Symmetrical and unsymmetrical faults analysis: Using Nigeria 330-KV grid as case study

When a short-circuit occurs in a power system, the magnitude of the fault currents, which is very high compared to the steady state current that flows in the power system, is determined by the reactance of the power system equipment (and the reactance of the ground if ground is involved). It is essential that symmetrical and unsymmetrical analysis of the power system network be undertaken to determine the magnitude of these fault currents and fault MVA (Megavolt-Ampere) so as to select adequate ratings of the power system protective devices - such as circuit breakers, relays and fuses. Otherwise, these protective devices will not operate properly in the event of faults thereby causing considerable damage to life and equipment. In this paper, the Nigeria 330KV network, 32 bus system is considered using relevant data which was obtained from the Nigerian National Integrated Power Project (NIPP) inhouse Grid Study Team Report, and the Transmission Company of Nigeria (TCN). Power System toolbox in MATLAB was used to carry out the simulation analysis. Fault impedance was set to j*0.1, and the various kinds of faults were simulated at bus 32 (Calabar) and bus 6 (Ayede). Base MVA of 100 MVA and Base KV of 330 KV were used. The results obtained show that symmetrical three phase fault is the most severe kind at the transmission lines, while for faults occurring very close to the generating station or synchronous generator, single line to ground fault is the most severe.