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Inverter model: Input and Output

On the input side

(see also Inverter Operating Limits)

  • The inverter must find the Maximum Power Point of the array (MPP tracking), continuously adjusting the operating voltage to draw maximum available power from the array.
    This MPP tracking operates within a voltage window defined by the VmppMin and VmppMax parameters.
  • When the true MPP voltage of the array is outside of these limits:
    in "Limitation" mode, the voltage will be clipped to the VmppMin or VmppMax value (most of the modern inverters). The IL_Vmin and IL_Vmax losses will be the difference between the true MPP of the array and this clipped operating point power.
    in "Cut" mode, the inverter will simply stop working. The full MPP array power is lost.
  • Production begins when MPP power exceeds the power threshold Pthresh. Power below this threshold is accounted as IL_Pmin loss, typically less than 0.1% annually.
    PVsyst does not use a voltage threshold, as array voltage is not highly dependent on irradiance. Even at very low irradiance, array MPP voltage remains significant.
    Pthresh represents the power needed for internal inverter circuit consumption.
    Some manufacturers specify very low thresholds. Note that 0.5% of PNom corresponds to 5 W/m² irradiance (relative to 1000 W/m²). Remember that even heavily overcast conditions provide 30-50 W/m².
  • If MPP power exceeds acceptable input power (PnomDC), the inverter clips the operating point to the power corresponding to Pnom(AC).
    In "Limitation" mode, array voltage increases until reaching PnomDC. The difference from the theoretical MPP power is accounted as overload loss (IL_Pmax).
    In "Cut" mode, the inverter stops (only in very old models), so IL_Pmax equals full MPP power.
    These older inverters could not start under full irradiance and remained off until the next day.
    In any case, overload mode does not mean the inverter dissipates excess energy; rather, this energy is not produced by the array. Therefore, no extra heat or inverter wear occurs, making it a safe operating behavior.
  • The voltage needed to limit power to PnomDC may exceed VmppMax. In such cases, no valid operating range exists, forcing the inverter to stop and resulting in very high overload loss. Therefore, especially with oversized arrays, normal operating voltage should not be set too close to VmppMax.
  • When manufacturers specify a minimum voltage for achieving maximum power (VminPnom), this represents input current limitation, which PVsyst treats as such in simulation. Any resulting loss is accounted as IL_IMax.
  • When the manufacturer specifies a Nominal voltage (VmppNom), the sizing will take this condition into account as far as possible, but this is a "weak" condition.
  • The Absolute Maximum voltage VAbsMax is the limit voltage acceptable for the input electronics devices: it should never be overcome in any situation.
  • Some manufacturers specify array conditions: maximum ISC (STC) or array nominal power. These have no real physical basis, as the inverter itself selects the I/V curve operating point. However, they may be contractual (affecting warranty) and should be met in such cases.

On the output side

  • The primary parameter is nominal AC power Pnom, the maximum power the inverter can deliver to the grid under any conditions.
  • Some manufacturers also specify maximum AC power Pmax, achievable under specific conditions. This represents an increase in authorized nominal power when ambient temperature is favorable. See the page "Inverter: special output conditions"
  • Output (grid) nominal voltage VnomAC is used to determine any wiring losses (dependent on current). However, the specified maximum current value is not used.
  • PVsyst doesn't treat the Power transfer to the grid in details. However you can define an external transformer (for MV lines) and its associated losses.
  • The real grid voltage is not an accessible input to the simulation, and cannot be modeled of course. Therefore no output voltage limitations can be applied.
  • Grid operators may require a specified phase shift, resulting in reactive power production. However, the simulation output power is active power.
  • You can define auxiliary losses (fans, others), active from a specified power threshold, and night consumption. These values will be used as defaults in the Detailed losses.
  • Inverters may or may not have a transformer. Transformers are bulky and heavy devices that reduce efficiency by approximately 1% or more.
    Transformers reference the array voltage to ground or another reference, making it independent of grid voltage.
    However, inverters without transformers are incompatible with amorphous modules (risk of TiO2 layer damage).