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Inverter Operating Limits

The inverter input electronics assumes the function of choosing the operating point on the I/V curve of the PV array.

In normal conditions it will choose the maximum power point (MPPT tracking).

However there are limits in power, voltage and current. When attaining one of these limits, the inverter will clip the operating point on the intersection of the I/V curve and this limit.

Inv_Operating_Limits

Inverter losses

The power difference between the MPP of the arrays' I/V curve and the effective power of this operating point on the limit curves is accounted as inverter loss:
IL_Pmin Inverter Loss due to power threshold (not shown on this diagram)
IL_Pmax Inverter Loss over nominal inverter power, i.e.overload loss (intersection on the blue curve)
IL_Vmin Inverter Loss due to voltage threshold, i.e. when the array mpp voltage is below VmppMin
IL_Vmax Inverter Loss over nominal inverter voltage, i.e. when the array mpp voltage is over VmppMax
IL_Imax Inverter Loss due to the maximum input current. (operating intersection on the orange curve)

The current limit condition may be specified:

  • either explicitly as "Maximum current per MPPT"
  • or by defining:
    • VminPNom, the minimum voltage for attaining PNom: in this case we have
    • Imax = Pnom (DC) / VminPNom, where PnomDC = PnomAC / Efficiency.

Operating conditions for current limiting losses

The Current limiting loss is very often "preceded" (i.e. masked) by the overload loss.

Remember that when the Pmpp is outside the colored limits, the operating point is shifted on the I/V curve of the array, up to its intersection with a limiting line (see Pmpp => POper), i.e. towards higher voltages, and therefore lower currents.

There are only very few situations for which the current limiting is really active: this may arise when the Pmpp of the array is between the IMax (orange) limit and the PMax (Blue) limit, (or slightly above the PMax at low voltage). This is only possible when you define a low voltage for your array, i.e. few PV modules in series.

Therefore in many cases when the operating (or nominal) current of the array is above the acceptable current for the inverter input, you will not see any Current loss during operation, but only Power overload.