ABSTRACT

Quality electricity supply in Nigeria is yet to be enjoyed by all due to certain technical and non-technical factors. It is important to note that every power system consists of various expensive equipment which need to be protected from dangerous fault currents. A properly coordinated power system protection is that which isolates only the faulted part of the network. For proper coordination, there should be a time delay between the primary and secondary protection system. This time gap between primary and secondary protection is known as Coordination Time Interval (CTI). The data for the modeling and simulation of  overcurrent relay coordination for a typical 132/33 kV transmission station is gotten from the Umuahia 132/33 kV substation of the Transmission Company of Nigeria (TCN). The Electric Transient Analyzer Program (ETAP 16.0) software was used for the modeling and analysis. A three-phase short circuit test was conducted on the network at three different locations namely: Afara 33 kV feeder, the 33 kV bus section and at the 132 kV bus. The overall maximum peak short circuit current of 20.15 kA was recorded. Afterwards, a detailed sequence of operation of the station’s overcurrent relays was carried out at these locations for standard inverse relay characteristics, very inverse relay characteristics and extremely inverse relay characteristics. Results showed that for a fault injected at Afara 33 kV feeder, relays R16 and R21’s, time current curve (TCC) for the standard inverse relay setting, very inverse relay setting and extremely inverse relay setting curves, indicated a fault current of 3.141 kA, 3.153 kA and 3.153 kA respectively which lasted for 3.51 s, 3.48 s and 3.48 s on the network respectively. Similarly, the results showed that for a fault injected on the 33 kV busbar, R16, R17, R18 and R19’s, time current curve (TCC) for the standard inverse relay setting, very inverse relay setting and extremely inverse relay setting curves, indicated a fault current of 7.746 kA which lasted for 3.57 s, 3.57 s and 3.28 s on the network respectively. Moreover, it was shown that when a fault was injected at 132kV bus, R1 and R2’s, time current curves (TCC) for the standard inverse relay setting, very inverse relay setting and extremely inverse relay setting curves, indicated a fault current of 2.843 kA which lasted for 3.25 s, 3.47 s and 3.48 s on the network respectively. Finally, it was seen that it is best to use relays with the extreme inverse characteristics at points closer to the source - as a faster trip time of 0.0952 s was recorded at the 132kV side, whereas the very inverse relay setting and standard inverse relay setting characteristics should be used downstream, as trip times of 3.51 s and 1.9 s respectively were recorded at locations farther from the source.