In some big PV installations (in the MWp range), the transformer is not part of the inverter, but an external device directly connected to the MV grid.
The main losses associated with the transformer are:
 The iron losses (mostly due to hysteresis and eddy currents in the core) are proportional to the square of the core flux, i.e. to the square of the voltage. As we have a constant grid voltage, this is considered as a constant loss. PVsyst proposes a default value of 0.1% of the nominal power.
 The ohmic losses, often named copper losses, either in the primary and in the secondary windings. These may be represented by an equivalent resistance, and the loss will be computed as R * I² during the simulation. Therefore the yearly resistive loss will always be lower (in percentage) than the nominal loss at STC.
NB: The iron loss remains active and constant during the whole connecting time, and may represent a significant energy loss.This appears as negative E_Grid system yield during the night.
Therefore it may be economically profitable to foresee a switch for disconnecting the transformer from the grid during night. The option "Night disconnect" is available for the simulation.
Parameter determination
The transformer manufacturer usually specifies on the datasheets:
 a nominal power PNomTrf
 a global loss under this nominal power PGlobLossTrf [kW] or FGlobLossTrf [%] = PGlobLossTrf / PNomTrf
 an iron loss, i.e. global loss at null power PIronLssTrf [kW] or FIronLssTrf [%] . = PIronLssTrf / PNomTrf
=> we can deduce the resistive loss as the difference PResLssTrf = (PGlobLossTrf  PIronLssTrf), which behaves as the square of the PoperTrf.
In PVsyst the transformer losses are specified as percentages of the system output Power when running at STC, named PnomAC (appearing as "Pac" on the Ohmin losses dialog).
NB: this PnomAC = PnomPV (STC) * Inverter efficiency at PnomPV. This is a definition for the calculations. This should be considered as not affected by the inverter or grid injection limitations.
Passing to the PVsyst parameters, referenced to PnomAC:
 PIronLss(PVsyst) = PIronLssTrf [kW].
 FIronLss(PVsyst) = PIronLssTrf [kW] / PnomAC = FIronLssTrf * PNomTrf / PNomAC
 PResLss(PVsyst) = PResLssTrf * PNomAC ² / PNomTrf ²
 FResLss(PVsyst) = PResLss(PVsyst) / PnomAC = FResLssTrf * PNomAC / PNomTr,
i.e. the opposite behaviour !!! (the higher the PnomAC, the higher the resistive loss factor, but the lower the Iron loss factor).
NB: If there is a second HV transformer in the circuit, you should calculate the loss percentages in the same way, and add them to the MV transfo contribution.
As an example:
Transformer parameters from the datasheets:
 PNomTrf = 1.5 MW
 PIronLssTrf = 1,5 kW (i.e. 0.1% of PNomTrf)
 PGloblossTrf = 16.5 kW (under nominal power PnomTrf)
=> The resistive loss is PGloblossTrf  PIronLssTrf = 15 kW (i.e. 1.5% of PnomTrf).
In PVsyst, if we have a PV system of 1.05 MWp with an inverter efficiency = 95% @PNom
=> Pnomac = 1 MW (the Pnomac value appears as "Pac" on the AC circuit inverter loss dialog).
Applying the previous expressions:
 PIronLss(PVsyst) = 1.5 kW
 FIronLss(PVsyst) = 1.5 kW / 1 MW = 0.15%
 PResLss(PVsyst) = 15 kW * (1 MW / 1.5 MW)² = 15 kW * 0.444 = 6.66 kW
 FResLss(PVsyst) = 6.66 kW / 1 MW = 0.66%
The values 0.15% and 0.66% are those to be introduced in PVsyst.
