SMA SB 1100LV - Manual (Page 29)

SMA Technologie AG Installation
Installation Guide SB1100LV-11:SE2006 Page 29
How to avoid reverse current in the modules?
First, we must know that today's state-of-the-art bypass diodes for module construction
do not affect reverse current in the module, but only reduce the effects of any shading
which may occur.
The following standard methods of preventing or reducing reverse current to the
modules exist.
1. String technology
All components in a string (modules, cable cross-section, plug connectors) must be
designed for the remaining generator short-circuit current as reverse current. This is
always the case if no more than two strings are connected in parallel, as the resulting
reverse current of a (defective) string cannot exceed the value of the short-circuit current
of the (intact) string.
2. String diodes
String diodes connected in series to the individual strings prevent any form of reverse
current in the corresponding string. Disadvantage: The diode is permanently connected
in series to the corresponding generator string, which means that the current of the string
in question always flows through it, leading to correspondingly high permanent losses.
If the diode fails, the protective function may be lost or the entire string may fail.
3. String fuses
The string fuses connected in series to the individual strings allow the reverse current in
the corresponding string to be limited to a permitted maximum value. The losses at the
string fuses are significantly lower than at the string diodes. The failure of a string fuse
can be detected by monitoring the fuse or via "intelligent" fault monitoring of the solar
generator.
For cost-effective solutions, only the first option is suitable. The PV input terminals are
approved for 62 A per connection. The system planner must ensure that this value is not
exceeded.