Question 3:

A 1000-MVA 20-KV, 60-Hz three-phase generator is connected through a 1000-MVA

20-KV A/345-kV Y transformer to a 345-kV circuit breaker and a 345-kV transmission

line. The generator reactances are X"=0.17, X = 0.30, and Xd = 1.5 per unit, and

its time constants are T = 0.05, T=1.0, and TA = 0.10 s. The transformer series

reactance is 0.10 per unit; transformer losses and exciting current are neglected. A

three-phase short-circuit occurs on the line side of the circuit breaker when the gener-

ator is operated at rated terminal voltage and at no-load. The breaker interrupts the

fault 3 cycles after fault inception. Determine (a) the subtransient current through

the breaker in per-unit and in kA rms; and (b) the rms asymmetrical fault current the

breaker interrupts, assuming maximum dc offset. Neglect the effect of the transformer

on the time constants.