In-depth analysis of the four major problems of the junction box of photovoltaic power generation

　　Recently, the photovoltaic industry has caused some discussions on the safety of high-current junction boxes, mainly focusing on the safety concerns of high-current junction boxes under high radiation in summer. For these discussions, the industry has actually had many professional answers before.

　　1. Is there a need for higher standards under high-irradiation conditions in summer?

　　Recent discussions have pointed out that 1100 and 1200W/m2 levels of light intensity often occur in summer, and there may be higher radiation conditions. Therefore, higher safety standards are required to ensure the work safety of the junction box.

　　In fact, the relevant IEC standards have fully considered the above situation and adopted a safety factor of 1.25 times for the selection of junction boxes and components, which is reasonable and in line with reality.

　　For occasional higher irradiance conditions, the junction box certification and test will not only test the temperature under the rated current, but also evaluate the heat-resistant safety of the junction box at 1.25 times the rated current, so there is no need to worry about safety. It is understood that the component factory has also prepared junction box selection requirements for customers with higher safety factors, such as junction boxes with rated currents of 28A and 30A.

　　2. Is the current IEC standard for double-sided modules reliable?

　　The more intensely discussed issue is the safety of the high-current junction box of the bifacial module. Some people believe that the formula Isc×(1+30%×ϕ)×1.25 in the IEC standard for double-sided modules has increased the double-sided coefficient, and pointed out that the design margin for the selection and design of the junction box of high-current modules is insufficient to ensure safety.

　　After confirming with the certification authority, although the new IEC standard adds the consideration of double-sided power generation, the certification body does not yet have exact requirements on what backside radiation parameters should be used when selecting parts. From the current point of view, it is reasonable for the new standard to conduct reliability tests according to the irradiance of 300W/m2 under extreme conditions. According to the requirements of different certification rules, a junction box with a higher rated current can be selected to match, so as to meet different test conditions and the needs of special customers.

　　3. Does the 25A junction box meet the requirements of 210 components?

　　Considering the stress irradiance of 300 W/m2 under extreme conditions, the maximum output short-circuit current of the 210 double-sided module is Isc×(1+30%×ϕ)=22.56A. Choosing a 25A junction box can fully meet the needs of use . Moreover, the normal working current of the component, Impp, is 17.3A instead of Isc 18.4A. Based on this calculation, the final output current is lower.

　　So, when the component is under high irradiance, hot spot effect occurs, can the junction box meet the requirements?

　　Let's analyze it: the back and the front of the bifacial module are in parallel. In addition to the parallel connection of the upper and lower circuits, the front and back of a single battery are also connected in parallel. When the hot spot effect occurs on the front battery, the back battery can still work normally and shunt. In the actual power generation process, unless it is deliberately blocked, the hot spot effect on the front and back of the same string of batteries at the same time is extremely low; while the upper and lower strings of the same diode are connected in parallel, and the hot spot effect at the same time is even lower.

　　The rated current of the junction box is not as large as possible. It is the most reasonable to choose the most suitable part specifications. Excessive high allocation will only transfer costs to owners and customers.

　　4. What is the real situation of the dual-chip voltage drop difference?

　　Regarding this issue, there have been rumors that the difference between the dual cores is 50 millivolts, which is obviously not in line with common sense and the actual process level.

　　Previous data has pointed out that the VF of two chips on the same wafer after packaging does not exceed 3mV. This difference is only about 1% in the voltage drop of the entire diode, which is within the test error range.

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