SMC Networks Sunny Mini Central SMC 5000A - Manual (Page 30)

Electrical Connection SMA Technologie AG
Page 30 SMC50A_60A-IEN073320 Installation Guide
6.3.3 Reverse Current
Advice on Generator Configuration for PV Systems Using the Sunny
Mini Central
In contrast to the Sunny Boy string inverters, three and more strings are usually
connected in parallel in the PV generator of a system using the Sunny Mini Central. 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
solar module being subjected to so-called reverse current, which may be considerably
higher than the normal maximum current (short circuit) 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
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 UPV 0, 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.
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
causes 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-
connected 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