Question

A 1000MVA, 20kV, 60 Hz three phase generator is connected to an infinite bus through a 1000MVA, 20kV? / 345kVY transformer to a 345kV circuit breaker and a 345kV transmission line. The generator reactances are X``d=0.17, X`d=0.30, and Xd=1.5 per unit on a 1000MVA, 20kV base. Generator time constants are T``d=0.05 seconds, T`d=1.0 seconds, and TA=0.10 seconds. The transformer series reactance is 0.10 per unit on the transformer ratings as a base. Neglect transformer losses, excitation current, and transformer time constants. The far end of the line is considered to be an infinite bus operating at 1.0 per unit voltage on the transformer ratings as a base. The line reactance is +j0.20 per unit on the same base as everything else. A three phase short circuit occurs on the line side of the transformer when the generator operates at no load. Breakers interrupt the fault three cycles after it begins. Determine the subtransient current through the transformer in per unit and the rms asymmetrical fault current that the breakers interrupt assuming maximum dc offset.

Answer 1