PERFORMANCE EVALUATION OF OVERCURRENT RELAY IN A DISTRIBUTION SUBSTATION FOR IMPROVED PROTECTION AND COORDINATION

Abstract

The paper examined Overcurrent relay protection and coordination in the Igwuruta distribution
system. As power system infrastructure advances in size and complexity, fault current level also increases in the
system due to an increase in load demand, and increasing network interconnections. In order to mitigate the
fault current level to the barest minimum, and also ensure un-interruption in the power supply it is pertinent to
ensure the protection scheme is designed in such a way that the protective relays are properly coordinated to
provide both primary and backup protection that can isolate any faulty section and prevent it from escalating in
the distribution system. A properly coordinated scheme should ensure the protective device close to the point of
fault operates first during fault occurrence and if it fails due to variation in operating time, the backup protection
scheme operates to isolate the fault. The network was modeled and simulated in Electrical Transient Analyzer
Program ETAP 19.01 Software using over current relay 51 which is one of the most commonly used protection.
The result obtained from base case simulation shows that the sequence of operation was not in order as circuit
breakers upstream from the point of fault tripped first during a fault in the following sequence CB4: 92ms, CB5:
100ms, CB1: 365ms and CB3: 392ms. However, after fine-tuning the relay setting using the TCC curve, the order
of tripping followed sequentially and there was proper backup protection in the following sequence CB1: 92ms,
CB3: 100ms, CB4: 365ms, and CB5: 392ms.