Relay Coordination Analysis in SPDC Forcados Distribution Network for Operation, Planning and Future Expansion

Power protection system is increasingly challenging as fault current level increases in electrical power system due to increase in static load, introduction of heavy-duty electric motors and increase in network interconnections. In order to coordinate relays effectively, load flow study and short circuit current analysis are carried out for proper selectivity. This analysis formed basis for optimal setting of Over Current Relays (OCRs) via Standard Inverse Timedelay (SIT). The detailed survey on the existing definite time delay with current time grading amounted to higher relay current-time configuration at the upstream in the steps of 0.5s up to a minimum time of 3s and with steps of 25% rated current increment to 250%, the upstream at this setting experienced thermal overload which was resolved via SIT. The inadequate relay definite time characteristics setting causing thermal overload (I2R) of the 25MVA and incessant tripping was resolved with SIT relay coordination scheme using Electrical Transient Analyzer Program (ETAP). This method also accommodated the associated circuit breakers response breaking capacity time (380ms to 500ms) which was a major achievement as compared to other inverse time delay characteristics such as Very Inverse Time (VIT), Extremely Inverse Time (EIT) etc.