As shown in Figure, a 25-MVA, 13.8-kV, 60-Hz synchronous generator with X"d = 0:15 per unit is connected through a transformer to a bus that supplies four identical motors. The rating of the three-phase transformer is 25 MVA, 13.8/6.9 kV, with a leakage reactance of 0.1 per unit.
Each motor has a subtransient reactance Xd" = 0.2 per unit on a base of 5 MVA and 6.9 kV. A three-phase fault occurs at point P, when the bus voltage at the motors is 6.9 kV.
Determine:
(a) the subtransient fault current,
(b) the subtransient current through breaker A,
(c) the symmetrical short-circuit interrupting current (as defined for circuit breaker applications) in the fault and in breaker A.