Views: 4 Author: Site Editor Publish Time: 2022-01-10 Origin: Site
In recent years, with the rapid development of the photovoltaic industry, how to increase the power generation and reduce the cost of power generation under the premise of ensuring long-term stable and safe operation has become a difficult problem faced by the photovoltaic industry. On the one hand, photovoltaic technology experts seek solutions through the introduction of new designs, new processes, new materials and new technologies. On the other hand, these new elements also put forward higher requirements for the quality of photovoltaic modules and components. As the most commonly used and most important supporting components of photovoltaic modules, how photovoltaic junction boxes and connectors respond to the opportunities and challenges under this "big" trend has become a microcosm of the continuous development of the entire component industry.
This article starts with the related failure phenomena of photovoltaic junction boxes and connectors, hoping to see the essence through the appearance of failure, and to promote the entire industry to pay attention to and deepen the quality and safety of photovoltaic components under the new situation by drawing inferences from one point to another.
In the photovoltaic power generation system, the connector is a long-term current-carrying component, and it is particularly important to maintain its long-term stability. This undoubtedly puts forward higher requirements for the design, manufacture and installation of the connector and the selection of spare parts. Problems in any link will cause the connector to heat and deform or even burn during use. Next, we analyze the reasons behind the abnormal heating of the connector through several typical failure cases.
1. Abnormal contact
Under normal circumstances, the internal metal parts of the connector can be divided into three parts as shown in the figure below. When the metal pins are coupled, they rely on the drum spring in the middle to provide sufficient contact. Displacement, it will lead to abnormal contact during coupling, although it will not affect the current conduction, long-term current-carrying conditions will easily cause abnormal heating of the contacts and changes in contact resistance, aggravate the heating of the contacts, and eventually lead to the deformation of the connector burn. When the connectors of different manufacturers are used in the photovoltaic power station for mutual matching, the risk of failure will be greatly increased due to structural differences and technical barriers between different products.
2. Connection of terminals and cables
Photovoltaic DC connectors are basically cables with multi-strand copper wires. How the cables and metal terminals are connected and the quality of the connection will affect the long-term safety of the connectors, such as the picture below. The crimp connection method must ensure that all copper wires are completely and tightly wrapped, which imposes strict requirements on the specifications of the cable, the size of the crimping part of the terminal, and the crimping process. Once there is a condition that does not match, there will be The copper wire may be exposed as shown in the left figure below, or the crimping part shown in the right figure below may be damaged. In addition to relying on visual inspection to identify, the introduction of compression ratio can also more intuitively test the quality of crimping, and to the greatest extent avoid potential safety hazards caused by such internal connection problems during long-term use of the product.
3. Reasonable selection
The determination of rated current is also very important. If the connector passes current beyond its own current carrying capacity for a long time, it will accelerate the aging of materials and accessories and shorten its service life. Therefore, understanding the performance of the cables, metal terminals, shell materials and other parts and components of the connector, and matching them in a targeted manner can largely avoid the risk of blindly upgrading the current of the connector.
Like the connector, the metal parts inside the junction box also need to carry current for a long time. In addition, a bypass diode is required to achieve its functionality. Therefore, the failure of the junction box can be roughly divided into two parts.
1. Cables and metal terminals
If the terminal and the cable cannot provide complete and close contact, a large current will easily cause local abnormal heating for a long time, which will accelerate the aging and deformation of the material in this part. In the actual use process, in addition to the connection of metal parts and cables inside the junction box, it is also necessary to pay attention to various connections such as metal parts and welding ribbons, metal parts and diodes, and metal parts and metal parts. The quality of any kind of connection will be related to the safety and stability of the junction box during the long-term power-on process.
When the diode is bypassing, in addition to the current flowing in, what other factors will cause abnormal heating?
The diode on the far right of the junction box has its pins desoldered during a power-on test. After analysis, it is found that the metal sheet at the bottom of the desoldering diode is connected to the cable, but the pressure block for fixing the cable cannot provide sufficient locking force, and the fixing hole of the metal sheet is deformed by heat, so the stress will be transmitted to the diode through the metal sheet. , With the continuous energization and heating of the diode, the solder that originally fixed the diode pins will gradually dissolve and lose with the high temperature, which is easy to induce the desoldering of the diode. Therefore, it is necessary to keep the diode and the metal sheet in stable contact to avoid being affected by stress as much as possible. The structural design of the box itself and the selection and installation of spare parts have put forward higher requirements.
The introduction of glue will seal the entire internal space of the junction box, improve the pollution level, and also change the way that the heating components inside the box rely on air for heat exchange. However, if the glue is not filled enough to cause air residue inside the box, it will in turn affect the safety of the junction box. As shown in the figure above, the air remaining under the metal sheet will expand once it is heated, which will not only easily lift the upper metal sheet and the diode, but also cause the junction box to bulge and deform, which will lead to the diode desoldering and current interruption. one of the incentives. In addition, the two mediums of glue and air that exist inside the junction box also affect the heat dissipation effect, resulting in uneven heating of the junction box material, especially the bottom material, resulting in local overheating and deformation of the box material. Since different glues have differences in fluidity, setting time, etc., it is also the key to strictly follow the product specifications and user manuals and improve the glue quality.
There are many types of solder paste, the difference in melting point between different solders, whether the distribution of solder is uniform and sufficient, whether there are impurities or air residues on the contact surface, and whether the metal material or surface coating in contact with solder is conducive to contact with solder, etc. These factors will ultimately affect the overall quality of welding. From the above two photos of the bottom of the diode after desoldering, we can clearly see the condition of the entire welding surface after power-on, which undoubtedly puts forward higher requirements for the quality of welding. In addition, whether we can find a better solution or adopt means of increasing auxiliary fixation, etc., can become a topic of our deeper discussion.
The above content introduces the current status of junction boxes and connectors, as well as the changes that have occurred in these products in recent years. The photovoltaic industry is constantly developing, and photovoltaic technology and photovoltaic products are also advancing with the times and constantly adapting to the requirements of the development of the entire market. But no matter how it changes, quality will be our common goal.