difference between selecting multi-mppt or not

[osven]

I am modeling a project for the university. and I have the doubt what is the difference of selection or not multi-mppt.

A colleague told me that it is mandatory to have it selected but when I select it it never gives me the number of panels I need to install. but when it is not selected it works better for me. What would be the difference? Thank you very much for the information.

[dtarin]

You do not need to use it, but it comes down to your installation and how you want to model. Using the MPPT feature will be dependent on your OND file definition, string size, module. If for example you have 1 inverter with 2 MPPT, you will need to enter "2" where it says Nb of MPPT inputs to equal 1 full inverter. What this does is it then takes the max PV current of the inverter and divides it by the number of MPPT inputs, and models each of these inputs separately from one another. Generally PVsyst prefers an equal number of strings per MPPT input, according to the max current per MPPT input ("balanced"). You will need to consider the current and voltage on each input to be within the inverter's spec to minimize clipping losses.

In some cases, the results are the same with or without it if the system is balanced and within the inverter's spec. In other cases, if you have an odd number of strings or unequal strings across the MPPT inputs, you can end up with very high (and possibly unrealistic) clipping losses. For example, say you have 5 strings on 1 inverter with 4 MPPT inputs; you could end up with 10% clipping losses if you model with MPPT enabled. But if you disable this feature, and just model 5 strings on 1 inverter, you would end up with only 2.8% clipping losses. So which is correct?

It comes down to your installation. For string inverters, you can have them installed so the MPPT inputs are independent of one another. In this case, you will use the enable MPPT feature. If your inputs are jumpered (connected together internally), they will share the incoming current from all strings, and you will disable MPPT feature.

Another use for the MPPT feature is if you have different modules or string sizes connected to one inverter (1 string of 18 modules, 1 string of 17 modules, etc). Generally it is better to not have it enabled, and reflects actual PV plants as they're typically built. In all projects I have modeled that have gone to construction, string inverter inputs are jumpered and not operating independently.