Junction boxes and their impact on solar module operation

In recent months, leading manufacturers such as Jinko Solar and Longi Solar have introduced new products with different wafer sizes and power ratings to the market. The focus was mainly on maximizing the rated power. The technical background was also discussed to some extent. However, negative facts that can occur with constant maximization were rarely discussed. This includes, for example, the junction box installed in PV modules, which has to cope with new challenges due to higher currents. An example of this is the reaction time of the junction boxes, which should be shortened along with the increase in power. In recent months, however, it has been observed that this “faster” response time could not be applied in part to both old and new junction boxes. It is suspected that this is due to different design components and processing of the junction boxes concerned. If such a reaction time cannot be applied, failures and quality problems occur more frequently.

In this article, we’d like to break down for you what technical issues and challenges manufacturers can face when putting high-powered solar modules into service.

What is the important role of junction boxes and how can we achieve sufficient quality despite high power?

In this article , we will answer these and other questions.

Junction boxes as an important indicator for the safe operation of a PV module

Junction boxes play a major role in the functionality of a PV module. They ensure that the current from a solar module can flow in the right direction. Defects at the junction box can lead to heat accumulation with harmful consequences for the module.

Within the following picture you can have a closer look at such a defect:

Source: https://www.pv-magazine.com/magazine-archive/current-affairs/ 

As mentioned in the section above, according to Roman Giehl, Technical Business Development Manager at Jinko Solar Europe, the increased electrical current poses the greatest challenges. In addition to shortening the response time and improving the heat load of junction boxes, connectors and diodes also need to be adapted. According to Shawn Qu, chairman and CEO of Canadian Solar, in order to handle higher currents and meet reliability requirements according to IEC standards, the junction box design must be adapted in both structure and its diodes.

Larger module sizes require new solutions due to increase in electrical current

JA Solar, in particular, has been addressing the impact of higher currents on individual PV system components in recent months. In a white paper, the company noted, among other things, that the largest modules currently on the market require junction boxes that should carry a 30-amp rating. However, in this case, the safety margin would still be less than for smaller modules, which have a 25-amp rating. To ensure the current carrying capacity of the junction box diode, it is recommended that the rated power of the junction box for monofacial modules be greater than 1.25 times the ISC STC value of the PV generator. In addition, a comparison set up by JA Solar showed that 25 amp diodes allow 182-mm modules to operate with a 15% safety margin. Conversely, 210-mm modules, which consist of a 25-amp diode, exceed a safety margin of around 11%. 210-mm modules, with a 30-amp diode, operate at a 7% margin.

Based on JA Solar’s published findings, Jinko Solar chose the 182-mm wafer because it offers a significant increase in energy yield over previous module formats with minimal disruption to other components in the module and system. According to Jinko, this M10 format is compatible with existing junction box designs.

Canadian Solar, on the other hand, released primarily large-area solar modules with 210-mm wafers this year. Here, optimism is particularly high that the respective necessary design changes will take into account the problems of higher currents in the long run. The Chinese manufacturer QC Solar is even prepared to meet the respective requirements for modules requiring junction boxes with a rated power of 30 amps or higher with its new product 3Q-Axis.

Quality adjustments to junction boxes, poses new challenges for manufacturers

Even though manufacturers of junction boxes are currently doing everything they can to meet the quality requirements, it will only become clear in the next few years whether the higher currents or the lower safety margins will lead to problems later on during operation.

Current tests show that problems are already occurring to some extent in initial applications of high-power solar modules with the respective junction boxes. The most frequent faults observed included leakage currents from cells and ribbons, which were due to faults arising in the respective junction box. In addition, diode failures occurred more frequently, resulting in the loss of at least one third of the module power. These failures could be traced back to the intermediate loss of the cover of the respective junction box. Mostly, the loss of the lid occurs when wafer sizes increase and the lid finally becomes too small. In common junction box designs, the electrical components are sealed in a potting material. This potting material should protect the respective components even if the lid falls off. However, this material is exposed to different outdoor conditions over time, which can cause decay. Because of the decay, moisture can eventually penetrate to the internal elements. Even just a few drops of dew can lead to hours of power loss. This poses a real risk to large-scale solar module systems.

Regular quality checks can lead to rectification and prevention of faults in the long term

In order to be able to rectify emerging faults in junction boxes as quickly as possible, it may well make sense to put workers on it, for example to replace the respective covers or diodes in a project. The module manufacturer is responsible for regularly checking the quality of junction boxes. According to Mark Hutchins, editor of PV Magazine, manufacturing costs should not be skimped on here. However, according to Alex Barrows, research director at manufacturing analysis firm Exawatt, the reality is that manufacturers are choosing to cut costs by going for a lower-cost option.

To ensure junction box quality, a sample of junction boxes must be taken from a shipment, according to George Touloupas, director of technology and quality at CEA. This sample should then be tested in the incoming quality control (IQC) laboratory for the following criteria:

  • Appearance
  • Size
  • Connection strength between cables
  • box and connector
  • Cable length
  • Water protection
  • Insulation
  • Forward voltage drop of bypass diodes

If this test cannot be passed against the IQC criteria, the incoming shipment must be immediately sub-isolated. It is also important to separate the affected products from the technically sound products and clearly label them. A second, more detailed inspection then determines whether the defective products can continue to be used within production or must be returned to the supplier.

The last precautions of a quality control on the part of the manufacturer refer both to the correct connection of the respective junction box and to the production of a potting, which is to protect the components inside the box.

Even with a junction box that appears to be of good quality, malfunctions can still occur. Here, the cause could be that the soldering was not done correctly or that the potting agent has not cured properly. For this reason, Touloupas says process control is just as important as material control.

Conclusion

The trend toward large-area solar modules is unstoppable. Despite the risks associated with high-power modules, a large number of the major Chinese module manufacturers are convinced that troubleshooting in the junction box area is possible in the long term. Above all, it is important to carry out regular quality checks on the part of the manufacturers and to invest more in the production of the products. Lower-cost alternatives for the production of junction boxes can lead to further defects. Only in the coming years will we find out whether the quality controls applied are sufficient to reduce defects in high-power solar modules in the long term.

References

 
Mark Hutchins ? PV-Magazin; Current affairs; 07.09.2021; https://www.pv-magazine.com/magazine-archive/current-affairs/ 

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