Inverter Operating Limits

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Inverter Operating Limits

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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.  




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_PminInverter Loss due to power threshold  (not shown on this diagram)
IL_PmaxInverter Loss over nominal inverter power, i.e.overload  loss (intersection on the blue curve)  
IL_VminInverter Loss due to voltage threshold, i.e. when the  array mpp voltage is below   VmppMin
IL_VmaxInverter Loss over nominal inverter  voltage, i.e. when the  array mpp voltage is over   VmppMax
IL_ImaxInverter 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.