SMA SSM8-21-BS - Manual (Page 12)

4 Product Description SMA Solar Technology AG
12 SSMxx-21-IA-BS-JP-en-20 Installation Manual
Block Circuit Diagram of the Sunny String-Monitor SSM8-21-BS-JP/SSM16-21-BS-JP
Figure 4: Block circuit diagram of the Sunny String-Monitor SSM8-21-BS-JP/SSM16-21-BS-JP
4.2 String-Current Monitoring
The string currents are monitored via the measurement PCB String-Monitor Unit. The String-Monitor Unit measures the
string currents and continuously calculates the mean values of the string currents. The string currents are compared with
the mean values. If a string current exceeds or falls short of the mean value by the set tolerance value, the warning 380
SMU is displayed.
With central inverters, you can use the user interface to set the parameters for the string-current monitoring and read off
the mean values (see operating manual of the respective central inverter).
The total failure of a string can be detected via the parameters set. Marginally increased string currents are reliably
detected over several query intervals and distinguished from typical current fluctuations of the PV array.
The sections below describe:
• the conditions under which the string current is monitored
• the intervals at which data is transmitted
• the key parameters for the sensitivity of the string-current monitoring.
Requirements for Activating the String-Current Monitoring
☐ The DC power P
DC
of the PV array exceeds 5% of the nominal AC power P
AC nom
of the Sunny Central.
P
DC
> 0.05 * P
AC nom
☐ The mean value of the string currents of the individual groups l
M String
is equal to or greater than 0.5 A.
l
M String
≥ 0.5 A
Error Sum
The error sum is a measure of the sensitivity of string-current monitoring. The error sum can be parameterized via the
tripping time and the tolerance and can be calculated using the following formula: