SMA SB 1100LV - Manual (Page 27)

SMA Technologie AG Installation
Installation Guide SB1100LV-11:SE2006 Page 27
6.3 Reverse current
Advice on generator configuration for PV systems using the Sunny Boy
SB 1100LV
The Sunny Boy SB 1100LV operates with very high input currents. 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 cause wrongly directed module current, which can lead to a PV
module being subjected to so-called reverse current, which may be several times more
than the normal maximum current (short circuit current) of the PV 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 (Voc) of the individual parallel 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 Voc, even in this situation
no significant reverse current occurs.
Under fault-free operation of a correctly laid out 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 (VMPP)
of the remaining generator elements. The internal diode structure of the solar cells
6. Pull the cable(s) of the PV generator through the
PG screw fitting on the bottom of the inverter.
DC + must be pulled through the PG screw fitting
on the left; DC - through the one on the right.
7. There are two terminal blocks for DC connection
on the left-hand side of the inverter. Connect
DC + to the terminal block on the left and DC -
to the terminal block on the right. The terminals
can hold cables with a cross-section of up to
25 mm². If you are using a core end sleeve, it
may not exceed a cross-section of 16 mm².
DC + DC -
DC + DC -