SMA SMC 5000 Installation - Manual (Page 32)

Installation SMA Technologie AG
Page 32 SMC50_60-11:SE2006 Installation Guide
How can one avoid reverse currents within the modules?
Bypass diodes that are currently state of the art in module design have no impact on
the reverse current within the module but only reduce the effects of possible shadow-
ing.
You have the following possibilities to cope with the reverse current problems:
1. String Technology
All components of a string (modules, cables, plug connectors) must be designed for
the remaining generator short-circuit current as reverse current. This always applies
in case of parallel connection of a maximum of two strings as the resulting reverse
current of a (damaged) string can achieve at most the value of the short circuit current
of the (faultless) string.
2. String Diodes
Due to so called string diodes connected in serial circuit with the individual strings
only the reverse current within the respective string is avoided. Disadvantage: the di-
ode is permanently in serial circuit with the respective generator string and the re-
spective string current flows through it. Respectively high permanent losses result. In
addition, failure of the diode may cause a loss of the protective function or failure of
the entire string.
3. String Fuses
String fuses connected in serial circuit with the individual strings allow to limit the re-
verse current in the respective string to the permitted maximum. The losses at the
string fuses are lower than the losses you would observe with string diodes. A string
fuse failure can be identified by a fuse control or an "intelligent" failure control of the
PV panel.
The first option is to be considered for the design of a cost-efficient plant. The Sunny
Mini Central 5000/6000 is therefore delivered with MC and Tyco plug connectors
that have a rating of 30 A current.
Design Tips
The following is specifically to be checked and/or ensured:
1.
Do all strings have the same number of modules connected in serial circuit?
2. What is the maximum reverse current in a defective string under nominal condi-
tions?
Example: Generator consisting of 4 strings of modules with short circuit current
of 5 A maximum reverse current is 3 x 5 A = 15 A.
3. Are the modules suitable for such a reverse current?
4. Are the plug connections of the modules and the inverter suitable for such a re-
verse current?
5. Is the string cabling suitable for such a reverse current?