Ohmic Losses - Dc cables

[Michalis Angeli]

I would like to ask if there is a different in the way I definede the DC cables in Ohmic Losses dialog.

Actually, i have an inverter with 9 mppt inputs. Should I define seperate the length of dc cables for each individual mppt (for ex. 23 panels for one String)

Or can I define the length of dc cable as the average length of all strings?
What differents may have in simulation that scenario.

I attach some pictures to be more specific.

[Linda Thoren]

Dear Michalis Angeli,
In the Wiring layout you should define the average length for one string (blue) and the distance to the main box to the inverter (green). If you have multiple strings and/or inverters this will be reflected in the Global array and the Field global wiring resistance. See example below. It is this number that will be used in the simulation. 

If you wish to simulate a more pessimistic scenario, to be on the safe side, you can artificially increase the Loss fraction at STC  to, for instance 0.8%, compared to the calculated values of 0.65 from the example. 

[Michalis Angeli]

In this example, you have 7 inverters with 9 Mppt inputs and therefore overall 63 strings. The Field global wiring resistance is the whole resistance for all strings and inverters?

I would like you to calrify me, if is necessary to create differents subbarays in define system dialog in order to define one by one the lengths of each string and mppt input?
Or i can define a value (cable length) for all strings.

In our projects we don't use Main so I don't define any value there I keep it equal to 0.

Thank you!

[Lazare Fesnien]

Dear Michalis, 

In your case you must define the average length of a string.

Then PVsyst will calculate the resistance of the DC circuit taking into account the number of strings in your system.

At this location, the number of MPPTs is not taken into account in the calculation of DC losses. We will take into account the number of inverters and strings

Regards

[Michalis Angeli]

Because i want to be totaly correct, I attach some pictures from my project. Can you tell me if is ti correct how i define the cables lengths?  It's about a big scale pv park. In this case I have an inverter with 8 strings. I defined an average length 33 m.
Pleas icnore the section, the correct is 6mm^2.

Is ti correct? Isn't?

Edited September 21, 2023 by Michalis Angeli

[Linda Thoren]

According to your attached picture, the average length of one string (including 23 modules) is 33m and no distance from the main box to the inverter. We've identified a bug that will be addressed in the next version. It is marked here that you have 8 inverters, even though you've indicated in your message that you only have one (I assume with 8 MPPT inputs, causing PVsyst to incorrectly register 8 inverters). If you want to be sure to take all the losses into account, it is always possible to slightly increase the Loss fraction at STC from the calculated values, as suggested earlier.

[Michalis Angeli]

Okay thank you very much. According with the bug that you referer. Is not a problem at present time, right? 
I can ran my simulation in the way you suggested me ?

Thank you.

[Linda Thoren]

Yes you can run your simulation.

[Michalis Angeli]

Hello again Linda, i want to clarify something. That you say here " the average length of one string (including 23 modules) is 33m" , you mean the length of dc cable right?
Because i defined it as 33 m, considering that length. In our case we don't use main box and in this way we consider the length of one stirng the distance between panel and ivverter.

Thank you.

[Linda Thoren]

Hello Michalis,

Yes that is correct, you should consider the average length of the (DC) cables in the string. The Ohmic Losses tab is divided in one section for the DC circuit (in the box with light green background colour) followed by AC losses after the inverter (in the box with a grey background colour)  

[Michalis Angeli]

Okay thank you very much!