String box: how to increase system security?

String boxes usually concentrate the protections and parallelism points of PV systems, when used. However, their functionality can go far beyond a direct current panel.

It is possible to complement protection with non-mandatory extra devices, provide robust monitoring systems or even facilitate operation and maintenance with remote shutdown systems.

Monitoring

In larger plants where there is no major risk of shading or uneven dirt, it is common to use inverters with a few centralized MPPTs, in order to reduce the number of cables and simplify installation.

It is inevitable in this construction that there will be points of parallelism of multiple DC circuits in a conductor with a larger section that will go to the inverter. This construction, in principle, would not allow individual monitoring of each of the series, which could be a difficulty in the future.

There are already several string box solutions on the market with string current sensors, which, together with the wired or wireless communication system, send information individually from the series, thus greatly improving monitoring of plant generation and fault diagnosis. The device below is an example of an individual string monitoring module that can be added to a string box.

Remote shutdown

Another function that makes the daily lives of plant operators easier is the possibility of remotely turning off the string box. This is a feature that can increase the safety of the installation during maintenance, as the sectioning status of each string box can be checked by the operator and maintenance team, and, in case of emergencies, the sectioning and reconnection can be carried out remotely.

It is also possible, for example, to remotely turn off and on again to test failure hypotheses and try to locate the set of defective modules, or the string box that is causing alarms in the inverter.

AFCI

The arc is a phenomenon of high energy intensity, that is, it heats the surrounding air and easily radiates heat, thus increasing the risk of fire significantly. Arcs are difficult to detect and interrupt, however, there are specific devices for their detection and sectioning, the AFCI.

Although the arc detection and sectioning device (AFCI – Arc Fault Circuit Interrupter) is not mandatory in the installations, its use is highly recommended. The arc can appear in 3 different situations, namely:

• Series arc – when a conductor or MC4 connector breaks or separates while current is passing;
• Parallel arc – when there is a break in the insulation and cables from different circuits create an electrical arc between them;
• Arc to ground – when there is a double insulation fault grounding two different poles simultaneously.

As it is not a mandatory item, not all inverters have this protection integrated or activated by default. In cases where the inverter does not provide this protection, it is recommended to add an AFCI device to the solar DC circuits.

The device to be added can be external, that is, purchased separately, or integrated into the string box. The string box below, model BHTZ-11/11 from manufacturer ZJBENY is an example of an AFCI device integrated into the string box, that is, it can complement an inverter without this function. The device detects the arc and activates the string switches, simultaneously sending a signal through the communication system to the system operator.