Power systems of various components, ranging from generating, transmission, and distribution components are quite expensive, so that good protection, reliable and economical to avoid internal and external damage that can interfere with the main transformer components in the Geothermal Power Plant Unit 4 Kamojang managed by PT Pertamina Geothermal Energy. Disorders that occur in the main transformer of various types so that required major protection in the transformer that is a differential relay to protect in order to avoid the disturbance that occurred. This study discusses the comparison of different relay settings with actual relay differential settings data on Geothermal Power Plant Unit 4 Kamojang and simulation with ETAP 12.6 software. The problems that occur in the main transformer of geothermal power plant Unit 4 Kamojang is the difference of slope 1 actual data setting with mathematical calculation that is actual data setting 40% and mathematical calculation data 8.5% cushion. Please note the minimum limit of 1 is 5%. The slope change 1 is performed by PT Pertamina Geothermal Energy so that the differential relay is not very sensitive to the fault current that occurs within the differential relay protection zone.