How to Choose a Solar Panel Connection Box?
The solar panel connection box is the connector between the solar panel and the charging control device, and is an important part of the solar panel. It is a cross-disciplinary comprehensive design that combines electrical design, mechanical design and material science to provide users with a combined connection scheme for solar panels.
The main function of the solar connection box is to output the electrical energy generated by the solar panel through the cable. Due to the particularity and high price of solar cells, solar junction boxes must be specially designed to meet the requirements of solar panels. We can choose from five aspects of the function, characteristics, type, composition and performance parameters of the junction box.
1. The Function of Solar Panel Connection Box
The basic function of the solar connection box is to connect the solar panel and the load, and draw the current generated by the photovoltaic panel to generate electricity. Another function is to protect the outgoing wires from hot spot effects.
(1) Connection
The solar junction box acts as a bridge between the solar panel and the inverter. Inside the junction box, the current generated by the solar panel is drawn out through terminals and connectors and into the electrical equipment.
In order to reduce the power loss of the junction box to the solar panel as much as possible, the resistance of the conductive material used in the solar panel junction box should be small, and the contact resistance with the busbar lead wire should also be small.
(2) Protection Function of Solar Connection Box
The protection function of the solar junction box includes three parts:
1. Through the bypass diode is used to prevent the hot spot effect and protect the battery and solar panel;
2. The special material is used to seal the design, which is waterproof and fireproof;
3. The special heat dissipation design reduces the junction box and the The operating temperature of the bypass diode reduces the loss of solar panel power due to current leakage.
2. The Characteristics of PV Junction Box
(1) Weather Resistance
When the photovoltaic junction box material is used outdoors, it will withstand the test of climate, such as damage caused by light, heat, wind and rain. The exposed parts of the PV junction box are the box body, the box cover and the MC4 connector, which are all made of weather-resistant materials. At present, the most commonly used material is PPO, which is one of the five general engineering plastics in the world. It has the advantages of high rigidity, high heat resistance, fire resistance, high strength, and excellent electrical properties.
(2) High Temperature and Humidity Resistance
The working environment of solar panels is very harsh. Some operate in tropical areas, and the daily average temperature is very high; some operate in high altitude and high latitude areas, and the operating temperature is very low; in some places, the temperature difference between day and night is large, such as desert areas. Therefore, photovoltaic junction boxes are required to have excellent high temperature and low temperature resistance properties.
(3) UV Resistant
Ultraviolet rays have certain damage to plastic products, especially in plateau areas with thin air and high ultraviolet irradiance.
(4) Flame Retardancy
Refers to the property possessed by a substance or by treatment of a material that significantly delays the spread of flame.
(5) Waterproof and Dustproof
The general photovoltaic junction box is waterproof and dustproof IP65, IP67, and the Slocable photovoltaic junction box can reach the highest level of IP68.
(6) Heat Dissipation Function
Diodes and ambient temperature increase the temperature in the PV junction box. When the diode conducts, it generates heat. At the same time, heat is also generated due to the contact resistance between the diode and the terminal. In addition, the increase in ambient temperature will also increase the temperature inside the junction box.
Components inside the PV junction box that are susceptible to high temperature are sealing rings and diodes. 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 in the diode, and the reverse current will double for every 10 °C increase in temperature. Reverse current reduces the current drawn by the circuit board, affecting the power of the board. Therefore, photovoltaic junction boxes must have excellent heat dissipation properties.
A common thermal design is to install a heat sink. However, installing heat sinks does not completely solve the heat dissipation problem. If a heat sink is installed in the photovoltaic junction box, the temperature of the diode will temporarily decrease, but the temperature of the junction box will still increase, which will affect the service life of the rubber seal; If installed outside the junction box, on the one hand, it will affect the overall sealing of the junction box, on the other hand, it is also easy for the heatsink to be corroded.
3. Types of Solar Junction Boxes
There are two main types of junction boxes: ordinary and potted.
Ordinary junction boxes are sealed with silicone seals, while rubber-filled junction boxes are filled with two-component silicone. The ordinary junction box has been used earlier and is easy to operate, but the sealing ring is easy to age when used for a long time. The potting type junction box is complicated to operate (it needs to be filled with two-component silica gel and cured), but the sealing effect is good, and it is resistant to aging, which can ensure the long-term effective sealing of the junction box, and the price is slightly cheaper.
4. The Composition of Solar Connection Box
The solar connection junction box is composed of box body, box cover, connectors, terminals, diodes, etc. Some junction box manufacturers have designed heat sinks to enhance the temperature distribution in the box, but the overall structure has not changed.
(1) Box Body
The box body is the main part of the junction box, with built-in terminals and diodes, external connectors, and box covers. It is the frame part of the solar connection box and bears most of the weather resistance requirements. The box body is usually made of PPO, which has the advantages of high rigidity, high heat resistance, fire resistance, and high strength.
(2) Box Cover
The box cover can seal the box body, preventing water, dust and pollution. The tightness is mainly reflected in the built-in rubber sealing ring, which prevents air and moisture from entering the junction box. Some manufacturers set a small hole in the center of the lid, and install the dialysis membrane in the air. The membrane is breathable and impermeable, and there is no water seepage for three meters underwater, which plays a good role in heat dissipation and sealing.
The box body and box cover are generally injection molded from materials with good weather resistance, which have the characteristics of good elasticity, temperature shock resistance, and aging resistance.
(3) Connector
Connectors connect terminals and external electrical equipment such as inverters, controllers, etc. The connector is made of PC, but PC is easily corroded by many substances. The aging of solar junction boxes is mainly reflected in: connectors are easily corroded, and plastic nuts are easily cracked under low temperature impact. Therefore, the life of the junction box is the life of the connector.
(4) Terminals
Different manufacturers of terminal blocks terminal spacing is also different. There are two types of contact between the terminal and the outgoing wire: one is physical contact, such as tightening type, and the other is welding type.
(5) Diodes
Diodes in PV junction boxes are used as bypass diodes to prevent hot spot effects and protect solar panels.
When the solar panel is working normally, the bypass diode is in the off state, and there is a reverse current, that is, the dark current, which is generally less than 0.2 microampere. Dark current reduces the current produced by a solar panel, albeit by a very small amount.
Ideally, each solar cell should have a bypass diode connected. However, it is very uneconomical due to factors such as price and cost of bypass diodes, dark current losses and voltage drop under operating conditions. In addition, the location of the solar panel is relatively concentrated, and sufficient heat dissipation conditions should be provided after the diode is connected.
Therefore, it is generally reasonable to use bypass diodes to protect multiple interconnected solar cells. This can reduce the production cost of solar panels, but can also adversely affect their performance. If the output of one solar cell in a series of solar cells is reduced, the series of solar cells, including those that are working properly, are isolated from the entire solar panel system by the bypass diode. In this way, due to the failure of one solar panel, the output power of the entire solar panel will drop a lot.
In addition to the above issues, the connection between a bypass diode and its adjacent bypass diodes must also be carefully considered. These connections are subject to some stresses that are the product of mechanical loads and cyclical changes in temperature. Therefore, in the long-term use of the solar panel, the above-mentioned connection may fail due to fatigue, thereby making the solar panel abnormal.
Hot Spot Effect
In a solar panel configuration, individual solar cells are connected in series to achieve higher system voltages. Once one of the solar cells is blocked, the affected solar cell will no longer work as a power source, but become an energy consumer. Other unshaded solar cells continue to carry current through them, causing high energy losses, developing “hot spots” and even damaging the solar cells.
To avoid this problem, bypass diodes are connected in parallel with one or several solar cells in series. Bypass current bypasses the shielded solar cell and passes through the diode.
When the solar cell is working normally, the bypass diode is turned off in reverse, which does not affect the circuit; if there is an abnormal solar cell connected in parallel with the bypass diode, the current of the entire line will be determined by the minimum current solar cell, and the current will be determined by the shielding area of the solar cell. Decide. If the reverse bias voltage is higher than the minimum voltage of the solar cell, the bypass diode will conduct and the abnormal solar cell will be shorted.
It can be seen that the hot spot is solar panel heating or local heating, and the solar panel at the hot spot is damaged, which reduces the power output of the solar panel and even leads to solar panel scrapping, which seriously reduces the service life of the solar panel and brings hidden danger to the power station power generation safety, and the heat accumulation will lead to solar panel damage.
Diode Selection Principle
The selection of the bypass diode mainly follows the following principles: ① The withstand voltage is twice the maximum reverse working voltage; ② The current capacity is twice the maximum reverse working current; ③ The junction temperature should be higher than the actual junction temperature; ④ Thermal resistance small; ⑤ small pressure drop.
5. PV Module Junction Box Performance Parameters
(1) Electrical properties
The electrical performance of the PV module junction box mainly includes parameters such as working voltage, working current, and resistance. To measure whether a junction box is qualified, electrical performance is a crucial link.
①Working voltage
When the reverse voltage across the diode reaches a certain value, the diode will break down and lose unidirectional conductivity. In order to ensure the safety of use, the maximum reverse working voltage is specified, that is, the maximum voltage of the corresponding device when the junction box works under normal working conditions. The current working voltage of the PV junction box is 1000V (DC).
②Junction temperature current
Also known as working current, it refers to the maximum forward current value that is allowed to pass through the diode when it works continuously for a long time. When current flows through the diode, the die is heated and the temperature rises. When the temperature exceeds the allowable limit (about 140°C for silicon tubes and 90°C for germanium tubes), the die will overheat and be damaged. Therefore, the diode in use should not exceed the rated forward operating current value of the diode.
When the hot spot effect occurs, current flows through the diode. Generally speaking, the larger the junction temperature current, the better, and the larger the working range of the junction box.
③Connection resistance
There is no clear range requirement for the connection resistance, it only reflects the quality of the connection between the terminal and the busbar. There are two ways to connect the terminals, one is clamping connection and the other is welding. Both methods have advantages and disadvantages:
First of all, the clamping is fast and maintenance is convenient, but the area with the terminal block is small, and the connection is not reliable enough, resulting in high contact resistance and easy to heat.
Secondly, the conductive area of the welding method should be small, the contact resistance should be small, and the connection should be tight. However, due to the high soldering temperature, the diode is easy to burn out during operation.
(2) Width of Welding Strip
The so-called electrode width refers to the width of the outgoing line of the solar panel, that is, the busbar, and also includes the spacing between the electrodes. Considering the resistance and spacing of the busbar, there are three specifications: 2.5mm, 4mm, and 6mm.
(3) Operating Temperature
The junction box is used in conjunction with the solar panel and has strong adaptability to the environment. In terms of temperature, the current standard is – 40 ℃ ~ 85 ℃.
(4) Junction Temperature
The diode junction temperature affects the leakage current in the off state. Generally speaking, the leakage current doubles for every 10 degree increase in temperature. Therefore, the rated junction temperature of the diode must be higher than the actual junction temperature.
The test method of diode junction temperature is as follows:
After heating the solar panel to 75(℃) for 1 hour, the temperature of the bypass diode should be lower than its maximum operating temperature. Then increase the reverse current to 1.25 times ISC for 1 hour, the bypass diode should not fail.
6. Precautions
(1) Test
Solar Junction boxes should be tested before use. The main items include appearance, sealing, fire resistance rating, diode qualification, etc.
(2) How to Use the Solar Junction Box
① Please make sure that the solar junction box has been tested and qualified before use.
② Before placing the production order, please confirm the distance between the terminals and the layout process.
③When installing the junction box, apply glue evenly and comprehensively to ensure that the box body and the solar panel backplane are completely sealed.
④Be sure to distinguish the positive and negative poles when installing the junction box.
⑤ When connecting the bus bar to the contact terminal, be sure to check whether the tension between the bus bar and the terminal is sufficient.
⑥ When using welding terminals, the welding time should not be too long, so as not to damage the diode.
⑦When installing the box cover, be sure to clamp it firmly.