Loop Primary System—Radial Secondary System

This system consists of one or more “PRIMARY LOOPS” with two or more transformers connected on the loop. This system is typically most effective when two services are available from the utility as shown in Figure 1.1-29. Each primary loop is operated such that one of the loop sectionalizing switches is kept open to prevent parallel operation of the sources. 

When secondary unit substations are used, each transformer may have its own duplex (2-load break switches with load side bus connection) sectionalizing switches and primary load break fused switch as shown in Figure 1.1-30 or utilizing three on-off switches or a four-position sectionalizing switch and vacuum fault interrupter (VFI) internal to the transformer saving cost and reducing footprint. 

When pad-mounted compartmentalized transformers are used, they are furnished with loop-feed oil immersed gangoperated load break sectionalizing switches and Bay-O-Net expulsion fuses in series with partial range back-up current-limiting fuses. By operating the appropriate sectionalizing switches, it is possible to disconnect any section of the loop conductors from the rest of the system. In addition, it is possible to disconnect any transformer from the loop.

Figure 1.1-29. Loop Primary—Radial Secondary System

Figure 1.1-30. Secondary Unit Substation Loop Switching

Figure 1.1-31. VFI / Selector Switch Combination

Figure 1.1-32. Pad-Mounted Transformer Loop Switching

Figure 1.1-33. Basic Primary Selective—Radial Secondary System

A key interlocking scheme is normally recommended to prevent closing all sectionalizing devices in the loop. Each primary loop sectionalizing switch and the feeder breakers to the loop are interlocked such that to be closed they require a key (which is held captive until the switch or breaker is opened) and one less key than the number of key interlock cylinders is furnished. An extra key is provided to defeat the interlock under qualified supervision.

In addition, the two primary main breakers, which are normally closed, and primary tie breaker, which is normally open, are either mechanically or electrically interlocked to prevent paralleling the incoming source lines. For slightly added cost, an automatic throw-over scheme can be added between the two main breakers and tie breaker. During the more common event of a utility outage, the automatic transfer scheme provides significantly reduced power outage time. 

The system in Figure 1.1-29 has higher costs than in Figure 1.1-28, but offers increased reliability and quick restoration of service when

1) a utility outage occurs,

2) a primary feeder conductor fault occurs, or

3) a transformer fault or overload occurs.

Should a utility outage occur on one of the incoming lines, the associated primary main breaker is opened and the tie breaker closed either manually or through an automatic transfer scheme. When a primary feeder conductor fault occurs, the associated loop feeder breaker opens and interrupts service to all loads up to the normally open primary loop load break switch (typically half of the loads). Once it is determined which section of primary cable has been faulted, the loop sectionalizing switches on each side of the faulted conductor can be opened, the loop sectionalizing switch that had been previously left open can then be closed to all secondary unit substations while the faulted conductor is replaced. If the fault should occur in a conductor directly on the load side of one of the loop feeder breakers, the loop feeder breaker is kept open after tripping and the next load side loop sectionalizing switch manually opened so that the faulted conductor can be sectionalized and replaced. Under this condition, all secondary unit substations are supplied through the other loop feeder circuit breaker, and thus all conductors around the loop must be sized to carry the entire load connected to the loop. Where separable load break or non-load break connectors are used, they too must be sized to handle the entire load of the loop. Increasing the number of primary loops (two loops shown in Figure 1.1-33) will reduce the extent of the outage from a conductor fault, but will also increase the system investment.

When a transformer fault or overload occurs, the transformer primary fuses open, and the transformer primary switch manually opened, disconnecting the transformer from the loop, and leaving all other secondary unit substation loads unaffected. A basic primary loop system that uses a single primary feeder breaker connected directly to two loop feeder switches which in turn then feed the loop is shown in Figure 1.1-34. In this basic system, the loop may be normally operated with one of the loop sectionalizing switches open as described above or with all loop sectionalizing switches closed. If a fault occurs in the basic primary loop system, the single loop feeder breaker trips, and secondary loads are lost until the faulted conductor is found and eliminated from the loop by opening the appropriate loop sectionalizing switches and then reclosing the breaker.

 

Figure 1.1-34. Single Primary Feeder—Loop System