Properties of photovoltaic junction boxes

1. Weathering resistance

Weathering resistance refers to the comprehensive damage caused by materials such as coatings, plastics, rubber products, etc. applied outdoors to withstand the test of climate, such as light, heat, wind and rain, bacteria, etc. Its resistance is called weather resistance.

The parts of the junction box that are exposed to the environment are the box body, box cover, and connector (PC), which are all made of materials with strong weather resistance. Currently, the most commonly used material is PPO (polyphenylene oxide), which is one of the five universal engineering plastics in the world. It has the advantages of high rigidity, high heat resistance, flame retardancy, high strength, and excellent electrical performance. In addition, polyether also has advantages such as wear resistance, non-toxic, and pollution resistance. The dielectric constant and dielectric loss of PPO are one of the smallest varieties in engineering plastics, almost unaffected by temperature and humidity, and can be used in low, medium, and high-frequency electric field fields. The load deformation temperature of PPO can reach above 190 ℃, and the embrittlement temperature is-170 ℃.

2. High temperature and humidity resistance

The working environment of the components is very harsh, some of which work in tropical regions with very high daily average temperatures; Some work at very low temperatures, such as in high-altitude and high-latitude areas; Some areas have a very large temperature difference between day and night, such as desert areas. Therefore, it is required that the junction box should have excellent high-temperature and low-temperature resistance performance.

The weather resistance test is shown in the table below:

3. UV resistance

Ultraviolet rays have certain damage to plastic products, especially in the plateau where the air is thin and the ultraviolet Irradiance is high.

4. Flame retardancy

Flame retardancy refers to the properties of a substance or material that significantly delay flame spread after treatment.

The flame retardant level increases gradually from HB, V-2, V-1 to V-0:

HB: UL94 and CSA C22.2 No 0.17 are the lowest flame retardant ratings in the standard. For samples with a thickness of 3 to 13 millimeters, the combustion rate is required to be less than 40 millimeters per minute; Samples less than 3 millimeters thick have a combustion rate of less than 70 millimeters per minute; Or turn off before the 100mm mark.

V-2: After two 10 second combustion tests on the sample, the flame extinguishes within 60 seconds. Flammable materials can fall.

V-1: After two 10 second combustion tests on the sample, the flame extinguishes within 60 seconds. No combustibles should fall.

V-0: After two 10 second combustion tests on the sample, the flame extinguishes within 30 seconds. No combustibles should fall.

5. Waterproof and dustproof

Standard: The IP level of dust and water resistance specified in GB 4208 2008 "Degrees of Protection Provided by Enclosures (IP Code)" is expressed as:

The first X represents the dust level

0: No protection

1: Prevent large solid intrusion

2: Preventing intrusion of medium sized solids

3: Prevent small solids from entering and invading

4: Prevent solid objects larger than 1mm from entering

5: Prevent harmful dust accumulation

6: Completely prevent dust from entering

The second X represents the waterproof level

0: No protection

1: Water droplets dripping into the shell have no effect

2: When the shell is tilted to 15 degrees, water droplets drop into the shell without affecting it

3: Water or rainwater from a 60 degree angle

4: The liquid splashed onto the shell from any direction has no harmful effect

5: Rinse with water without any harm

6: Can be used in the environment inside the cabin

7: Can withstand immersion in water for a short period of time (1m)

8: Immerse in water for a long time under certain pressure

The waterproof and dustproof level of general junction boxes is IP65.

6. Heat dissipation

The main factors that increase the temperature inside the junction box are the diode and ambient temperature. Diodes generate heat when conducting, and at the same time, heat is also generated due to the contact resistance between the diode and the wiring terminal. In addition, an increase in ambient temperature will also cause an increase in the internal temperature of the junction box.

The components in the junction box that are easily affected by high temperatures are sealing rings and diodes. High temperature will accelerate the aging rate of the sealing ring, affecting the sealing performance of the junction box; There is a reverse current inside the diode, which doubles for every 10 ℃ increase in temperature. The reverse current reduces the current generated by the component and affects its power. Therefore, the junction box must have excellent heat dissipation or be designed with special heat dissipation.

A common heat dissipation design is to install heat dissipation fins. However, installing the heat sink did not completely solve the heat dissipation problem. Because if a heat sink is installed inside the junction box, although the diode tube temperature is temporarily reduced, it will still increase the temperature of the junction box, affecting the service life of the rubber sealing ring; If installed outside the box, it will not only affect the overall sealing of the junction box, but also easily corrode the heat dissipation fins.