SUBSTATIONS

The purpose of a substation is to switch circuits on and off and to change the generated

Figure 5-8.\Elements of a power distribution system.

power to the proper voltage and frequency necessary for transmission. Usually, a substation is used to connect the generating plant to the transmission lines. The substation transforms the generated voltage to transmission voltage and protects the generating plant against any faults on the transmission lines. Another substation is used to transform the transmission voltage to that of the distribution voltage. The distribution substation protects the substation and transmission lines against any faults occurring on the distribution feeders. At many advanced bases, the source of the power will be generators connected directly to distribution centers, thus eliminating the need for substations.

PRIMARY FEEDERS

Primary feeders are those conductors in a distribution system that are connected from the substations and that transfer the power to the distribution centers (fig. 5-8). They may be arranged as radial, loop, or network systems and may be overhead or underground.

Radial Distribution System

A representative schematic of a radial distribution system is shown in figure 5-9. You will note the branching out of the independent feeders to several distribution centers without intermediate connections between feeders.

The most frequently used system is the radial distribution system because it is the simplest and least expensive system to build. It is not as reliable as most systems, however, because a fault in the main feeder may result in an outage on all loads served by the feeder.

Service on this type of feeder can be improved by installing automatic circuit breakers that will reclose the service at predetermined intervals. If the fault continues after a predetermined number of closures, the breaker will be locked out until the fault is cleared and service is restored by hand reset.

Loop, or Ring, Distribution System

The loop (or ring) system of distribution starts at the substation and is connected to or encircles an area serving one or more distribution transformers or load centers; the conductors of the system return to the same substation.

Figure 5-9.\Radial distribution system.

The loop system (fig. 5-10) is more expensive to build than the radial type, but it is more reliable. It may be justified in an area where continuity of service is of considerable importance\at a medical center, for example.

In the loop system, circuit breakers sectionalize the loop on both sides of each distribution transformer connected to the loop. The two primary feeder breakers and the sectionalizing breakers associated with the loop feeder are ordinarily controlled by pilot wire relaying or directional overcurrent relays. Pilot wire relaying is used when there are too many secondary substations to obtain selective timing with directional overcurrent relays.

A fault in the primary loop is cleared by the breakers in the loop nearest the fault, and power is supplied the other way around the loop without interruption to most of the connected loads. If a fault occurs in a section adjacent to the distribution substation, the entire load may have to be fed from one direction over one side of the loop until repairs are made. Sufficient conductor capacity must be provided in the loop to permit operation without excessive voltage drop or overheating of the feeder when either side of the loop is out of service. If a fault occurs in the distribution transformer, it is cleared by the breaker in the primary leads, and the loop remains intact.