The PLN Unit Layanan Transmisi dan Gardu Induk (ULTG) Yogyakarta has three transformer units in which each transformer has a capacity of 60 MVA, to serve loads in Bantul particularly and has an interconnected system because it is also a Basecamp substation in Yogyakarta. The PLN ULTG Yogyakarta can convert the voltage of 150 KV to 20 KV which in case it uses a step-down Transformer and then be channeled through the 20 KV feeder. For the maximizing the role of substation, especially in PLN ULTG Yogyakarta, each transformer must be installed with a protection system device. The protection system device to protect the transformer from failure is Relay Protection, detect a failure and send the signal to a circuit breaker (CB) or Pemutus Tenaga (PMT) to trip. The sort of Relay Protection device is Over Current Relay (OCR) and Ground Fault Relay (GFR). OCR function is detecting excess current and GFR is used to detect phase disturbances to the ground. Furthermore, this analysis uses a software called ETAP 16.0.0 for a simulation and testing the sensitivity of protection system at PLN ULTG Yogyakarta when got a failure. In order to be expected analytic result can show protection system in normal or abnormal condition