How to choose the model and specification of the junction box

How to choose the model and specification of the junction box

The photovoltaic junction box is a kind of connector of the solar cell module. Its primary function is to export the electric energy generated by the solar cell module through the cable. Due to the particularity of the use of solar cells and their expensive value, photovoltaic junction boxes must be specially designed to meet the use requirements of solar cell modules.
In the photovoltaic power generation system, if the photovoltaic junction box is improperly selected, the solar panel may be burned, or the photovoltaic system may collapse. But, the saying goes, "Don't lose the big because of the small." So how should we choose the junction box?

1. Connection
The junction box acts as a bridge between solar modules and control devices such as inverters as a connector. Inside the junction box, the current generated by the solar module is drawn out and introduced into the electrical equipment through the terminal block and the connector.
To minimize the power loss of the junction box to the component, the conductive material used in the junction box requires a slight resistance, and the contact resistance of the lead wire of the bus strip should be small.

2. Protection
The protection function of the junction box includes three parts; one is to prevent the hot spot effect through the bypass diode and protect the cells and components; the second is to seal the design with unique materials for waterproof and fireproof; the third is to reduce the working temperature of the junction box through unique heat dissipation design, Reduce the temperature of the bypass diode, thereby reducing the power loss of the component due to its leakage current.

3. Weather resistance
Weather resistance refers to materials such as coatings, plastics, rubber products, etc., that are used outdoors to withstand the test of climate, such as extensive damage caused by light, cold and heat, wind and rain, bacteria, etc., tolerance is called weather resistance.
The parts of the junction box exposed to the environment are the box body, the box cover, and the connector (PC). They are all made of materials with solid weather resistance. The most commonly used material is PPO (polyphenylene ether), one of the world's top five universal One engineering plastics. It has the advantages of high rigidity, high heat resistance, flame retardancy, high strength, and excellent electrical properties. In addition, polybenzyl ether also has the advantages of wear resistance, non-toxicity, and pollution resistance. The dielectric constant and dielectric loss of PPO is one of the most miniature varieties in engineering plastics, and it is hardly affected by temperature and humidity. Therefore, it can be used in low, medium, and high-frequency electric fields. The deformation temperature under the load of PPO can reach above 190℃, and the embrittlement temperature is -170℃.

4. high temperature and humidity resistance.
The working environment of the components is very harsh—for example, some work in tropical areas. The average daily temperature is very high; some work at shallow temperatures, such as high altitudes and high latitudes; some work in substantial temperature differences between day and night, such as desert areas. Therefore, the junction box must have excellent high-temperature and low-temperature resistance.

5. UV resistance
Ultraviolet rays damage plastic products, especially in the plateau area where the air is thin and the ultraviolet irradiance is very high.

6. Flame retardancy
Flame retardancy refers to the property possessed by a substance or by treatment of a material to significantly delay the flame's spread.
The flame retardant grade increases step by step from HB, V-2, V-1 to V-0:
HB: The lowest flame retardant rating in UL94 and CSA C22.2 No 0.17 standards. For samples 3 to 13 mm thick, the burning rate is less than 40 mm per minute; for example, less than 3 mm thick, the blazing speed is less than 70 mm per minute; or it is extinguished before the 100 mm mark.

7. Waterproof and dustproof
Standard: IEC62852 /UL6703 "Enclosure Protection Level (IP Code)" provides the IP level of dust and water resistance, and the available junction box has a water and dust resistance level of IP65.

8. heat dissipation
The main factors that increase the temperature in the junction box are the diode and the ambient temperature. Diodes generate heat when they are conducting, and at the same time, heat is also caused due to the contact resistance between the diodes and the terminals. In addition, the increase in ambient temperature will also increase the temperature inside the junction box.
The components in the junction box that are easily affected by high temperature are sealing rings and diodes. The high temperature will accelerate the aging speed of the sealing ring and affect the sealing performance of the junction box; there is a reverse current inside the diode, and the reverse current will double for every ten °C increase in temperature, and the reverse current will reduce the current generated by the component, which affects the power of the element. Therefore, the junction box must have excellent heat dissipation or make a unique heat dissipation design.
A typical thermal design is to install a heat sink. But installing heatsinks doesn't completely solve the heat dissipation problem. Because the heat sink is installed inside the junction box, although the tube temperature of the diode is temporarily reduced, it will still increase the temperature of the junction box and affect the service life of the rubber sealing ring; if it is installed outside the box, on the one hand, it will affect the overall junction box. On the other hand, the tightness of the heat sink will also quickly cause the heat sink to be corrupted.

In general, the primary information for selecting photovoltaic junction boxes should be the current size of the components, one is the maximum working current, and the other is the short-circuit current. First, of course, the part's maximum current can output during short-circuit current, according to the short-circuit current. Therefore, the rated current should have a relatively significant safety factor. On the other hand, the safety factor is more minor if the junction box is calculated according to the maximum working draft.
The scientific basis for selecting photovoltaic junction boxes should be based on the change of the current and voltage of the cells that should be taken out with the light intensity. Therefore, it is necessary to know which are the modules you produce are used in and how much light is the strongest in this area, and then Compare the current curve of the cell with the light intensity, check the maximum possible current, and then select the rated current of the photovoltaic junction box.

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