AC Cable Losses

[MarcoMalagnino]

Dear Mermoud,

I have noticed that the ac cable losses, as well as the external transformer losses, depend in PVsyst on the Pdc of the pv generator defined in the simulation through the definition of the loss fraction coefficient. In order to calculate this losses you consider the Pac as follow: Pac(STC) = Pdc(STC) x european efficiency of the inverter.

Now my questions: which is the Pac(STC) I have to use for the definition of these losses? For what I know, the design of the ac cable is done considering the maximum power from the inverter (or from the transformer, if it is a central inverter with transformer) and the ac voltage. Why don't you take into account the inverter limit and then the inverter maximum power as Pac(Stc) of the system?

I know that it is just a definition problem but it can cause different losses according to the different interpretation of the Pac(STC).

Thanks in advance.

Best reegards,

Marco

[André Mermoud]

This reference of the AC loss with respect to Pmpp(STC) is a default value, it is a first guess proposed by the software for a quick estimation of a reasonable value.

The real relevant value is based on the Rwiring value, that you should ideally calculate for your actual wiring installation.

The resulting ohmic wiring loss is indeed expressed as a percentage of the Energy remaining at this point of the simulation.

You can have a look on our FAQ "How to determine the Wiring loss Parameter",

and also How to determine the parameters for external transfo loss ?.

[MNWild]

Hi MarcoMalagnino,

Curious what exact efficiency number is used internally to PVsyst to calculate Pac(stc)=Pdc(stc)*(euro efficiency) ? You stated it's the euro efficiency, is it a specific one, or is it dynamic? I ask because there are 3 different voltage levels for efficiency in ond and I've tried to figure out which efficiency is used by backing into from the existing 8760 loss calculation, but no luck.

Yes, this exact AC loss problem especially for transformers has been on my radar as well for many years now. I even sent a study to PVsyst folks to illustrate issue, showing how if a user types in a percentage ac loss the no load loss are increasingly overestimated and load losses are increasingly underestimated as the dc/ac ratio of the project goes up. All because of exactly what you stated, Pac(stc)=Pdc(STC)*some efficiency is used for the ac base for calculating actual KWac losses instead of correctly using point of interconnect as the base or the inverter limitation at terminals as the base. The only workaround I have is post processing ac losses which is cumbersome but required many times since it puts large utility projects at risk $millions of dollars to pass a performance test if AC losses aren't modeled more accurately. You can trick PVsyst as an alternative but then everyone involved will ask why the input % Load losses in PVsyst is higher than actual or will ask why the input % no load loss is low and doesn't match design.

My theory is that the ac loss section was programmed initially with typical designs 8-10 years ago which had dc/ac ratio's of 1.0, PVsyst used that as a "reasonable assumption", but no longer is this accurate and the logic has been forgotton.

[msleiman8]

Hello,

I have noticed a strange difference in the way PVsyst v7.0 handles AC ohmic losses. I used variants previously simulated on PVsyst v6.78 and ran the simulation on v7.0. Observations listed below:

1- In detailed losses summary the selected AC ohmic loss (%) is now higher in v7.0 while the suggested cable length and size remain the same. However, in the loss diagram the actual considered loss throughout the year for AC ohmic is actually lower on v7.0.

2- The parameters used in v6.78 were 1% AC ohmic and in loss diagram turns out as 0.85%, when using v7.0 it shows 1.2% AC ohmic and in loass diagram 0.68%.

The thing is, I tried this on several existing variants and the results were not consistent, nevertheless, the AC ohmic loss value always changes. Sometimes the change is more significant than others.

Appreciate your clarification.