Domestic photovoltaic power plants commonly use a two-stage combiner method: the photovoltaic strings are first combined through the combiner box, and then the current is combined to the DC distribution cabinet (or the DC side of the inverter).
Different projects require different numbers of combiner boxes, with 16-circuit combiner boxes being the mainstream design for large-scale photovoltaic power plant arrays. This design is currently widely used in photovoltaic power plants across various terrains, landforms, and climates in China. However, comparative analysis of the number of circuits is lacking. This paper will use the following model to analyze and compare theoretical conditions (natural terrain at zero degrees Celsius) and practical conditions (the relevant design values are the actual engineering values of a mountain photovoltaic power plant):
The photovoltaic array is located at a latitude of 23°. The photovoltaic modules are 60-cell polycrystalline silicon 250W modules, with Vmppt=30.7V and Imppt=8.15A. There are 22 strings connected in parallel, with 182 parallel connections. The cable from the modules to the combiner box is 1×4mm². The combiner boxes are available in 8-circuit and 16-circuit configurations, with corresponding quantities of 24 and 12 units. The cross-section of the combiner box outgoing cables is 1×35mm² and 1×70mm².
To simplify the model in the analysis, this paper takes the photovoltaic module as an ideal current source, that is, the voltage is constant and the current changes with environmental parameters.
Based on the relevant parameters and calculations using classical theoretical formulas, the conclusions are as follows:
1) Under theoretical conditions, the theoretical loss of the selected combiner box is not much different from that of the selected combiner box when the calculated diameter is the same. Considering the difference between the cable parameters and the actual situation, it can be assumed that the theoretical loss is the same under various conditions.
2) In actual operation, the connection length of cables increases significantly under complex terrain conditions. Therefore, compared to the theoretical condition, cable loss increases substantially. Furthermore, due to the large installation area of multi-circuit combiner boxes, the cable loss in multi-circuit combiner boxes is significantly greater than the cable loss in individual circuits.
Based on the above analysis, it is recommended that large grid-connected photovoltaic power plants with complex terrain conditions adopt 8-circuit combiner boxes; under terrain conditions where the photovoltaic array does not significantly increase the land area occupied, combiner boxes with more circuits can be selected.
