As the core primary combining device in a photovoltaic array, the combiner box plays a crucial role, serving as the starting point for a photovoltaic power station's power supply. The selection of key components within the combiner box directly impacts its operational stability. Therefore, when building a photovoltaic system, the combiner box selection process cannot be overlooked.

❒ Importance of combiner box

The importance of the combiner box lies in its role as the starting point for a photovoltaic power plant's power supply. The selection of its components has a significant impact on stability . Its central position in the photovoltaic system means any equipment failure can lead to instability in the entire system, so it must be given great attention during design and selection.

❒ Structure and principle

A PV DC combiner box, a box that connects PV strings and is equipped with necessary protective devices, facilitates parallel connection of PV strings. It connects PV strings in parallel and is equipped with fuses and lightning arresters for safety . This equipment is typically deployed outdoors, and IP65 or higher protection is a  requirement.

❒ Protection level

Since the equipment is usually placed in an outdoor environment, having an IP65 or higher protection level is a basic requirement to ensure that the equipment can operate normally under various outdoor conditions.

02Selection of key components

❒ Fuse selection

In the application of PV DC combiner boxes, fuse selection is crucial. Currently, China's regulations in this area are not yet complete, so we can refer to the relevant provisions of the US National Electrical Code ( NEC , 2011 edition). For floating-ground PV DC combiner systems, it is recommended to install fuses on both the positive and negative poles of the strings. The fuses must comply with NEC standards for both the positive and negative poles , and should have a rated voltage of at least 1500V .

❒ Short-circuit current calculation

In a PV DC combiner box, if there are n combined paths and the short-circuit current of each path is Isc (here Isc refers to the short-circuit current of the module under STC conditions), then if a short circuit occurs between the positive and negative terminals of a particular string, the short-circuit current that the string fuse will withstand will be ( n-1 ) Isc . For example, if the PV DC combiner box has 24 paths, if a short circuit occurs in one of the strings, the current that the string fuse will withstand will reach 23Isc .

❒ DC circuit breaker

In a photovoltaic DC combiner box, a DC circuit breaker is a key protection device, typically used to protect the output current of several series-parallel connected groups. To meet the 1500V/1000V withstand voltage requirements, a 4P or 3P DC circuit breaker is generally used .

03Lightning protection and monitoring

❒ Lightning arrester selection

To ensure safe system operation, a lightning arrester (SPD) must be installed at the output of the PV DC combiner box. This device must provide dual positive and negative lightning protection, and its specifications must strictly meet relevant requirements. The SPD must have high discharge capacity and disconnect functionality , and the total protection level must be less than 4kV when connected in series .

❒ Monitoring device

To ensure users can accurately and in real time monitor the operating status of PV strings and thus guarantee efficient operation of the solar photovoltaic power generation system, PV DC combiner boxes are typically equipped with monitoring devices. These monitoring devices must be capable of real-time monitoring of parameters such as current and voltage, and ensure stable operation through self-powered operation and communication lightning protection. 485 communication is used. To prevent overvoltage caused by induced lightning from being introduced into the PV DC combiner box through the communication lines, a signal lightning arrester must be installed at the 485 interface.