Explain solar junction box and its functions in detail

1. What is a solar junction box

Solar junction box is an important component of solar cell modules. It is also the connector between the solar cell and the solar cell charging control device. At the same time, the solar junction box is a comprehensive cross-field design. The top 10 photovoltaic battery companies in the world are able to provide users with a combination connection scheme for solar panels.

2. Classification of solar junction box

There are two main types of solar junction boxes: non-potted and potted. The ordinary junction box is sealed with a silicone sealing ring, and the glue junction box is filled with two-component silicone to pot, in order to prevent outdoor aging, most of them are now filled with glue junction box.

In addition, according to the functional application, it can also be divided into monomer junction box, three-part junction box (currently used in half a piece), conventional junction box, intelligent junction box (intelligent shutdown), etc.

3. Design principle of solar junction box

No matter how solar junction box changes, the basic structure remains unchanged, including the box, the cover, connectors, terminal blocks, diodes, etc. Some junction box manufacturers have designed heat sinks to enhance the temperature dissipation in the box, and some junction box manufacturers have made other detailed designs. Such as chips, etc., but the overall structure has not changed.

● Box

The box body is the main part of solar junction box, built-in terminal blocks and diodes, external connectors and box covers, is the frame part of solar junction box, and bears most of the weathering requirements. They have the advantages of large rigidity, high heat resistance, flame retardant, wear resistance, non-toxic, pollution resistance, small dielectric constant, high strength, and excellent electrical properties.

● Box lid

The lid of the box plays the role of sealing the box, waterproof, dustproof and anti-pollution. The tightness is mainly reflected in the built-in rubber sealing ring, which prevents air and moisture from entering the inside of the junction box. Some manufacturers set small holes in the center of the box lid, and dialysis membranes are installed in the air.

The membrane is breathable and impermeable, and there is no water penetration for three meters underwater, which plays a very good role in heat dissipation and sealing. The box body and lid are generally made of injection molding materials with good weather resistance, good elasticity, temperature shock resistance and strong aging resistance.

● Connector

Connectors connect terminal blocks and external powered devices such as pv inverter, controller, etc. Connectors are generally made of PC (polycarbonate) or PPE (polyphenylene ether), but PC is easily corroded by a variety of substances.

The aging of solar junction box is mainly reflected in the fact that the connector is easy to be corroded, and the low temperature impact of the plastic nut is easy to crack. Therefore, the service life of the junction box is reflected in the life of the connector.

● Diode

The diode in the solar junction box is used as a bypass diode to prevent the hot spot effect and protect the component. In addition, masking one cell has a different effect than masking half of each of the two cells. So when masking is unavoidable, try to make as many solar batteries as possible to shade, each solar battery with as little shadow as possible.

4. The function of a solar junction box

● Connecting role

Solar junction boxes act as connectors to bridge solar modules to control devices such as inverters. Inside solar junction box, the current generated by the solar modules is drawn out and directed into the electrical equipment through terminal blocks and connectors.

In order to minimize the loss of module power by the junction box, the conductive materials used in the solar junction box require small resistance and small contact resistance of the bus lead wire. Considering the economic benefits, it is mainly used for relatively large power generation systems rather than home energy storage.

● Protective effect

The protective effect of solar junction box includes three parts: Bypass diodes prevent hot spot effects and protect cells and components. Designed for waterproof and fireproof protection by special material sealing. The operating temperature of solar junction box is reduced by a special heat dissipation design and the temperature of the bypass diode is reduced. This reduces the loss of component power by leakage current.

● Resistance properties

• Weather resistance

Materials such as coatings, plastics, rubber products, etc., are used outdoors to withstand the test of climate. Such as the comprehensive damage caused by light, cold and heat, wind and rain, bacteria, etc., its tolerance is called weather resistance. The exposed parts of the solar junction box are the box, the lid and the connector, which are all made of weather-resistant materials.

At present, the most commonly used materials are PPO (polyphenylene ether), PPE (polypropylene), one of the general engineering plastics. They have the advantages of large rigidity, high heat resistance, flame retardant, wear resistance, non-toxic, pollution resistance, high strength, and excellent electrical properties. In addition, polyphenylene ether also has the advantages of wear resistance, non-toxicity and pollution resistance.

• High temperature and humidity resistance

The working environment of components is generally very harsh. Some work in the tropics, where the average daily temperature is very high. Some operating temperatures are very low, such as high altitude areas and high latitudes. Some have very large temperature differences between day and night, such as desert areas. Therefore, solar junction box is required to have excellent high temperature resistance and low temperature resistance.

• UV resistant

Ultraviolet rays have a certain damage to plastic products, especially in the plateau area, the air is thin, and the ultraviolet irradiance is very high. Flame retardancy refers to the property that a substance has, or that the material has after treatment, significantly delays the spread of flames.

• Heat dissipation

The main factors that increase the temperature in the solar junction box are the diode and the ambient temperature. The diode generates heat when it is on. At the same time, heat is generated due to the contact resistance of the diode and the terminal block. In addition, the increase in ambient temperature will also increase the internal temperature of the solar junction box.

The components of the solar junction box that are susceptible to high temperatures are seals and diodes. High temperature will accelerate the aging speed of the sealing ring and affect the tightness of the junction box. There is a reverse current inside the diode, and for every 10°C increase in temperature, the reverse current doubles, and the reverse current reduces the current generated by the component, affecting the power of the component.

Therefore, the solar junction box must have excellent heat dissipation, or do special heat dissipation design. A common heat dissipation design is to install a heat sink. But installing a heatsink does not completely solve the heat dissipation problem.

Because if a heat sink is installed inside solar junction box, although the tube temperature of the diode is temporarily reduced, it will still increase the temperature of the solar 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 tightness of the solar junction box, and it will also be easy to corrode the heat sink.

5. Considerations for testing and use

● Test

The junction box should be tested before use, and the main items are appearance, sealing, fire rating, whether the diode is qualified, etc.

● Application

• Before use, make sure the junction box has been tested and qualified.
• Before placing a production order, please determine the terminal block spacing and determine the typesetting process.
• When installing solar junction box, the gluing should be uniform and comprehensive to ensure that the box body and the back plate are completely sealed.
• Be sure to distinguish between cathode and anode.
• When connecting a bus belt, be sure to check whether the tension between the bus belt and the terminal block is sufficient.
• When using soldered terminal blocks, do not solder for too long to avoid damaging the diode. Be sure to clamp firmly when installing the lid.

6. Common undesirable phenomena of solar junction box

Common faults of photovoltaic module junction boxes include: aging and deformation of the box, virtual welding in solar junction box, bypass diode breakdown failure, junction box burning, junction box detachment from silica gel, etc.

● Analysis of common fault principles of junction boxes

• Component welding process quality problems

There is virtual soldering at the connection between the diode pin and the copper conductor and the bus bar and the copper conductor in the junction box. When the PV module has shadow occlusion or other problems that cause the bypass diode to turn on, the virtual solder will heat up.

When the heat accumulation at the virtual weld exceeds the heat deflection temperature of solar junction box insulation material, the junction box will undergo aging deformation. The longer the bypass diode is open, the greater the risk of deformation and aging of solar junction box, when the temperature is greater than the upper limit of the diode junction temperature, the high temperature will cause thermal breakdown damage to the bypass diode, and even burn the junction box.

• Component sealing process quality issues

There is contamination in the adhesive process between the junction box and the PV module backplane, resulting in the failure of solar junction box and the silicone in the later stage.

• Shadow occlusion andcracking

The photovoltaic module is under the conditions of shadow occlusion, cracking, local hot spot and other conditions for a long time, so that the bypass diode is in a continuous working state for a long time, which causes the temperature of the bypass diode junction to rise.

When the junction temperature accumulates to a certain extent, thermal breakdown failure occurs in the bypass diode. If not treated in time, when the heat accumulates to the deformation temperature of the junction box insulation material, the junction box will be deformed and aged, and the serious situation will cause solar junction box to burn.

• Lightning strike

When a photovoltaic module is struck by lightning, the bypass diode will be instantly broken down by high voltage. When the rain clears, the diode heats up as normal component current flows through the failed diode for a long time. When the heat accumulates to a certain extent, it will cause the aging and deformation of the junction box, and even cause solar junction box to burn.

7. Conclusion

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