In photovoltaic (PV) systems, both combiner boxes and junction boxes play a vital role. These two electrical components are critical to ensuring proper electrical connection and safety within the system, but they differ significantly in their specific functionality and technical implementation. Understanding these differences is critical for system designers and installers as it can improve the efficiency, stability and safety of the system. This article provides a detailed analysis of the function, application, structure and maintenance of combiner boxes and junction boxes, explaining their importance in photovoltaic systems.

Functions and applications of combiner boxes and junction boxes

Although combiner boxes and junction boxes are both components of a photovoltaic system, they have different functions and different applications.

-Function of the combiner box: The main function of the combiner box is to combine the currents from several photovoltaic module strings (photovoltaic panel strings) into a larger current. This combined current is then transmitted through one or more thicker cables to the inverter for processing. In large photovoltaic arrays, combiner boxes can significantly reduce the number of cables, simplify wiring, and reduce system cable losses. Additionally, combiner boxes are often equipped with fuses or circuit breakers to prevent excessive current from any one string from damaging the system. Advanced combiner boxes can also be equipped with monitoring equipment to track the current and voltage of each string, allowing for rapid troubleshooting and ensuring optimal system performance.

-Functions of the junction box: On the other hand, the junction box is mounted on the back of a single PV module and is responsible for connecting the positive and negative cables of the module to the rest of the array. Bypass diodes within the junction box protect the module by allowing current to bypass shielded or damaged parts of the module, preventing voltage drops or hot spot effects from affecting the module’s performance. Junction boxes also provide basic protection, ensuring that electrical components can withstand harsh environmental conditions.

Structure and application scenarios of combiner boxes and junction boxes

There are significant differences in the structure and application scenarios of combiner boxes and junction boxes, which affects their use in photovoltaic systems.

-Structure and application of combiner box: Combiner box is usually installed between the photovoltaic array and the inverter to collect current from different module strings. The internal structure of the combiner box is relatively complex because it includes not only current combining functions, but also protection devices such as fuses, isolators and potential monitoring modules. With the development of technology in the solar energy industry, more and more combiner boxes are equipped with remote monitoring functions, allowing them to be connected to operation and maintenance platforms via the Internet, thus facilitating real-time monitoring of photovoltaic system performance. Combiner boxes are widely used in large-scale photovoltaic power stations or distributed photovoltaic projects, especially when multiple modules are connected in series and parallel.

-Structure and application of junction box: The junction box is small in size and is usually installed on the back of each photovoltaic module. Its internal structure is relatively simple, mainly composed of connecting terminals and bypass diodes. Because junction boxes are exposed to outdoor environmental challenges, such as high and low temperatures, moisture, and UV radiation, junction boxes must be designed with excellent protection capabilities. High-quality junction boxes usually have a high level of protection, such as IP65 or higher, to ensure long-term operation in harsh conditions. The junction box is a critical component at the module level, ensuring that the electrical output of each module is efficiently transmitted to the rest of the array.

Comparison of the functions of combiner box and junction box

Although combiner boxes and junction boxes are both responsible for electrical connections in a photovoltaic system, they play different roles.

– System level vs. module level roles: The combiner box operates at the system level, collecting currents from multiple module strings and transmitting them to the inverter, which simplifies system wiring, optimizes cable layout, and reduces losses. Therefore, the combiner box is a vital electrical hub in the entire photovoltaic system. However, junction boxes operate at the module level, ensuring that each module’s electrical output is properly connected to the rest of the array. In addition, it provides basic protection for each module against blackout or partial failure.

– Comparison of safety and protection features: Combiner boxes often contain fuses or circuit breakers that protect the system by cutting off excess current in any one string. Some advanced combiner boxes also include isolators, allowing for safe disconnection of power during system maintenance. In contrast, junction boxes rely on their internal bypass diodes for protection. When part of the module is blocked, the bypass diode allows current to flow around the affected part, preventing damage to the module due to hot spot effects. So while combiner boxes provide protection at a system level, junction boxes primarily protect individual modules.

Selection and maintenance of combiner boxes and junction boxes

Proper selection and maintenance of combiner boxes and junction boxes are critical to ensuring the long-term stability and performance of your photovoltaic system.

-Selection of combiner box: When selecting a combiner box, the system size should be considered to determine the appropriate current capacity. Additionally, consideration must be given to whether the system requires monitoring capabilities. For large photovoltaic power plants, combiner boxes with remote monitoring capabilities can provide real-time data on system performance and help with maintenance work. Additionally, selecting a combiner box with adequate overcurrent protection devices, such as fuses or circuit breakers, is critical to protecting the system from potential faults.

-Junction box selection: The selection of junction box depends largely on the specifications of the photovoltaic module and the usage environment. A high-quality junction box should be highly resistant to water, dust, and heat to ensure reliable long-term performance. In addition, bypass diodes within the junction box should be carefully selected to ensure that they effectively protect the module in the event of blackout or partial failure.

– Maintenance of combiner boxes and junction boxes: Both combiner boxes and junction boxes require regular maintenance to ensure optimal performance. Fuses and circuit breakers in the combiner box should be checked regularly to ensure they are functioning properly and to prevent system failure due to aging components. In addition, monitoring modules, if any, should be tested regularly to confirm their data collection and transmission capabilities. As for the junction boxes, although they are mounted on the back of the PV modules and are more difficult to access, it is necessary to regularly visually inspect their appearance and protective seals to ensure that they remain waterproof and dustproof, especially in severe weather conditions.

Practical application case studies

To better understand how combiner boxes and junction boxes are used in real-world photovoltaic systems, let’s look at some practical examples.

-In large-scale ground-mounted photovoltaic power stations, combiner boxes with intelligent monitoring functions are usually used to ensure the smooth operation of the power station. Such power plants may have hundreds or even thousands of PV module strings, and without combiner boxes, the number of cables would make wiring complex and maintenance challenging. By using combiner boxes to gather current, the system design is simplified and overall safety and maintenance efficiency are improved.

-In small-scale distributed photovoltaic projects, the reliability of the junction box becomes critical. For example, in rooftop PV installations in urban environments, shading occurs frequently, making bypass diodes in the junction box critical to maintaining module performance. If the module is partially shaded, a bypass diode in the junction box prevents the blocked part from affecting the output of the entire module, ensuring stable energy production.

In conclusion

Although combiner boxes and junction boxes both play key roles in photovoltaic systems, there are significant differences in their functions, structures and application scenarios. Combiner boxes are critical at a system level, gathering current, protecting the system and providing monitoring capabilities, especially for large PV installations. Junction boxes, on the other hand, are critical at the module level to ensure proper electrical connections and protect individual modules from obstruction or failure. When designing and installing a photovoltaic system, careful selection of these two components is critical to ensuring the safety, reliability, and efficiency of the system. Regular maintenance is also necessary to keep the system running optimally over time.