Primary Selective System—Secondary Radial System

The primary selective—secondary radial system, as shown in Figure1.1-33, differs from those previously described in that it employs at least two primary feeder circuits in each load area. It is designed so that when one primary circuit is out of service, the remaining feeder or feeders have sufficient capacity to carry the total load. Half of the transformers are normally connected to each of the two feeders. When a fault occurs on one of the primary feeders, only half of the load in the building is dropped. 

Duplex fused switches as shown in Figure 1.1-33 and detailed in Figure 1.1-35 may be utilized for this type of system. Each duplex fused switch consists of two load break three-pole switches each in their own separate structure, connected together by bus bars on the load side. Typically, the load break switch closest to the transformer includes a fuse assembly with fuses.

Mechanical and/or key interlocking is furnished such that both switches cannot be closed at the same time (to prevent parallel operation) and interlocking such that access to either switch or fuse assembly cannot be obtained unless both switches are opened.

Figure 1.1-35. Duplex Fused Switch in Two Structures

One alternate to the duplex switch arrangement, a non-load break selector switch mechanically interlocked with a load break fused switch can be used as shown in Figure 1.1-36. The non-load break selector switch is physically located in the rear of the load break fused switch, thus only requiring one structure and a lower cost and floor space savings over the duplex arrangement. The non-load break switch is mechanically interlocked to prevent its operation unless the load break switch is opened. The main disadvantage of the selector switch is that conductors from both circuits are terminated in the same structure.

Figure 1.1-36. Fused Selector Switch in One Structure

This means limited cable space especially if double lugs are furnished for each line as shown in Figure 1.1-33. The downside is that should a faulted primary conductor have to be changed, both lines would have to be de-energized for safe changing of the faulted conductors. 

A second alternative is utilizing a threeposition selector switch internal to the transformer, allowing only one primary feeder to be connected to the transformer at a time without the need for any interlocking. The selector switch is rated for load-breaking. If overcurrent protection is also required, a vacuum fault interrupter (VFI), also internal to the transformer, may be utilized, reducing floor space. 

In Figure 1.1-33 when a primary feeder fault occurs, the associated feeder breaker opens and the transformers normally supplied from the faulted feeder are out of service. Then manually, each primary switch connected to the faulted line must be opened and then the alternate line primary switch can be closed connecting the transformer to the live feeder, thus restoring service to all loads. Note that each of the primary circuit conductors for Feeder A1 and B1 must be sized to handle the sum of the loads normally connected to both A1 and B1. Similar sizing of Feeders A2 and B2, etc., is required. 

If a fault occurs in one transformer, the associated primary fuses blow and interrupt the service to just the load served by that transformer. Service cannot be restored to the loads normally served by the faulted transformer until the transformer is repaired or replaced. 

Cost of the primary selective—secondary radial system is greater than that of the simple primary radial system of Figure 1.1-27 because of the additional primary main breakers, tie breaker, two-sources, increased number of feeder breakers, the use of primary-duplex or selector switches, and the greater amount of primary feeder cable required. 

The benefits from the reduction in the amount of load lost when a primary feeder is faulted, plus the quick restoration of service to all or most of the loads, may more than offset the greater cost. 

Having two sources allows for either manual or automatic transfer of the two primary main breakers and tie breaker should one of the sources become unavailable.

The primary selective-secondary radial system, however, may be less costly or more costly than a primary loop—secondary radial system of Figure 1.1-29 depending on the physical location of the transformers. It also offers comparable downtime and reliability. The cost of conductors for the types of systems may vary depending on the location of the transformers and loads within the facility. The cost differences of the conductors may offset cost of the primary switching equipment.