Reverse Current - SMA SMC 5000A Installation Manual

SMA
Technologie AG

5.3.3 Reverse Current

Advice on generator configuration for PV systems using the Sunny Mini
Central
In contrast to the "Sunny Boy" string inverters, in the PV generator of a system using the
"Sunny Mini Central", three and more strings are usually connected in parallel. This
does not sound particularly spectacular but it has practical consequences because, in
such large generators, certain faults which are totally uncritical in string systems must be
allowed for. Short circuits can cause wrongly directed module current, leading to a so-
lar module being subjected to so-called reverse current, which may be several times
more than the normal maximum current (short circuit current) of the solar module.
How does reverse current occur?
In principle, reverse current can only occur when modules are connected in parallel
and the open circuit terminal voltage (open circuit voltage UPV 0) of the individual pa-
rallel strings is different. In normal operation, this is adequately avoided when the
strings are of the same length. Since shadowing of the modules has no significant effect
on UPV 0, even in this situation no significant reverse current occurs.
Under fault-free operation of a correctly laid out PV generator, no excessive reverse cur-
rent 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.
In the worst case, the voltage on the faulty string may lie within the MPP voltage (UMPP)
of the remaining generator elements. The internal diode structure of the solar cells cau-
ses reverse current to flow through the faulty generator string that, depending on the
amount of current, may lead to excessive heating or destruction of the modules in this
string.
Among other symptoms, the following faults may lead to reduction of the open circuit
terminal voltage of a generator string and subsequent reverse current in parallel-con-
nected systems:
• Short circuit in one or more modules,
• Short circuit in one or more cells in a module,
• Double ground fault in a module and/or the cabling.
Installation Guide
Electrical Connection
SMC50A_60A-11:SE3706 Page 35