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Currently, when we come across the topic photovoltaic installation there are still many doubts about feasibility, reliability and what materials should be involved to bring system security and integrity.
This article focuses on Brazilian photovoltaic installations, where an essential part of this type of installation is the use of string boxes.
These boxes are used to combine multiple strings, protect the installation and home against overvoltage, in addition to having many other functions.
What are the special features of the PV Next string box?
O PVNext is a broad and global string box portfolio from Weidmüller and, from 2023, will also be manufactured in Brazil.
They are based on a modular concept and the application of an assembly design on printed circuit boards, meeting the objective of covering the largest possible number of variants with the smallest number of final products, in addition to reducing the biggest risk generators that are the connections.
For example, if the installation requires a version with only one input and one output, the two-input and two-output version can be used, ensuring high stock availability, flexibility of use and cost optimization.
PV NEXT’s MPPT and PCB philosophy
An essential aspect of the modular concept is the use of a limited number of printed circuit boards, or PCBs. The logic is simple: a PCB is needed for an MPPT.
Therefore, if the inverter has three MPPT's, a PV Next box with three similar PCBs will be required. Furthermore, for optimization there are already models with two MPPT's on a single PCB.
Adapting to the market for the purpose of several MPPT inputs, the boxes have on their PCB, three connection points for each input and three connection points for each output, all of which are interconnected.
To make this selection as easy as possible, Weidmüller offers a digital selection guide to find the right PV Next variant according to your inverter usage. This step-by-step guide will also be indicated at the end of the article.
How can I protect facilities against a power surge?
In accordance with regulations such as NBR IEC 61643 and EN 51643-32:2020, design and installation engineers agree that all photovoltaic installations in residential or commercial buildings must be protected with surge protectors (SPDs) on the DC side.
And depending on the type and conditions of the installation, it is necessary to define the overvoltage protection between Type II or Type I+II, remembering that this decision is subject to the risk analysis presented in NBR 5419. The PV Next portfolio offers the appropriate solution for both the types.
Example: The SMA Sunny Boy 5.0 inverter with two inputs (each with an MPPT) connected to a rooftop installation without a lightning protection system requires a PV Next with two PCBs and Type II overvoltage protection.
It also includes the essential component for protection, which is the disconnector. Furthermore, it is shown that only one slot per PCB is used. The other two remaining MPPT inputs remain unused and available.
The importance of external sectioning to the inverter
Even with the use of string inverters with on-board disconnectors, it is necessary to use a dedicated switch in a string box for the following reasons:
- In the event of a possible short between the positive and negative poles of a string, the string box with disconnector ensures total separation of the modules and the inverter, physically guaranteeing the useful life of the inverter and connections;
- In preventive or corrective maintenance, the use of a dedicated disconnector will guarantee safe operation when handling the strings and electrical connections of the PV system;
- Normally, the number of cycles of a dedicated disconnector, related to its useful life, is greater than those specified in the inverters;
- Use of specific equipment for the application of DC sectioning under load and which cannot allow any risk of electric shock to the user or creation of an electric arc posing a risk of fire.
Inverter availability
It is true that most inverters used in the residential and commercial market already have DPS incorporated, which theoretically meets the basic standards for photovoltaic installations, but does not necessarily mean that it is the best option for protecting the system.
When the use of this type of inverter is mentioned, we still compromise the availability of the installation if we choose not to use the string box.
The DPSs installed in the inverter are soldered on their control board. When the DPS acts during lightning or at the end of its useful life, causing system unavailability, a stop and, sometimes, even a complete replacement of the product is necessary to resume generation.
Meanwhile, in the string box it is possible to change just the component or just the actuated cartridge. In this matter, PV Next will guarantee an easy and quick way of installation and replacement when the DPS is compromised, using the PCB extraction system with an installation base,
Matching number of strings with PV Next
In addition to the overvoltage protection function, it is also possible to combine strings with all speaker models. The range extends to combining up to twelve input strings for six MPPTs, i.e. with two inputs each.
And as mentioned previously, these models allow the connection of up to three input channels per MPPT. All projects are certified according to IEC 61439-2.
Example: A 45 kWp rooftop installation is implemented with a DELTA M50 A. This inverter can support 58 kWp with an MPPT input.
Due to the high power and low number of inputs, installation requires a combiner box with three strings per MPPT. Thus, the solution can be visualized in the graph below.
Additional PV Next Resources
The modularity of the projects is also continued in other features of the portfolio:
- On all models, a functional earth connection is ready for use. It guarantees the proper functioning of the equipment and the electrical system to which it will be connected;
- Layouts can be ordered with the application of a Disconnector to allow control between DC on and off. In this profile, the solution design, including the number of PCBs and the box size, remains the same, only featuring cables between the PCB and the disconnector. In keyless designs, this step is connected with a cable (pre-installed). Keys can be easily locked for maintenance;
- String connection (input +/-) can be implemented in two different ways. The first option is to use the Weidmüller PV connector for a quick connection out of the box. The second is to pass the cables through cable glands and connect them inside the box. Inside the box, the connection can be made easily with the help of the PUSH-IN terminals as this technology only requires a stripped cable and this can be inserted directly using just a standard screwdriver;
- Combiner boxes are often installed under adverse weather conditions. This will lead to condensation inside the box. To get rid of this moisture, all PV Next boxes have a built-in pressure compensation element;
- The status of the surge protectors can be seen from their color indication (green = OK to operate, red = must be replaced). This only requires a visual check through the transparent box lid. Another alternative is to check the status on a monitoring system. For this purpose, all PV Next projects contain a Remote Contact, which allows a cable connection to the communication unit.
Guide to quickly and correctly find your PV Next model
Access the website by clicking here. See the most frequently asked questions below.
How many MPPTs are considered for your project? How many MPPTs do your inverters have?
It is possible to protect multiple inverters with a single PV Next box. This depends on the installer's philosophy.
How many inputs should the combiner box have?
With PV Next, you can handle two input strings (+/-) for up to six MPPTs. To discover the correct combination, the installation must be simulated or calculated. The required parameters are the power, voltage and number of solar panels and the power, voltage and number of inputs per MPPT of the inverter.
What type of surge protection is needed? Type I/II or Type II?
Generally, type I/II must be used if the building has an external lightning protection system; otherwise, type II must be used. Details about surge protection in combiner boxes can be found on the Weidmüller websites.
Does the installation require DC load sectioning?
According to IEC 60364-7-712, a load disconnector must be installed in the direct current location. This may be part of the inverter or part of the combiner box (suggested).
Would you like to prepare a plug&play installation or via PUSH-IN connectors?
Customers can decide on the connection type. You can choose between a pre-wired box with PV connector interface or a box with cable glands and connection via PUSH IN terminals inside. The second option requires inserting the cables inside the box at the installation location.
The opinions and information expressed are the sole responsibility of the author and do not necessarily represent the official position of Canal Solar.
