Multi-MPPT inverters

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Multi-MPPT inverters

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Many inverters have several MPPT inputs. Each MPPT input may be connected to one sub-array. This is done by selecting "Use Multi-MPPT feature" for normal multi-MPPT inverters, or automatically for inverters with unbalanced MPPT inputs.

Each sub-array should be homogeneous, i.e. same PV module model and same number of modules in series. If you have different configurations at the input of Multi-MPPT inverters, you should define a sub-array for each kind of configuration.

In the description of the inverter model, we distinguish 2 kinds of Multi-MPPT inverters:

Normal Multi-MPPT inverters

This is the great majority of multi-MPPT devices: each MPPT input has identical electrical requirements.

By default, PVsyst assumes that an inverter with e.g. 2 MPPT inputs behaves as 2 identical inverters of half the power. That is, each MPPT input will have a "nominal power" of half the power of the full inverter. The behaviour is analogous for more MPPT inputs per inverter. This is not a problem as far as you don't have overload losses, i.e. when each sub-array has an acceptable Power Ratio (less than 1.25 to 1.3).

However during operation, the real inverters have the possibility of sharing the total output nominal power between their different MPPT inputs. PVsyst allows to take this PNom power sharing into account, in different ways.

By selecting "Use Multi-MPPT feature" it is possible to consider the single MPPT inputs, instead of working with whole inverters. Below the inverter model selection, one thus selects a given number of inputs instead of a number of inverters.

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.