A substation transformer has a 10% impedance and an X/R ratio of 20 . The bulk power system
source behind it is strong and has only 1% impedance with an X/R ratio of 5 on the same base
impedance as the substation transformer. The transformer low side bus voltage is 1.0 per unit. If
a three phase bolted fault occurs on the main substation bus on the low side, then what is the
approximate fault current in per unit?
a. 50.5 per unit
b. 35.3 per unit
c. 11.7 per unit
d. 9.1 per unit
e. 5.25 per unit
f. 3.5 per unit
A substation transformer has a 10% impedance and an X/R ratio of 20. The bulk power system
source behind it is strong and has only 1% impedance with an X/R ratio of 5 on the same base
impedance as the substation transformer.The transformer low side bus voltage is 1.o per unit. If
a three phase bolted fault occurs on the main substation bus on the low side, then what is the
approximate fault current in per unit?
a. 50.5 per unit b. 35.3 per unit
C. 11.7 per unit d. 9.1 per unit e. 5.25 per unit f. 3.5 per unit
