Parallel Connection of Strings
SMA
Technologie AG
Page 14
SSM-TEN072310
Technical Description
4 Parallel Connection of Strings
4.1 Maximum Number of Strings per Measurement Input
While the SSM-C is able to connect several strings in parallel, the analysis in the SCC
is, however, limited to a maximum of nine, with a tolerance setting of 10 %. The
maximum number of strings which can be connected in parallel in order to achieve
reliable analysis in the SCC depends on the adjustable tolerance, and can be
determined as follows:
N = maximum number of strings which can be connected in parallel.
The parameter "tolerance" can be adjusted in the SCC.
4.2 Reverse Current
In contrast to the "Sunny Boy" string inverters, in the PV generator of a system using the
Sunny Central, e.g. using thin-film modules, many strings are connected in parallel. The
maximum number of connected strings depends on the module type, and thus must be
requested from the module manufacturer.
Short circuits can cause wrongly directed module current, leading to a string being
subjected to so-called reverse current, which may be considerably higher than the
normal maximum current (short circuit) of the string. Thus, the string fuses protect the
faulty string. The largest permissible string fuse depends on the module type, and must
be selected in accordance with its defined curve.
In principle, reverse current can only occur when modules are connected in parallel
and the open circuit terminal voltage (Voc) of the individual parallel strings is different.
In normal operation, this is avoided adequately when the strings are of the same length.
Since shadowing of the modules has no significant effect on Uoc, even in this situation
no significant reverse current occurs.
In failure-free operation of a correctly dimensioned PV generator, no excessive reverse
current can occur!
Reverse current can only occur due to a fault in the solar generator (e.g. short circuit in
one or more modules) that causes the open circuit terminal voltage of a module string
to be significantly lower than the open circuit terminal voltage of other parallel strings.
N = 0.9 / tolerance (%)
Information
The maximum number of strings which can be connected in parallel is limited
to nine, if the tolerance is set to 10 %.
Thus, if the number of strings connected in parallel is larger than nine, failure
of a string cannot be registered or identified in the SCC.
