With the operating current of PV modules continuing to increase, reliable junction boxes are essential to ensure a 20-30 year service life. This article will discuss what factors need to be considered when selecting a junction box for large-format modules with different operating currents.
Junction box rated current
According to the by diode test specifications of IEC 61215, the rated current of a junction box should be greater than 1,25 × Isc for a monofacial module. For a bifacial module, the bifacial gain should also be considered as per the following equation (IEC61215\IEC61730:2021):
Ijbox = Isc x (1 + 30% * f) x1,25
Where 'Isc' is the short-circuit current of the module and f is its bifaciality factor. Bifaciality is normally around 70% ± 5%, and to allow a reasonable safety margin for the junction box, the current adopted for different modules is shown in the following table:
It can be seen that for modules based on 156,75mm, 166mm and 182mm wafers, there is a sufficient safety margin for the junction box. However, for a bifacial module based on a 210 mm wafer, there is no safety margin remaining, which indicates that in some cases, the module operating current will be greater than the rated current, resulting in possible failure of the module box. junction.
Diode design analysis of a junction box
For 182mm-based modules, a 25A junction box with a by diode for a given cell group is reliable, with a sufficient safety margin (17,5%), as indicated above. However, if the operating current of a module increases to 18,4A, the safety margin for the 25A junction box becomes insufficient, requiring a significant increase in diode size.
Therefore, for an 18,4 A module, a parallel association of diodes is generally adopted, which has the disadvantage of the risk of unequal distribution of the current between the diodes and their possible failure. The figure below is a typical forward bias voltage curve of a 150-thousandths-of-an-inch Schottky diode, model PANJIT 30SQ045. If the forward voltage drop is VF = 50 mV, the difference in the current ing through the two diodes will be 10 A, which could cause their failure.
Junction box reliability analysis
As the junction temperature is critical to the reliability of the junction box, several junction temperature tests were performed according to the following table for LONGi's Hi-MO 5 bifacial module, based on a 182 mm wafer. It can be seen that the 25 A junction box for the 182 mm module ed all tests even when the test current was increased to 26 A.
Furthermore, in order to simulate a complete set and outdoor aging factors (high temperature and humidity), a junction temperature test under PCT 48h (high pressure aging test) and current 26 A was carried out as established in table below. It can be seen that a 25 A junction box for a module size of 182 mm can be considered safe in outdoor conditions.
In summary, when selecting a reliable junction box for a large current module, safety margin, diode selection and performance under harsh conditions are all factors that need to be considered. LONGi adopted the 25A junction box for its Hi-MO 5 module after thorough analysis and testing, making the product compatible with long life expectancy, low LCOE (levelized cost of energy) and high performance.
