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Will PV Current Imp Cause Inverter Tripping/Shutdown?

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    #16
    Originally posted by DL-PE View Post
    Based on the original query by TheElectrician, and replies to that query from others, I'd like to clarify several points that have been posted, most of which are excerpted for brevity:

    * electrofelon
    There is a potential issue with DC/ac ratios being too high, where the point on the I/V curve needed to clip that power is outside of the inverter's capabilities. It seems that with some basic measurements and discussion with the manufacturer would identify of that is what is going on. I also seems to me that specifying a DC/ac ratio that is acceptable is too crude, and one would need to look at where the array parameters sit within the inverter's capabilities.

    DL-PE:
    No. This is incorrect. There is no point at which the DC-to-AC ratio can be "too high". Inverters will use as much DC power as is needed to attain their maximum rated AC output power. The rest goes unused. Yes, this may result in the inverter operating at a point on the array's voltage-current curve that isn't exactly at the "maximum power point", but this is harmless. The only "damage" is to the bank account of the system owner, when they pay much more for the system than was necessary.
    .
    Electrofelon is correct on this, with a caveat. In theory, the upper limit of the inverter's MPPT tracking window can be less than the voltage needed to bring the PV array current down to a level that the inverter can use if the inverter needs to curtail the PV array output. In this instance, the inverter would have to shutdown to protect itself from the excess current. Many PV system modeling programs, using the published MPPT window, will model the inverter shutting down under this condition and we can see a significant reduction in production in the output report.

    The caveat is that the actual inverter MPPT tracking window is usually wider than the published window. So the upper MPPT voltage cutoff is often high enough to keep the inverter running even with high DC/AC ratios. The published MPPT tracking window is used in determining the inverter's average efficiency so there is a motivation for the inverter manufacturer to limit the MPPT window to give a higher average efficiency across the window. It's taken me a considerable amount of time talking to the right people inside inverter companies and with software developers to get this worked out since it's not published information and shows how efficiency numbers are being legally, if opaquely, manipulated.

    SMA for one has started listing this practice in the inverter datasheet. If you look at the datasheet for the CORE1 inverter there are two MPPT windows listed, a rated MPPT window, and a MPPT operating window that is wider. The rated MPPT is used for efficiency rating.
    Last edited by pv_n00b; 07-25-19, 12:40 PM.

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      #17
      Good info, thanks. Also, in theory the inverter could go either way to clip power. Do manufacturers pick one direction?
      Ethan Brush - East West Electric. NY, WA. MA

      "You can't generalize"

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