Interference cannot be avoided in an electrical protection system. This interference can affect the performance of the equipment used both inside and outside the equipment. Therefore, a protection system is needed to prevent the equipment from being damaged or avoiding things that are not desired. In this study aims to determine the quantity of interference with the performance of the 150 KV Substation Power Transformer protection system by using percentage description calculations to obtain data qualitatively. The data generated from the calculation of this percentage description of relay performance is said to be good if the calculation gets a minimum 90% result. Based on the results of research and calculations carried out from 2009 to 2018, there are criteria for disturbances that affect the performance of the 150 KV Substation Substation Transformer protection system, namely: technical problems, non-technical disturbances, and without causes.