In the photovoltaic power generation system, in order to reduce the connection between the photovoltaic cell components and the inverter, prevent the system from being struck by lightning, find faulty photovoltaic cell components in time, facilitate system maintenance, and improve the reliability of the photovoltaic power generation system. According to the requirements of SJ/T 11127-1997K Photovoltaic (PV) Power Generation System Overvoltage Protection-Guidelines” and GB/T 18479-2001 Terrestrial Photovoltaic (PV) Power Generation System Overview and Guidelines”, it is generally necessary to add a DC combiner device between the PV array and the inverter, that is, a photovoltaic DC combiner box. The combiner box is based on the DC voltage range input by the inverter, the photovoltaic cell modules are connected in series to form a photovoltaic cell series, and then several photovoltaic cell series are connected to the combiner box for confluence (parallel).
According to the above requirements, photovoltaic DC combiner boxes can be divided into two types: ordinary type and intelligent type. Among them, the ordinary photovoltaic DC combiner box has basic functions such as lightning protection and convergence, and the main equipment configured includes DC fuses, DC circuit breakers, lightning protection devices and other components. In addition to the basic functions of lightning protection and convergence, the intelligent photovoltaic DC combiner box can also monitor the operation of the photovoltaic array in real time, determine the faulty photovoltaic string, and locate and alarm it, and send alarm messages to the monitoring system through the communication interface of the device. The intelligent photovoltaic DC combiner box adds elements such as intelligent monitoring modules on the basis of the ordinary type. Smart photovoltaic DC combiner box is a kind of intelligent equipment for monitoring photovoltaic cell series, and it is a new type of equipment produced with the large-scale application of photovoltaic power generation. The following describes the functions and selection of the main originals.
The main function of the DC fuse is to protect the photovoltaic cell components. In order to avoid current backflow damaging the photovoltaic cell components, it is necessary to install a DC fuse in each photovoltaic cell string output loop. The breaking condition of the DC fuse is to effectively break the faulty photovoltaic cell without affecting the operation of other normal working photovoltaic cell series.
As the market’s demand for high-power photovoltaic modules continues to increase, the promotion and development of the application technology of photovoltaic power generation systems and wind power grid-connected power generation systems, and the use of high-voltage DC fuses to protect the safety of photovoltaic cell components has become an important international technical measure.
The photovoltaic DC combiner box must be equipped with photovoltaic DC fuses between the positive and negative poles of the photovoltaic cell string (DC withstand voltage should not be less than 1kV) to protect the electronic devices inside the combiner box from overcurrent hazards. According to the current characteristics of the photovoltaic cell, the specification of the fuse can be calculated from the short-circuit current of the photovoltaic cell assembly, and the recommended value is not less than 1.56 times the short-circuit current.
DC circuit breaker
In the photovoltaic DC combiner box, the DC circuit breaker can protect the photovoltaic cell array from overcurrent when a short circuit occurs on the secondary bus side (the DC input side of the inverter). The rated voltage is the maximum open circuit voltage of the photovoltaic array, and the rated current is the maximum rated current of the photovoltaic array. Generally, a dedicated DC circuit breaker is used in series with two poles to increase the DC withstand voltage and reach the voltage required by the system. If the selected circuit breaker protection current level is too large, it will not be able to provide the protection function in the event of a fault, otherwise the system may not work normally.