The development of technology that is currently happening, namely the availability of electrical energy must be met very well. The growing number of population in an area, the electricity demand will automatically increase, the greater the number of population development in a region, the greater the electrical energy burden is needed, so that the electric current that is used is also greater with increasing electrical energy load the longer it gets bigger, it can affect the workability of the transformer. Therefore the transformer used must be protected from overloading. Over Current Relay (OCR) and Ground Fault Relay (GFR), which are overcurrent security relays that will work because of the overcurrent installed on a high voltage, medium voltage network, and also on the power transformer security. OCR and GFR are relays that work when there is an increase in current passing through the relay. The function of the OCR and GFR is to secure electrical equipment against short circuit between phases, this study will analyze the coordination work of the Overcurrent Relay (OCR) system and the Ground Fault Relay (GFR) in the 150 kV Medari Substation. As well as using ETAP (Electric Transient And Analysis Program) 12.6 software as the main simulator, with the hope that the OCR and GFR relay work systems will still work to meet the standards for years to come.