will drop to roughly N*Vmp. Is that about right?

For 12x2 CS6P-230P panels in Phoenix, I'm calculating my "hot" Voc should be ~374V, and "hot" Vmp should be at least 277V. Does that sound ok for the above inverter specs?

Thanks,
Rob
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Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #2
    Re: Inverter Startup Question

    Rob,

    Since none of here (that I know of) work for SMA--You probably should give them a call and confirm your proposed configuration...

    There have been times when the on-line tools have given wrong information, and other times when vendors have "proprietary" information on the operation of their product that they will not release (or even read it themselves:roll:).

    At least you have the understanding of their product and how solar panels work (and are affected by temperature), so you should be able to get a clear and accurate response from the help line.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • miscrms
    miscrms Registered Users Posts: 15 ✭✭
    #3
    Re: Inverter Startup Question

    Ok, thanks very much!
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  • Ravs
    Ravs Registered Users Posts: 13 ✭
    #4
    Re: Inverter Startup Question

    I have a Sunpower 6000US inverter, same exact as yours except slightler higher capacity, and the inverter will try to tie to the grid at 300 volts DC input. Once it does, it will bring down the DC voltage of the panel to near 250 volts.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #5
    Re: Inverter Startup Question
    miscrms wrote: »
    Can anyone verify this for me?

    I'm looking at a Sunny Boy SMA5000US. The MPP tracking Voltage goes down to 250V, but the startup voltage is listed as 300V. I'm assuming that the array output will be near N*Voc until it passes 300V, then the inverter will fire up and the array output will drop to roughly N*Vmp. Is that about right?
    Correct, except that Voc will be altered by temperature (elevated by temperatures below 25 degrees C). You need to adjust it to the lowest possible temperature the array will experience when it is first hit by sunlight. The conservative approach would be to use the record low temperature for your location because the low for the night is usually right before dawn, and voltage jumps up very quickly upon first illumination and does not depend on strength of insolation. The corrected string Voc must not exceed the maximum voltage allowed by your inverter, else damage can result. Even if the inverter survives the overvoltage event, many inverters (I'm not sure about SMA's) have a "black box" which records the highest DC voltage that is presented to the inverter, and if it exceeds the published maximum, your warranty is voided.
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  • miscrms
    miscrms Registered Users Posts: 15 ✭✭
    #6
    Re: Inverter Startup Question

    Ok, thanks!

    Here's what I'm calculating:

    Case 1) Max Startup Voc, cell temp = record low -2C

    12 * (36.8 + (-27 * -0.34)% ) = 482.1 V (Inverter max = 600V)

    Case 2) Min Startup Voc, cell temp = record high 50C + 20C freestanding rise

    12 * (36.8 + (+45 * -0.34)% ) = 374.0 V (Inverter startup = 300V)

    Case 3) Max Vmp, cell temp = record low -2C

    12 * (29.6 + (-27 * -0.43)% ) = 396.4 V (Inverter Vmp max = 480V)

    Case 4) Min Vmp, cell temp = record high 50C + 20C freestanding rise

    12 * (29.6 + (+45 * -0.43)% ) = 286.5 V (Inverter Vmp min = 250V)

    If those calcs look reasonable, it seems like I should be fine!

    Panel Specs (CS6P-230P):
    Voc = 36.8V
    Vmp = 29.6V
    TCvoc = -0.34% / degC
    TCpmax = -0.43% / degC

    Rob
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