Understand stringboxes with multiple inputs and multiple outputs

The stringbox is a component widely used in photovoltaic systems. It has protection and sectioning functions, in addition to its main function, which is to unite the strings into a single arrangement, which will then be connected to the inverter's direct current input.

The figure below illustrates the general organization of a stringbox, where we notice the presence of protection elements (fuses and DPS – surge protection device) and sectioning.

The main function of the stringbox is performed by the positive and negative buses, to which the string poles are connected.

The function of the buses, as we said above, is to unite all the positive and negative poles of the strings and provide two terminals at the output (positive and negative) that will be connected to the inverter. At least this is the function of the traditional stringbox, if we can call it that.

There are variations of stringboxes that do not perform the main function of concatenating strings, but offer protection and sectioning functions. That's why we find stringboxes on the market that have multiple inputs and multiple outputs. The images shown below are of a stringbox that has two inputs and two outputs, with the following characteristics:

• Entries: 2;
• Outputs: 2;
• Maximum voltage: 600 Vdc;
• Dimensions 310 x 215 x 110 mm;
• IP65 housing with UV protection;
• DPS Beny, which meets the EN 50539-11 standard;
• Use of fuses as redundant protection.

As we can see in the figure above, the stringbox does not have internal buses. There are two independent entries for strings. Each string is connected to a positive and negative fuse and then passes through the disconnector switch (in the center). The “String 1” and “String 2” inputs are independent, as are the “MPPT 1” and “MPPT 2” outputs.

One of the uses of this type of stringbox is to allow the use of a disconnector switch external to the inverter, something that is highly recommended, as discussed in the webinar Webinar: Stringbox – standards and good practices.

Furthermore, the stringbox allows the use of a surge protector device (SPD) external to the inverter. In this case, as the strings are independent, two surge protectors are used. Some inverters found on the market today already have internal surge protectors, which would theoretically eliminate the need for a stringbox protector.

In any case, the presence of the stringbox is interesting, as it makes the photovoltaic system compatible with any type of inverter (with or without internal DPS). Furthermore, even if the inverter already has internal DPS, the stringbox DPS provides additional protection and offers greater ease of inspection and maintenance, as discussed in the article Stringbox with DPS is necessary or not? What does NBR 16690 say?.

The most common case of a stringbox with multiple inputs would be the 2/2 (two inputs and two outputs), which we talked about above. However, many other variations can be found, depending on the type of photovoltaic system. Below we exemplify a 3/2 stringbox (two inputs and three outputs). In the image below we identify the existence of inputs for 3 strings, while on the other side we have two outputs.

In this specific project, the stringbox offers sectioning and surge protection individually to “String 1”, connected to the “MPPT 1” output, while the “String 2” and “String 3” inputs are concatenated by buses connected to the fuse holders (positive and negatives) present in these entries.

The “MPPT 2” output then corresponds to the “String 2” and “String 3” inputs and is associated with a second surge protector and a second disconnector switch. Other variations are possible, such as 4/2 (four inputs and two outputs), 6/2 (six inputs and two outputs) etc., depending on the needs of the photovoltaic system in question. Below we illustrate, for example, what a 6/2 stringbox is:

Having 6 inputs and 2 outputs, in this example, means that the stringbox can receive 2 groups of strings, each with 3 strings connected in parallel. Each group is connected to its own surge protector and individual disconnector switch. The “MPPT 1” and “MPPT 2” outputs are independent.

The 6/2 stringbox in this last example is suitable for inverters that have two MPPT inputs or for photovoltaic systems that employ 2 inverters, with each MPPT input (or each inverter, as the case may be) being able to receive 3 parallel strings.