Domestic photovoltaic power plants usually use the two-level confluence method: After the photovoltaic string passes through the confluence box once, it is then converged to the DC power distribution cabinet (or the DC side in the inverter).

Different projects have different choices for the number of loops in the combiner box. The mainstream design of large-scale photovoltaic power plant arrays chooses a 16-loop combiner box. The above schemes are currently widely used in photovoltaic power plants of various topography, landforms and climatic conditions in China. Lack of comparative analysis of the number of circuits. This article will use the following models to analyze and compare in the theoretical state (the natural terrain is zero degrees) and the field state (the relevant design value is the actual engineering value of a mountain photovoltaic power station):

Unit MWp photovoltaic array. Latitude of photovoltaic array: 23 °; photovoltaic module is 60 pieces of polysilicon 250W module, of which Vmppt = 30.7V, Imppt = 8.15A; the number of strings is 22, and the number of strings in parallel is 182; the cable from the module to the junction box is 1 × 4mm22; the junction box adopts 8 and 16 loop junction boxes, the corresponding number is 24 or 12, and the cross-section of the outlet cable of the junction box adopts 1 × 35mm22 and 1 × 70mm22 respectively.

In order to simplify the model in the analysis, this paper takes the photovoltaic module as the ideal current source, that is, the voltage is constant, and the current changes with environmental parameters.

After consulting the corresponding parameters and using classic theoretical formulas, the conclusions are as follows:

1) In the theoretical state, what kind of combiner box is used. When the calculated caliber is the same, the difference between the theoretical loss and the choice of the combiner box is not large. Considering the difference between the cable parameters and the actual, it can be considered that the theoretical loss is consistent in various states.

2) In the field state, the connection length of the cable is greatly increased under complex terrain conditions. Therefore, compared with the theoretical state, the cable loss is greatly increased at the same time. Due to the large laying range of the multi-circuit junction box, the cable loss of the multi-circuit junction box is significantly greater than the cable loss of the loop.

Based on the above analysis, it is recommended to use an 8-circuit junction box for large-scale grid-connected photovoltaic power plants under complex terrain conditions; a junction box with more loops can be used under the terrain conditions where the photovoltaic square array does not significantly increase the footprint.

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