Primary Distribution

Primary distribution systems include three main types: radial systems used in low-load-density areas with simple, economical feeders; loop systems designed for high-reliability areas that reconnect to the substation providing two-way feed; and primary network systems found in high-density urban areas with interconnected feeders from multiple substations. Each type balances cost, reliability, and service quality differently.

Automatic reclosing devices manage temporary faults caused by lightning flashovers or momentary contact incidents. These devices momentarily open the circuit during a fault and then promptly reclose it, restoring power supply without permanent interruption. This approach effectively handles transient faults while maintaining distribution reliability and automation in the system.

Sectionalizing fuses isolate faulted sections of the distribution system, preventing widespread outages and protecting equipment. Shunt capacitor banks manage voltage levels throughout the network, ensuring stable power delivery. Together, these components enhance system reliability by containing faults and maintaining voltage within acceptable ranges for consumers.

Loop systems provide a two-way feed by reconnecting to the distribution substation, ensuring power supply even during faults. Reclosers and tie switches isolate faulted sections while maintaining service to unaffected areas. This redundancy contrasts with radial systems, where a single fault can interrupt service to all downstream customers.

The 15-kV class voltage is predominant among U.S. utilities and is replacing older 2.5- and 5-kV classes. Higher 25- to 34.5-kV classes are used in high-density urban areas and rural regions with long feeders. Three-phase, four-wire multigrounded systems are widely employed for balanced power delivery with neutral grounding.

Rural areas typically use overhead lines with distribution transformers, fuses, and switches mounted on poles for economy. Urban areas often employ underground cable systems for reliability and aesthetics, offering significant protection from weather-related disruptions despite higher installation costs. The choice depends on load density, geography, and service requirements.

Primary network systems consist of a grid of interconnected feeders from multiple substations, providing the highest reliability and service quality. Faults are isolated by circuit breakers and fuses, and smaller unit substations can replace conventional distribution substations at selected network locations. This configuration ensures continuous power supply and flexibility in managing loads.