Inverters with Unbalanced MPPT inputs
Some special inverters (namely in the Tripower series of SMA) have 2 MPPT inputs with very different powers. In practice this is very useful as you can define an array without much constraints about the module number on the main input, and one string with the remaining of your modules to be installed - whatever their number - on the secondary input. This feature is part of the Inverter's definition.
With these very special unbalanced inverters, when defining a sub-array you will have the choice between "Main" and "Secondary" MPPT inputs.
In principle you should define sub-arrays for the same number of "Main" and "Secondary" inputs (i.e. use all the available inputs). However the button "adjust" gives the opportunity of not using the secondary input. Please check with the manufacturer that this is really possible with the real inverters you are using.
At the design time, the nominal powers of each input are evaluated according to the maximum currents specified for each MPPT inputs. This may sometimes lead to unacceptable overload losses. If this warning is red (error), you have to increase the allowed overload energy loss in the "Project's" parameters (button "Albedo-Settings").
At the Simulation time, the power sharing (as mentioned above for normal MPPT inputs) is automatically performed as function of the PV modules connected in each sub-array. The Pnom of each MPPT input is evaluated just before the simulation.
Example of use (tutorial):
Suppose you have to build a PV system using 155 PV modules of 250 Wp, i.e. 38.75 kWp.
1. - Choose the inverter(s), according to a reasonable PNom ratio of 1.25: you need inverters for PNom(ac) = 38.75 kW / 1.25 = 31 kW. 2 inverters with unbalanced MPPT, of PNom = 15 kW should be well suited.
2. - In the "System" part, Define 2 sub-arrays,
3. - First one: define "Main" input with 6 strings of 20 modules (i.e. 120 modules), and 2 "Main" inputs: you get a PNom ratio = 1.25, quite correct.
4. - There are 35 modules left to be attributed. This will correspond to 2 different "Secondary" inputs, so you have to increase the number of sub-array to 3.
5. - Sub-array #2, define 1 "secondary" input, and attribute 18 modules.
6. - Sub-array #3, define 1 "secondary" input, and attribute 17 modules.
7. - Now the Warning "The inverter power is strongly undersized" appears in red, because the Overload loss is over 3% (depending on the meteo). You have to increase the "Limit overload loss for design" in the project's definitions.
Now your system is ready for the simulation.
Best regards | Cordialement
Lazare Fesnien – www.pvsyst.com