Analysis of DC Fuse of PV System

01 Technical requirements for fuse protection in photovoltaic power plants

According to the standard IEC61730-2 [2] requirements: photovoltaic modules must have the ability to withstand reverse current overload. The requirements for the reverse overcurrent test are as follows: If a photovoltaic module with a maximum fuse rated current of 15A is used, it needs to be tested at 20.25A (1.35 times) for 2 hours. The thin paper should have no burning or burnt spots, and the MST17 test (wet leakage current test) should meet the same requirements as the initial test.

Therefore, photovoltaic modules certified in accordance with IEC61730-2 should have the ability to withstand 1.35 times the reverse current for 2h.

In response to the above requirements, from enhancing the safety of photovoltaic modules and extending their life, protective measures should be taken for the DC bus on the side of the photovoltaic inverter (or DC combiner box). Therefore, the IEC60269-6 [3] standard has the following technical requirements for photovoltaic fuses (15A): "It needs to be blown within 1 hour at a current of 21.75A (1.45 times)." In the UL2579 [4] standard, it is required to "fuse within 1 hour at a current of 20.25A (1.35 times)."

From the perspective of comprehensive component reverse current overload capability (IEC61730-2), the requirements of the UL standard are more reasonable, and the IEC60269-6 standard will also revise the contradictions of its requirements: for the rated current below 32A (including 32A) When the fuse is required to pass 1.35 times the rated current, it should be blown within 1h. When designing DC fuse for photovoltaics, mainstream fuse manufacturers that responded quickly to technology also updated their product parameter requirements for the above contradictions.

Therefore, when the fusing current of the fuse is not greater than the photovoltaic module can withstand the reverse current limit and the fusing time is not greater than the photovoltaic module can withstand the reverse current time, the fuse can realize the protection of the photovoltaic module.

02
Application status of fuses in photovoltaic systems

In order to avoid equipment loss caused by a short circuit on the DC side, at present, mainstream technical solutions add fuses on the DC side. At the same time, international mainstream inverter equipment manufacturers also require additional fuses on the DC side.

For the existing two-channel series-parallel equipment solution in each MPPT, according to the requirements of the above-mentioned technical standards, it is still necessary to configure over-current protection devices such as fuses to protect the equipment. There are potential safety hazards if overcurrent protection devices such as fuses are not used or only diodes are used for reverse current protection, which may cause the equipment to burn out or even cause a fire. Obviously, the DC side (or DC combiner box) busbar of the photovoltaic inverter is equipped with a fuse as the mainstream solution for the design of the DC side of the photovoltaic power station.

03
Possible problems with fuse protection

Similar to the circuit breakers and other protective devices in various power projects, the "breakpoint" is inevitably increased due to the configuration of the fuse, but the current technical means can avoid the loss of power due to equipment damage.

1. For the string inverter, because of its own string detection capability, it can complete the current and voltage collection of each string; it can realize the rapid detection of the fuse; replace the equipment in time to reduce the string open circuit time.

2. For photovoltaic power stations composed of centralized inverters, the current smart combiner box also has string detection capabilities. The above problems can also be avoided.