PROBLEM
A 20 MVA, 6.6 kV three phase alternator connected through a 3 cycle oil circuit breaker has sub-transient reactance, transient reactance and synchronous reactance of 0.1 p.u, 0.15 p.u and 0.8 p.u respectively. It is running on load with rated voltage develop across the terminals when three-phase symmetrical short circuit occurs just beyond the CB. Determine: (1) the steady short circuit current (2) the initial symmetrical current (3) the maximum possible d.c. component of the short circuit current (4) the making capacity of the CB in kA (5) the rms value of the symmetrical breaking current (6) asymmetrical breaking current and (7) the interrupting capacity in MVA.
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