Electrical energy is the one of capital energy in human life which must be continue
to exist and constantly supplying energy with minimal disruption. The one of
equipment in electrical energy distribution from power plant to consumers is power
transformers which is the functions is to increase or decrease the voltage whitout
changing the frequncy. The purpose of this study was to determine the reliability of
differential relays as protection of power transformers in the PLN 150 kV Kalibakal
substation, with comparison methode between the result of mathematical
calculation with transformers data spesification in substation. A transformers
capacity is 60 MVA and a rating current of 253,034 A on the voltage side of 150
kV and 1905,25 on the side of 20 kV, so the selected ratio is 300/5 on the sides of
150 kV and 2000/5 on the side of 20 kV. The results of the calculation of the error
mismatch of 1.125% are still below the standard PLN value of 5%, and the setting
flow of the calculation results of 0.48 A is smaller than the standard specification
calculation data. . Then in testing the short circuit current with an assumption of
8000 A produces a differential current of 0.76 A which means that it is greater than
the current setting and will make a command for circuit breaker trip, while
assuming a short circuit interference of 7000 A, the differecintial current obtained
is equal to 0.19 A which means that it is smaller than the current setting, the relay
will not work and PMT will not trip. From the calculation results that make the
differential current to 0.48 A, the result is that the nominal current limit allowed to
pass through the current transformer is 8,330.92 A.