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Array voltage sizing according to inverter

PVsyst provides a graphical tool (button Show sizing) for the study and understanding of the sub-array sizing, concerning either the array voltage (number of modules in series), and the array power (number of strings).

The number of modules in series is based on the compatibility of the array voltage in different operating situations, with the input voltages range of the inverter (VmppMin, VmppMax, VAbsMax). The array voltages used in these evaluations are depending on design temperatures, defined in the project's settings.

The voltage conditions are visualized on the upper graph of this tool:

Sizing_Voltage

The string voltage has to match the following conditions:

  • The minimum array operating voltage (i.e. Vmpp at max. module operating temperature, 60°C by default) should be above the minimum inverter's operating voltage (Vmin of MPPT range).
  • The maximum array operating voltage (i.e. Vmpp at min. module operating temperature, 20°C by default) has to stay below the maximum inverter's operating voltage (Vmax of MPPT range).
  • The maximum array absolute voltage (i.e. Voc at min. temperature, -10°C by default) has to stay below the absolute maximum inverter's input voltage.
  • The maximum array absolute voltage (i.e. Voc at min. temperature, -10°C by default) should not overcome the maximum system voltage specified for the PV module.

When the desired array configuration doesn't match these requirements, the system is usually not properly sized.

The 2 first conditions are fuzzy conditions: if, during the simulation, the array voltage goes below the inverter limit VmppMin, or above the upper limit VmppMax, the inverter will clip the voltage to this limit, and this will result in (usually little) voltage threshold or over-voltage losses in the loss diagram.

The 2 last conditions are mandatory: this is a question of system safety. The voltage should never overcome the voltage limits of the inverter (VAbsMax) nor specified for the PV module, in any conditions. This is the reason why we have to use the lower temperature of the site. This condition is checked at design time: this is the main constraint when sizing the string length in a system.

Amorphous modules

For Amorphous or other thin film modules, the voltage values calculated by PVsyst are the stabilized ones after degradation. The initial values may be up to 10-15% higher during the first months. This should be taken into account when sizing the system, especially concerning the absolute maximum voltages for the inverter input or the module insulation.