SMA SSM16-11 - Manual (Page 18)

3 Product Description SMA Solar Technology AG
18 SSM16-24-IA-IEN105120 Installation Guide
3.3 String Fuses
The string fuses protect the modules of a PV plant from reverse currents.
Figure 3: The principle of reverse current
Reverse current can occur when different open-circuit voltages are applied to modules connected in
parallel. In this event, the reverse current can be much higher than the module's short-circuit current.
The higher the reverse current, the higher the heat will rise in the modules of the faulty PV string.
Pronounced warming can destroy the modules of the faulty PV string.
Reverse current can have the following causes:
• Short circuit in one or more modules
• Short circuit in one or more cells in a module
• Double ground fault in a module
• Double ground fault in the cabling
The string fuses trip when the string current is too high and thus protect the modules of the faulty PV
string. The Sunny String-Monitor informs Sunny Central Control of the failure of the PV string.
In the case of grounded PV arrays, the positive or the negative pole can be grounded at the inverter.
To do this, cylindrical bridges are fitted in the Sunny String-Monitor instead of fuses.
If a string fails, the current difference in the above example is 4.5 A, which equates to a deviation
of 90% from the mean value. If the settings in the above example remain the same, i.e. tolerance at
10% and tripping time at 10 minutes, the error sum of 20% is already reached after the first polling
interval of 5 minutes. The data logger now sends an "SMU" warning.