Inverter Model:  Input and Output

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Inverter Model:  Input and Output

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On the input side  (see also Inverter Operating Limits):

-The inverter should search for the Maximum Power Point of the array  (MPP tracking), i.e. permanently adjust the operating Voltage in order to draw the higher possible power from the array.
This MPP tracking can be performed in 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.  

-The production begins as soon as the MPP power is over the power threshold Pthresh of the inverter. The MPP power below this threshold is accounted as IL_Pmin loss. It is usually less that 0.1% over the year.
PVsyst doesn't manage a Voltage threshold, as the Array voltage is not very dependent on the irradiance. Even with a very low irradiance, the array MPP voltage becomes significant.
The Pthresh power may be understood as the power required for the Inverter internal circuits consumption.
Some manufacturers claim for a very low threshold. We can observe that 0.5% of PNom would correspond to an irradiance of 5 W/m²  (with respect to 1000 W/m²). Please remember that by very covered weather we have already 30 to 50 W/m² !

-If the MPP power is greater than the acceptable input power (PnomDC), the inverter will clip the operating point to the input power which corresponds to Pnom(AC).
in "Limitation" mode, the array voltage will be increased until reaching this PnomDC. The difference with respect to the MPP virtual power is accounted as Overload loss (IL_Pmax).
In "Cut" mode, the inverter will stop working  (very old inverters), so that the IL_Pmax  value is the full MPP power.
As these very old inverters could not start under full irradiance, the inverter was OFF until the next day.
In any case this overload operating mode doesn't mean that the inverter will dissipate the excess energy: simply this energy is not produced by the array. Therefore there will be no extra heat, no extra wear of the inverter, it is a safe behavior.

-The necessary voltage for limiting the power to PNomDC may be over the VmppMax value. In these cases there is no possible operating range, and the inverter has to stop, leading to very high overload loss. Therefore, especially with highly oversized arrays, the normal operating voltage should not be chosen too close to VmppMax.

-When the manufacturer specifies a Minimum voltage for getting the maximum power  (VminPnom), this corresponds indeed to an input current limitation, and PVsyst treats this case as such during the simulation. The eventual loss will be 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 also specify conditions on the PV array: a Maximum value for the ISC (STC)  or the Array nominal power. To our mind, these conditions have no real physical meaning since the inverter chooses itself the operating point on the I/V curve. However they may be contractual (affect the warranty), and in these cases they should be satisfied.


On the output side:

-The main basic parameter of the inverter is the Nominal AC power Pnom, that is the maximum power the inverter is able to deliver to the grid in any conditions.  

-Some manufacturers specify also a Maximum AC power Pmax, as a power which may be attained in specific conditions. This corresponds to an increase of the authorized nominal power when the ambient temperature is not too high. See the page  "Inverter: special output conditions"

-The output (grid) nominal voltage VnomAC is used for the determination of the wiring losses if any  (i.e. related to the current). But 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.  

-A specified Phase shift may be imposed by the grid operator.  This will give rise to the production of Reactive energy. But the output power resulting of the simulation is the 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 have a transformer or not. The transformer is a cumbersome and heavy device. It reduces the efficiency by about 1% or more.
The transformer allows to reference of the array voltage to the ground or any given voltage, and makes the array voltage independent on the Grid voltage.
But inverters with no transformer are not compatible with amorphous modules (damages the TiO2 layer).