Mitigating Feeder-Wide Blackouts through Optimal Protection Coordination: A Case Study of the Port Harcourt 132 kV Substation

Abstract

The study looks at Port Harcourt mains 132kV substation because most medium voltage feeders are faced with blackouts several times per year over the last couple of years, which has caused downtimes during blackout duration, besides the maintenance and operation cost impact. Load flow analysis is used for determining the full load current and determining the Current Transformer's (CT) pick-up current via Newton-Raphson's technique. Short circuit analysis was used to determine the short circuit impedance, short circuit current, and circuit breaker Sizing. Star protection and coordination analysis are used for analysing the system protection, operation and coordination. After performing the simulation for external fault scenario on three (3) medium voltage feeders: Abuloma, RSPUB 2 and Refinery 1 to evaluate its impact on the existing protection scheme, the result shows that the sequence of operation of the relays for Abuloma and Refinary1 feeder are well coordinated in the sense that the relays downstream from the point of fault tripped first at 316ms followed by the backup relay upstream of the substation at 600ms for an external fault. However, the sequence of operation for the RSPUB 2 feeder is mis-coordinated as the relay upstream of the substation tripped first at 450ms, followed by the differential relay protecting the power transformers (T2A) tripping on an external fault outside the protection zone at 600ms, before the overcurrent relay downstream at the point of fault tripping at 750ms. This means that the entire medium voltage feeder fed by transformer T2A is out of power for a fault that occurred downstream on a feeder. Therefore, a new relay setting coordination is required for the RSPUB 2 feeder. In order to mitigate this challenge, a completely new coordinated scheme was designed using the TCC curve to ensure the coordination starts from downstream of the point fault (RSPUP 2 Feeder) up to the generator station upstream. Also, the entire definite-minimum-time (DMT) scheme was replaced with the more flexible inverse-definite-minimum-time (IDMT) scheme. The new settings provide a backup for over current fault system that is streamlined and does not result in gross miscoordination. This work is essential to avoid damage to electrical equipment and personnel working at the station.