As shown in Figure 7.21, a 25-MVA, 13.8-kV, 60-Hz, synchronous generator with \(\mathrm{X}_{d}{ }^{\prime \prime}=0.15\) 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 and \(13.8 / 6.9 \mathrm{kV}\) with a leakage reactance of 0.1 per unit. Each motor has a subtransient reactance \(\mathrm{X}_{d}{ }^{\prime \prime}=0.2\) per unit on a base of \(5 \mathrm{MVA}\) and \(6.9 \mathrm{kV}\). A three-phase fault occurs at point \(\mathrm{P}\), when the bus voltage at the motors is \(6.9 \mathrm{kV}\). Determine

(a) the subtransient fault current,

(b) the subtransient current through breaker A, and

(c) the symmetrical short-circuit interrupting current (as defined for circuit breaker applications) in the fault and in breaker A.

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O Gen. -O O O O A P Motors