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How to use the inverters with very different MPPT inputs (Tripower of SMA) ?
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Author:  AndrĂ© Mermoud [ Tue Nov 06, 2012 10:57 am ]
Post subject:  How to use the inverters with very different MPPT inputs (Tripower of SMA) ?

There is indeed a problem with such inverters like the Tripower series of SMA, which have 2 different MPPT inputs: one for the main part of your system (usually 2-3 strings), and a secondary one where you can put the "remaining" of your modules in one string.

With Version 5
PVsyst V5 is indeed not foreseen for treating such assymmetric MPPT inputs, and you will have Power limitation conditions.
Nevertheles you can overcome the problem by modifying the number of MPPT inputs in the inverter definition.
Please redefine your inverter with a number of MPPT inputs equal to your total number of strings (and save it under another file name). After that, you can define one "main" subfield by (N-1) strings on (N-1) MPPT inputs, and the remaining string on the last MPPT input.
In the reality, the strings of the (N-1) first inputs will of course be connected together on the main "physical" MPPT input.

With Version 6
This has been implemented: such inverters with unbalanced inputs have a "Main" and a "Secundary" input.
In the "System" definitions, you should define 2 sub-arrays, one involving the "main" input (with usually several strings, all identical) and the other one the "Secondary" input (with usually one string, i.e. the "remaining" modules of the system).
If you have several inverters, you can define several "Main" and several "Secondary" inputs of course, eventually in sub-arrays of different kinds. But be sure that for each inverterthe whole system is using both Main and Secundary inputs (identical total numbers).

If you get a warning about the eventual oversizing:
If the inverter is "slightly" undersized (orange warning), no problem, you can perform the simulation.
If the inverter is "strongly" undersized (red warning) , you can modify the parameter for this warning, in the project's parameters, button "Albedo-Settings", item "Limit overload loss for design".
This is not necessarily significant, because:
- at the design time, the "nominal power" sharing between both inputs (for design criteria) is done according the the limit input current ratio of each input (for ex. 50A:12.5A, means 4:1 ratio).
- at the simulation time, the power sharing is adjusted according to the nominal powers of the modules effectively connected to each input, so that the overpower conditions will be met by both inputs in the same operating conditions. This is probably nearer to the real behaviour of these inverters. You should perform the full simulation for getting realistic results.

Example of use (version 6):
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 in over 5% (depending on the meteo). You have to increase the "Limit overload loss for design" in the project's definitions.
Now your system is ready fort the simulation.
NB: in the present time, with Unbalanced MPPT inputs you have to define one Main and one Secondary input for each inverter. However it seems that some manufacturers now allow for not using the secondary input, so we will suppress this requirement in a next version.

NB: Some people want to use these inverters with unbalanced inputs with only one input. Although this seems to be allowed by some manufacturers, you are not advised to do that.
Up to version 6.32, PVsyst required that you use each Main and each Secondary input of all inverters. If you wanted to define a configuration using one only input, you had to redefine a new inverter in the database, with only one MPPT input. Since version 6.33, you can inform PVsyst that you are using only the "Main" input using the button "Adjust".

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