Protection systems on the main components such as generators and main transformers in power systems serve as a means to protect the main components in the event of interference, so that the component can avoid damage. In this final project will discuss the value of setting current and reliability factor of differential relay which is applied to protect generator and main transformers. The relay will work if there is a current difference on CT1 on the primary side and CT2 on the secondary side in the protection zone, if the fault occurs outside the protection zone, the relay will not work. This research is simulate data of single-line diagram from PLTP Unit 5 owned by PT Pertamina Geothermal Energy area Kamojang by using ETAP (Electric Transient and Analysis Program) 12.6 with purpose to know reliability factor in differential relay of time delay (pu) given and be input to compare the value of current differential relay settings obtained by calculation according to IEEE standards (0,26 A) with actual setting data (0,33 A). In this simulation process shown the performance and selectivity of differential relay, current value on CT, and then time of trip differential relay in protection zone or outside protection zone. From the results of this simulation shown that when the internal interference differential relay will trip (0.10-1.00 pu), while the external interference relay does not trip.