Sebuah gardu induk yang terhubung dengan beberapa saluran transmisi dan memasok daya ke beberapa trafo saluran transmisi umumnya melalui daerah terbuka dan banyak mengalami gangguan arus hubung singkat. Di dalam gardu induk terdapat banyak peralatan listrik seperti trafo daya, trafo arus dan trafo tegangan. Trafo daya merupakan alat utama yang termahal pada gardu induk sehingga memerlukan sistem proteksi yang memadai, disamping itu di perlukan koordinasi proteksi antara saluran transmisi dengan trafo dalam gardu induk. Agar sistem proteksi berkerja dengan baik, maka perlu koordinasi peralatan-peralatan sistem proteksi pada trafo terhadap hubung singkat yang terdiri dari CB yang dilengkapi dengan rele dan CT. Perhitungan arus gangguan dan besarnya arus nominal transformator akan digunakan sebagai acuan dalam penentuan setting rele pengaman (dalam hal ini rele differensial, OCR dan GFR pada sisi sekunder). Rele diferensial sesuai standart PLN dengan minimum setting 30%. Rele OCR pada sisi outgoing tms sebesar 0,098, sisi incoming tms sebesar 0,037 dan sisi 150 kV tms sebesar 0,403. Rele GFR pada sisi outgoing tms sebesar 0,1, sisi incoming tms sebesar 0,235 dan sisi 150 kV tms sebesar 0,257.

A substation connected to several transmission lines and supplying power to several transmission line transformers generally passes through open areas and experiences many short-circuit disturbances. Inside the substation, there are many electrical devices such as power transformers, current transformers, and voltage transformers. Power transformers are the most expensive main equipment in substations, so they require an adequate protection system. In addition, protection coordination between transmission lines and transformers in substations is also required. For the protection system to work properly, coordination of protection system equipment on transformers against short circuits consisting of CBs equipped with relays and CTs is necessary. The calculation of fault current and the nominal current of the transformer will be used as a reference in determining the setting of the safety relay (in this case, the differential relay, OCR, and GFR on the secondary side). The differential relay complies with PLN standards with a minimum setting of 30%. The OCR relay on the outgoing side is 0.098, on the incoming side is 0.037, and on the 150 kV side is 0.403. The GFR relay on the outgoing side is 0.1, on the incoming side is 0.235, and on the 150 kV side is 0.257.