Influence of string-box size and the derating factor associated to the DC protection for PV solar panels

•A new method and calculation for the DC protection design for solar plants.•It studies the influence of the derating factors, loading cycle, string-box caused by irradiance variation for clouds.•A case study with laboratory test and new algorithms have an accuracy of 99.99%.•It improves the heat transfer in the string-box with 111% with KZS is 74.14% to KZS = 82.3%. In the last decade, the photovoltaic (PV) solar plants have been spreading around the world with low cost and a construction record time. In this perspective, the challenges have been centered in the inverters, panels, and medium voltage cables design, in order to obtain the best efficiency; however, one of the main elements is the fuse for the strings, in the DC side. Several manufacturers have detected problems in the operating regime of fuses due to the recommendations of the IEC standard 61439-1, 61439-2, 60269-6, 60269-1, 60469; our findings are the influence of the derating factors, loading cycle of the fuse in PV solar plants caused by clouds and string-box size; it has influence in the fuse design; a traditional IEC calculation is not an adequate value for the DC fuse, with fuses melt until 30% of the total fleet per year, in a regular PV Solar plant, but with a new consideration of string-box size and derating factor in the fuse design; with this methodology, the failure rate could be reduced, suddenly, with a result of to 0.2% failures per year, with an improvement of 1.06% of the derating factor and size of the string-box, with the algorithm for future development and design according the best performance in the PV solar plant as a complementary procedure in the international standard with an accuracy of 99.99%.