Question 1 The power system shown in Figure 1 develops a single-phase-to-earth fault at the point F on the 275kV terminals of the transformer at Station C. Ignoring the resistive components of the overhead-line impedances, determine the value of the earth-fault current at F and, in addition, the value of the current in the transformer neutral at Station C. All three generators have a rating of 275MW, 324MVA, with positive and negative sequence impedance values of j21.5% and j18.0% respectively. The generator step-up transformers have an impedance of j17.0% on their rating of 324MVA and the transformer at Station C has an impedance of j15.0% on its rating of 120MVA. The 275 kV over-head-line has a positive-sequence impedance of 0.034 + j0.325 ohms per km and a zero-sequence impedance of 0.15 + j0.813 ohms per km, the distances from Stations A, B and C to the tee point being 96km, 128km and 32km respectively. It can be assumed that the pre-fault phase-to-phase voltage at F is equal to the rated value of 275 kV, and that no loads are connected at Busbars A, B and C. Figure 1. 2 D 275kV iço Ⓒ questions_1708304124673.pdf

Fig: 1