Electrical power sistem consists of various components, ranging from generating components, transmission, and distribution are quite expensive, so it takes good protection, reliable and economical to avoid internal and external disturbances that can trigger damage to the components, especially in the busbar in switchyard Power Plant Geothermal Unit 4 area Kamojang managed by PT Pertamina Geothermal Energy. Difficulties that occur on the Busbar of various kinds so that the required primary protection in the Busbar is a relay differential to protect to avoid the disturbance that occurred. This study discusses the comparison of differential relay settings in theory with actual relay differential data on Geothermal Power Plant Unit 4 Kamojang area and simulation with ETAP 12.6.0 software. From the results of research, the current limit setting that flows on the busbar side voltage of 150kV is 2.5 Ampere according to actual data in the field, while the calculated data is 0 Ampere. The results of this study indicate that the difference between the actual data with the data results from the calculation in mathematical theory. Therefore, in order to obtain a better level of reliability, it is necessary to conduct a reexamination after consultation with the vendor. It is necessary to know the reliability of protection system on busbar in switchyard unit 4 PLTP Kamojang.