Photovoltaic Module Backsheet Burns Attributed to Misaligned Busbar Wires

A photovoltaic module in an outdoor single-axis tracking, utility-scale field experienced a backsheet burn that exposed the rear of the cells and started a grass fire. The opening in the backsheet developed due to increased heating where the busbar wires stringing the cells together were offset from their original solder pad locations. Several other examples of similar backsheet burns were found at this installation, and some cases also show that thermal stress caused the front glass to crack and shatter. Photoluminescence, electroluminescence and thermal imaging techniques document the module performance and cell damage. Ultraviolet fluorescence provides time-dependent information to distinguish an older cell crack that developed with the backsheet melting from a newer crack that formed during the arc flash fire event and a scrape likely from handling to remove and transport the module. Microscopy analysis identifies the metal composition of the solder as Sn-Pb and not a Bi-containing low-temperature variety. Laser isolation of a cell contact area with misaligned wire shows intermittent contact quality, suggesting the module likely passed factory screening and showed no high contact resistance prior to aging in the field.