hv trafo loss AC losses in transformers

[Kanagavel K]

Dear PVsyst Team,

I need some clarification on the AC Losses portion,

1) AC Transformer Losses portion While defining the losses from the datasheet tab (6600kVA Nom.Power) 3 Nos of Trafo is used in the project so total Nom.Power will be 19800kVA. here, the point to be noted is that no value of temperature is being defined in PVsyst. 

Then, how the Nominal power at STC calculation is performed in internal pvsyst (at transformer side).

2)  AC Wire Loss Inverter to Transfo  Inverter ratting Nominal power at 50deg is 275kVA and as per the power vs temp curve (image is attached here for the reference) 20 deg to 30deg the power is 330kVA only

Then, how the Nominal power at STC calculation is performed in internal pvsyst (at inverter side).

 

 

 

[André Mermoud]

In the AC losses part, the reference power may be either the STC power of the PV array, or the PNomac output of the inveters. This choice is done in the project's settings:

In this case the PNomac of the inverters is the nominal power (not the possible max. power at low temperature). 

NB: In your case the choice of the reference power is the array STC power (nameplate of the PV modules). 

However the reference power choice affects the ohmic loss parameter when expressed in terms of percentage. The basic definition of the ohmic losses is the cable Resistance. When operating, the ohmic loss is proportional to the square of the current. When expressed as percentage of the power it is proportional to the power. 

This is explained in detail the help https://www.pvsyst.com/help/project-design/array-and-system-losses/ohmic-losses/index.html.

As an example: 

You have defined a MV lines loss of 1% for a  PNom at STC = 8.2 MWp  (PV array). 

Now if you have a PNomac(inv) of, say, 6 MW, the relative loss at PNom(inv)  will be 1% * 6MW/8.2MW = 0.73%. 

You can check that if you change the referenc power in the project's settings, the relative losses will drop accodingly in this dialog. 

 

 

 

 

[shuwan]

On 6/11/2025 at 10:20 PM, André Mermoud said:

In the AC losses part, the reference power may be either the STC power of the PV array, or the PNomac output of the inveters. This choice is done in the project's settings:

In this case the PNomac of the inverters is the nominal power (not the possible max. power at low temperature). 

NB: In your case the choice of the reference power is the array STC power (nameplate of the PV modules). head basketball game

However the reference power choice affects the ohmic loss parameter when expressed in terms of percentage. The basic definition of the ohmic losses is the cable Resistance. When operating, the ohmic loss is proportional to the square of the current. When expressed as percentage of the power it is proportional to the power. 

This is explained in detail the help https://www.pvsyst.com/help/project-design/array-and-system-losses/ohmic-losses/index.html.

As an example: 

You have defined a MV lines loss of 1% for a  PNom at STC = 8.2 MWp  (PV array). 

Now if you have a PNomac(inv) of, say, 6 MW, the relative loss at PNom(inv)  will be 1% * 6MW/8.2MW = 0.73%. 

You can check that if you change the referenc power in the project's settings, the relative losses will drop accodingly in this dialog. 

Thank you for the detailed explanation regarding AC losses and the reference power selection. It's very helpful to understand how these choices impact ohmic losses.

Could you please elaborate on how different cable types might affect the overall resistance and thus the ohmic losses in the system?

[André Mermoud]

The cable type or installation mode  may affect the resistance only in terms of "Resistivity as function of the Temperature". 

In PVsyst, the resistivity is specified for a temperature of 50°C (alu or copper). This will be modifiable in a next version of PVsyst (probably in V 8.1). But in practice, we usually don't have any information about the cables temperature in the system during operation. 

NB: The resistivity variation is 0.39 %/°C. Applied to a loss which is usually of the order of 1% of the total energy, the effect is completely negligible in the simulation results.