SMA SMC 5000 Installation - Manual (Page 30)

Installation SMA Technologie AG
Page 30 SMC50_60-11:SE2006 Installation Guide
5.3 Reverse Current
Information about the design of generators for PV plants with
Sunny Mini Central
In contrast to the „Sunny Mini Central“ string inverters, mostly three and more strings
are connected in parallel within a PV generator of a „Sunny Mini Central 5000/
6000“ (SMC). In case of such large generators failures may occur that are uncritical
in string plants: module currents misrouted by short circuits can load a PV module with
a so called reverse current that may exceed the normal maximum current (short cir-
cuit current) of the PV module.
When can a reverse current occur?
In principal, a reverse current can only occur when modules are connected in parallel
and the open terminal voltage (open-circuit voltage Uoc) of the individual parallel
strings is different. In normal operation mode, this is mostly avoided due to the fact
that the strings are identical. Even in the special case of shadowing no considerable
reverse current occurs as shadowing has no significant impact on Uoc.
In normal operation no excessive reverse current can occur in a PV plant that has
been accurately designed!
Thus, a reverse current can only arise in case the open terminal voltage of a module
string is significantly below the open circuit voltage of the other strings connected in
parallel due to a failure in the PV generator (e.g. short circuit of one or several mod-
ules). At worst, the voltage applied to the defective string is in the range of the re-
maining generator's MPP voltage (UMPP). Due to the internal diode structure of the
PV cell a reverse current flows through the defective generator string. This reverse
current may cause an extreme heat all the way to the destruction of the string's mod-
ules depending on the current intensity!
The failures indicated below may result in reduction of the open terminal voltage of
a generator string and thus in a reverse current in case of connection in parallel:
• Short circuit of one or several modules,
• Short circuit of one or several cells in the module,
• Double ground fault of a module and/or the wiring.
Even if these failures are very unlikely and rare in the field precautions must be taken.
This failure involves a high potential for damages and dangers. All modules of the
affected Strings can be damaged and cause secondary damages due to the high