Protecting distribution substation assets — Modern protection schemes with microprocessor-based relays

Distribution substations at electric cooperatives and municipal utilities have historically involved simple protection schemes consisting of feeder circuit overcurrent, reclosing, and transformer protection, either with high-side fuses or differential and overcurrent protection. These protective devices have served to protect the transmission operator as much or more than the distribution substation. Modern microprocessor-based relays allow for much better protection schemes to protect the distribution substation assets. This paper analyzes several schemes that have recently been implemented at Mid-Carolina Electric Cooperative in South Carolina. Benefits include: Faster tripping timesall zones in the substation are protected with differential relays. - Reduced arc-flash hazards for personnel. - Reduced equipment damage during faults; Backup protection schemes for each piece of equipment in the substation. - Backup feeder protection via transformer differential relays. - Detection of failed feeder relay and failed feeder breaker trip coil. - Backup bus differential via transformer differential low-side overcurrent. - Redundant transformer differential relays, one of which also includes the bus in the differential zone; Superior fault analysis through satellite clock time synchronization to all substation relays; Communications to each relay via an Ethernet network that provides both SCADA communications and engineering access for event retrieval.