- Function and classification
The main function of the junction box is to connect the solar cell circuit inside the module with the external circuit through the positive and negative cables of the junction box, and transmit the electric energy. The junction box is glued to the backplane of the module with silicone. The junction box is equipped with bypass diodes to protect the battery strings. The design requirements of the junction box are very high, involving technologies in many fields such as electrical design, mechanical design and material science.
The junction box is mainly composed of three parts: the junction box body, the cable and the connecting terminal. The junction box body is generally composed of the following parts: base, conductive parts, diode, sealing ring, sealing silicone, box cover, etc. At present, there are many types of junction boxes on the market. From the point of view of whether to pour glue, there are glue filling and non-glue filling. Whether glue filling is generally determined according to the volume and safety performance requirements of the junction box. According to the connection method of the bus bars inside the junction box, it can be divided into snap-on type and welding type. Generally, the non-glue type adopts the clip-on type, and the glue-filled type needs to use the welding type because of the small internal space. The amine-filled junction box is small in size, low in cost, and the proportion of component failure and replacement is not high, so it has gradually become the mainstream in the market.
There are also integrated junction boxes and split junction boxes. There are one or more diodes and positive and negative cables in the integrated junction box. Generally, only one junction box can be used for one component. The split junction box contains multiple junction boxes, each junction box has a diode, and the positive and negative cables are distributed on the two junction boxes, so a component must have at least two junction boxes. The split junction box needs to be filled with glue because of its small size. The advantages of using a split junction box are as follows:
(1) It can simplify the connection method of the internal bus bars of the module stack, reduce the usage of the bus bars, reduce the overall circuit resistance of the module, and improve the output power of the module;
(2) The positive and negative lead-out lines of the components are located on the side of the component close to the frame, so only a short cable is required for the vertical installation and connection of the component, which greatly reduces the amount of system cables, reduces the series resistance of the system, and improves the system. power generation. However, when the module is installed horizontally, the cable length will increase, so the design needs to be considered comprehensively.
There are two types of bypass diodes inside the junction box: pin-type and patch-type. Generally, pin-type diodes are used when the size of the box is large, such as non-glue junction boxes; patch-type diodes are used when the box is small, such as glue-filling wiring. box. The reverse withstand voltage and heat resistance of the diode should be determined according to the use requirements of the components and related standards. The working principle of the bypass diode is as follows: the diode is connected in parallel with several cells. During the operation of the module, when a certain cell or several cells in the module are blocked by dark clouds, branches, birds and other obstructions, a hot spot occurs. The bypass diode in the junction box uses its own unidirectional conductivity to provide a bypass channel for the faulty battery string, and the current flows through the diode, so as to effectively maintain the performance of the entire component and obtain the maximum power generation. The working principle is shown in the figure. 1 shown. The commonly used diode is the Schottky diode, which has the advantage of lower voltage drop, which can reduce the power loss caused by the diode. The most ideal component should be a bypass diode in parallel with each cell, but due to process and cost considerations, in practical applications, a diode generally needs to protect 10-24 solar cells.
The early junction boxes were mainly imported MC and Tyco, which were expensive. After 2002, Zhejiang Renhe, Suzhou Kuike, Changzhou Laini, Yangzhou Tongling, and Changzhou Jiuding continued to break through the technical bottleneck and realized the localization of junction boxes. , and technology continues to improve. The PV-RH series products produced by Zhejiang Renhe in the early days accounted for 1/3 of the global total during the peak period. In the early days, most of the junction boxes were one-piece, non-glue-filled and snap-on type. It has become the mainstream, and the split junction box has also been widely used in many component designs.
- Technical requirements of junction box
The junction box is directly exposed to the air and will be exposed to wind, rain and sun for a long time, and it is small in size and has many internal assemblies, which poses a great test for its weather resistance, sealing and safety. During the operation of the photovoltaic system, it is also relatively common for junction box problems main aspects:
(1) The shell has strong anti-aging and UV resistance;
(2) Good heat dissipation performance to reduce the internal temperature of the junction box and the temperature of the component junction box position;
(3) Excellent sealing performance and low connection resistance to reduce the power loss caused by the junction box;
(4) The current matching capability of the diode;
(5) Reasonable electrical safety design, such as creepage distance, etc., to meet electrical safety requirements;
(6) Excellent flame retardant ability.
A simple junction box needs more than ten kinds of materials. Whether the material selection is reasonable directly affects the quality of the junction box itself, so the material of the junction box has always been valued by the industry.
The size and structural design of the junction box body have strict requirements, such as the creepage distance from the internal live body to the edge, the electrical clearance requirements, and the maximum system voltage and maximum operating current that can be tolerated. The junction box design should currently implement the latest IEC/EN 62790 standard. Table 3-9 shows the changes in the standards for photovoltaic junction boxes and photovoltaic connectors.
For photovoltaic cables, the industry currently uses the German 2pfg1162 standard. Since there has been no standard for photovoltaic cables, TUV certification uses 2pfg1162 as the photovoltaic cable test standard. Until October 27, 2017, EN 50618 was adopted as the photovoltaic cable test standard. cable standard. In addition to the appearance and size of the junction box that need to meet the product design requirements, there are also technical specifications for the diode products in the junction box. The technical indicators are generally used as routine test items for the junction box, and each batch of materials needs to be tested. To evaluate and test a new type of junction box, the easiest way is to see if the product is certified, and to perform other reliability tests at the same time, such as thermal cycling, high temperature and high humidity, and wet freezing tests based on the IEC 61215 standard (generally need to do components for reliability testing). The wet leakage performance of the component is also one of the important indicators for evaluating the junction box. In addition, there is also an ultraviolet resistance performance test. Generally, it is required that the junction box is free of damage, cracking, bending, discoloration, brittleness, aging and other phenomena after receiving ultraviolet radiation of 90kw.h/m”.
The junction box is different from other packaging materials. It must first obtain the TUV/UL. full set of certification for the junction box body, cables and connecting terminals before it can be used in the market. TUv certificate can be inquired online at www.tuv.com; according to the UL. Yellow card number, UL certificate can be inquired online through the following website: http://data-base.ul.com/cgi bin/XYV/template/LISEXT/1FRAME/index. htm. Each component manufacturer conducts the above-mentioned relevant tests according to different situations, and can only be put into use after passing the test.
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