Figure shows an isolated three-phase load supplied from a 33 kV/11 kV substation via a 30 km long three-phase 11 kV overhead line. The overhead line has the following series parameters:
Series inductive reactance = 0.28 Ω/km/phase
Series resistance = 0.22 Ω/km/phase
Line capacitance and shunt leakage effects may be neglected.
The line supplies a small industrial site fed from a 600 KVA, 11 kV/415 V delta/star transformer with an impedance of 6% (calculated on transformer rating) and negligible resistance. The transformer secondary is earthed via a 1Ω earthing resistor.
The site has a load of 450 kVA at a power factor of 0.7 lagging. The voltage at the 33kV bus at the substation is maintained at 33.1kV and the 33kV fault level at the substation is 2000 MVA.
Calculate the magnitude of the fault current at the transformer secondary terminals for the following types of fault:
(i) phase-to-earth fault
(ii) phase-to-phase fault
(iii) phase-to-phase to earth fault