Studer Innotec SA
VarioString
User manual V 2.3 17
The VS-120 possible operating range within different configurations are indicated in the graph below
(blue (A): one independent input, red (B): two inputs in parallel, green (C): two inputs in series).
4.3.2
Serial connection (chain or string) of PV modules
To get the optimum voltage, connect the PV modules in series to form a string with the desired
voltage.
When using the two entries of the VarioString VS-120 in series, the + terminal of the PV1 entry (6) should
be connected to the + pole of the PV generator. The – terminal of the PV2 entry (3) should be
connected to the – pole of the PV generator and a bridge should be made between the entries (4)
and (5).
The open-circuit voltage of the string must be lower than 600V under any temperature and irradiation
conditions. A 10 to 20% margin is recommended in order to cover these unknowns. When the two
inputs are used in series on the VarioString VS-120, the maximum open-circuit voltage should not
surpass 900V.
To establish the maximum numbers of the PV modules connected in series in the same string, use the
following formula: 600V/(Uoc*1.1). (Example: 600/(22*1.1)=24.79). The result must be rounded down
(i.e. 24 modules - of 22 Voc - in series for the above example).
The table below gives an idea of the possible arrangements for common PV modules according to
the number of cells or their type:
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