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Basics Of Subtransmission Systems

Subtransmission systems are those circuits thatsupply distribution substations. Several differentsubtransmission systems can supply distributionsubstations. Common subtransmission voltagesinclude 34.5, 69, 115, and 138 kV. Higher voltagesubtransmission lines can carry more power withless losses over greater distances.

Distribution circuits are occasionally supplied byhigh-voltage transmission lines such as 230 kV;such high voltages make for expensive high-sideequipment in a substation. Subtransmission circuitsare normally supplied by bulk transmission lines at

subtransmission substations.

For some utilities, one transmission system servesas both the subtransmission function (feedingdistribution substations) and the transmissionfunction (distributing power from bulk generators). There is much crossover in functionality and voltage.One utility may have a 23-kV subtransmission system supplying 4-kV distribution substations.

Another utility right next door may have a 34.5-kV distribution system fed by a 138- kV subtransmissionsystem. And within utilities, one can find a variety of different voltage combinations.

Of all of the subtransmission circuit

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FIGURE 1.15 Radial subtransmission systems

arrangements, a radial configuration is thesimplest and least expensive (see Figure1.15). But radial circuits provide the mostunreliable supply; a fault on thesubtransmission circuit can force aninterruption of several distributionsubstations and service to manycustomers. A variety of redundant

subtransmission circuits are available,including dual circuits and looped ormeshed circuits (see Figure 1.16).

The design (and evolution) ofsubtransmission configurations dependson how the circuit developed, where theload is needed now and in the future, whatthe distribution circuit voltages are, wherebulk transmission is available, whererights-of-way are available, and, ofcourse, economic factors.

Most subtransmission circuits areoverhead. Many are built right along roadsand streets just like distribution lines.Some especially higher voltagesubtransmission circuits use a privateright-of-way such as bulk transmissionlines use. Some new subtransmissionlines are put underground, asdevelopment of solid-insulation cableshas made costs more reasonable.

Lower voltage subtransmission lines (69,34.5, and 23 kV) tend to be designed and

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FIGURE 1.16 Looped subtransmission system

ope a e as a e s u o es, wradial or simple loop arrangements,using wood-pole construction alongroads, with reclosers and regulators,often without a shield wire, and withtime-overcurrent protection.

Higher voltage transmission lines (115,138, and 230 kV) tend to be designed

and operated like bulk transmissionlines, with loop or mesh arrangements,tower configurations on a private right-of-way, a shield wire or wires forlightning protection, and directional orpilot-wire relaying from two ends.Generators may or may not interface atthe subtransmission level (which canaffect protection practices).

SOURCE: Electric Power Distribution

Equipment And Systems

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