In this study, 238 power transformers (PTs) with operating voltages between 150-500 kV and capacities in the range of 60-167 MVA are tested for partial discharge (PD). Thirty-eight of the PTs exceeded the pre-determined value of 70 pC at 1.5 maximum voltage of the PT. Failure analysis of the failed 38 PTs showed that 21% of the PD sources originate from the winding cylinder, which is formed by bonding the edges of a pressboard with an adhesive. Therefore, in this study, the effect of casein- and polyvinyl-based adhesives used in winding cylinders was investigated through standard PD measurements. The results indicate that pressboards bonded with casein-based adhesives exhibit PD values 20-25 pC lower than those bonded with polyvinyl-based adhesives (50-400 pC) at identical applied voltages (7.5-25 kV or 3-10 kV/mm). The excellent performance of casein-based adhesives is mainly attributed to its porous structure, which accelerates the impregnation of oil into the adhesive and pressboard, as revealed by scanning electron microscopy observations. This practical study will be beneficial in the process of designing and manufacturing PTs and highlight materials for producing PTs with low PD values.