SMC Networks Sunny Mini Central SMC 5000A - Manual (Page 32)

Electrical Connection SMA Technologie AG
Page 32 SMC50A_60A-IEN073320 Installation Guide
How Can Reverse Current in the Modules be Prevented?
First we must explain that bypass diodes which are the current state-of-the-art in module
construction do not influence the reverse current in the module, but only reduce the
effect of any shadowing which may occur.
In order to prevent or to limit the reverse current into the modules, the following
standard methods can be applied:
1. String technology
All components in a string (modules, cable cross-section, plug connectors) must be
designed as reverse current for the remaining generator short-circuit current. In case of
parallel connection of maximum two strings, this is always the case, since the resulting
reverse current of a (defect) string can reach no higher value than the value of the short
circuit current of the (intact) string.
2. String diodes
The so-called string diodes connected in series with the individual strings prevent any
reverse current in the corresponding string. Disadvantage: The diode is always
connected in series to the corresponding generator string, the string current flows
through it and leads to correspondingly high continuous losses. Moreover, the failure
of the diode may cause the loss of the safety function or lead to the failure of the entire
string.
3. String fuses
String fuses connected in series to the individual strings can limit the reverse current in
the corresponding string to the permitted maximum. The losses at the string fuses are
significantly lower than in string diodes. Failure of a string fuse can be detected by
monitoring the fuse or by intelligent fault monitoring of the solar generator.
Only the first option is viable for low-cost systems.