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Received Oct 26, 2017; Accepted Jan 9, 2018
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1. Introduction
Microcracks refer to the invisible cracks that cannot be easily perceived by the naked eye when a wafer is subjected to mechanical or thermal stress. There are several stages related to the generation of microcracks [1–8]: (i) the cutting process of an ingot or crystal bar due to a local uneven force; (ii) the cell or module fabrication process due to external factors; (iii) improper module installation; and (iv) the power plant operation period due to external factors such as wind or ground subsidence. Since the microcracked silicon wafer is not completely broken apart, microcracks can be detected only through the electroluminescence (EL) test [9]. The presence of microcracks may cause part of the cells to be inactive, leading to the loss in the output power and safety hazard of the module [9].
In a crystalline silicon cell, current is collected from fingers to the busbar and then through the string connector to the output from the junction box. The generated current of a cell is proportional to the cell active area. The inactive area can be judged by whether the current collection from the finger to the busbar is blocked or not. According to the inactive area of the cell, the number of microcracked cells, and the impact on the output power of modules, microcracks can be divided into three categories: microscopic microcrack, general microcrack, and serious microcrack. Modules with seriously microcracked cells generally need to be replaced in a power station, and those with general microcracks will not affect the power output in the initial stage and will be disposed according to their working condition. Microscopic microcracks generally refer to the microcracks that are single or partial flakes located not at the busbars and basically do not cause failure of the area, and the power degradation of the module with microscopic microcracks should meet the industry standard (i.e., the first-year power degradation less than 2.5%). Therefore, it becomes necessary to develop the means of quantifying the risk of power loss in PV modules...