Question

A 3-phase, 1000-MVA, \(20-\mathrm{kV}, 60-\mathrm{Hz}\) synchronous generator is connected to an infinite bus as shown below. All impedances are given in pu values. The generator delivers \(0.9\) pu power at unity power factor to the infinite bus whose voltage is at 1 pu.

If phase 'A' of circuit 2 (CCT 2) between buses H1 and H2 is suddenly opened at point ' \(\mathrm{F}\) ' near bus H1 due to the opening of a circuit breaker, determine:

(a) [10 marks] The internal voltage of the generator prior to the circuit breaker opening.

(b) [15 marks] The resulting phase ' \(\mathrm{B}^{\prime}\) and phase 'C' currents in circuit 2 (in pu) after the circuit breaker opening.

Generator internal impedances: \(X_{1}=1.8\) pu, \(X 2=0.25\) pu, \(x 0=0.04\) pu

Infinite bus internal impedance: Negligible.

For transformers and transmission lines, assume \(\mathrm{X} 1=\mathrm{X} 2\) (positive sequence reactance is equal to negative sequence reactance).

\(\mathrm{X} 0\) is the zero sequence reactance.