Using MPPT Causing Inverter Loss Over Nom Inv Power to Increase

[teopb]

I am running into an issue where using the MPPT feature is causing PVsyst to model significantly higher (multiple percent) Inverter Loss over nominal inv. Power.

Total inverter sizing does not change and the array is being modeled as uniform. The only thing changing is going from 58 inverters to 174 MPPT inputs (for a solectria 50TL with 3 MPPT inputs).

Which is a correct reflection of what actually happens?

Thank you.

[André Mermoud]

Please explain what you mean by "using the MPPT feature".

All inverters use indeed the MPPT feature in a "native" way.

[teopb]

Selecting the checkbox that says "Use multi-MPPT feature", and then using 90 for the "nb of inverters" instead of 30 (for an inverter with 3 MPPT).

[Bruno Wittmer]

The difference you are observing could be caused by a number of strings that is no multiple of the number of MPPT inputs, or even not a multiple of the number of inverters.

When doing a simulation with no multi-MPPT feature, PVsyst will just distribute equally the power of all strings on all inputs of the inverter. If the number of strings is not a multiple of the number of inverter inputs, you will get a fractional number of strings per input. This is a simplification, since in reality you cannot connect a fractional number of strings to one MPPT input, and the result will be an approximation in this sense.

With multi-MPPT feature activated however, PVsyst will try to match an integer number of strings to each MPPT entry. By default, it is assumed that each MPPT input can handle the same fraction of the total inverter power. This can give raise to strongly overpowered MPPT inputs.

There are inverters with multiple MPPT inputs, which can distribute the total nominal power in a non-equal way among the inputs. When there are different numbers of strings at each input, these inverters can make a better use of the total power limitation, by assigning more nominal power to the inputs with more (or longer) strings. To simulate this kind of inverters, PVsyst allows you to specify explicitly the power that each inverter input will handle. This is called 'Power sharing' in the system dialogue. You should only use this feature if you are sure that the inverter you are using is able to the different MPPT inputs with different nominal power.

In such a case, to describe the system correctly in PVsyst, you need to create several sub-arrays, one for each type of input configuration. Different input configuration means that any of 'Mod. in series', 'Nb. strings' or the power share differs. After creating the sub-arrays, you can adjust the 'Power Sharing' that is accessible through the corresponding button in the 'system' dialogue.

The smallest losses are obtained, if the Pnom ratio is the same for all inputs of an inverter. The overload losses will always be higher than in the simulation without multi-MPPT feature, which is a kind of 'theoretical limit' of perfectly balanced MPPT inputs.

[teopb]

I see. Thank you for the response.

[spaniagua]

Power sharing is not working properly. When I open power sharing and make all pnoms equal it changes the pnom but when i close the power sharing window a different power sharing is configured. When i reopen the power sharing window PVsyst has specified its own power sharing ratios that I did not want. PVsyst does not use the full value of the inverters nominal power. the configuration is 3 60kw inverters but when PVsyst assigns its own power sharing the total nominal power is 160kW when it should be 180kW. Why is PVsyst doing this. I previously had this simulation run in version 6.53 and the power sharing used the full value of all three inverters. This bug is causing an increase in inverter losses that were not previously present when using the previous version of PVsyst. Please fix this