Electrical Power Plant Design: Distribution System

WEEK 13: DISTRIBUTION: EQUIPMENT

Distribution: Equipment: Voltage Control

Fuse Cutouts consists of a short piece of metal having low melting characteristics which will melt at a rated temperature and cut out the section of the circuit which is endangered, allowing the rest of the circuit to remain energized. A primary fuse cutout is connected between the primary lines and the transformer to protect the transformer from overloads and to disconnect the transformer from the primary lines in case of trouble. Fuse cutouts are usually mounted on the crossarm or on the pole by bolts and a steel bracket.

Fuse Cutouts Types There are three main types of fuse cutouts, the principle upon which each is constructed is the same: a fuse ribbon makes a connection between two contacts, either the line and the transformer, or the main line and that portion to be connected. These are: door-type, open-type, and repeater-type.

The Door Type Cutout, also called as the Expulsion-type, the fuse is mounted inside the door in such a manner than when the door closes, the fuse engages two contacts, one on the top of the box and the other on the bottom. To open this cutout, the door is pulled open and allowed to hang downward from the box. The fusible element is enclosed in a fiber tube; when the fuse blows or melts because of excessive current passing through it, the resultant arc attacks the fiber tube, producing a gas which blows out the arc. This device is generally limited to circuits operating at voltages under 5kV.

The Open-Type Cutout is essentially the same as the door type. except that the fiber tube enclosing the fusible element is exposed in the open, rather than enclosed in a porcelain box. This arrangement enables larger currents to be interrupted without confining as much the attendant violent expulsion of gases, which can generally destroy the cutout. The tube drops when the fuse blows, indicating that the fuse has blown. This cutout is often used on distribution circuits operating at voltages over 5kV, but are also used lower than this rating.

The Repeater-Type Cutout is used in conjunction with circuit fuses. The repeated fuse is usually of the open type and consists of two or three mechanically arranged so that when the first fuse blows and drops, the action places the second fuse automatically in the circuit. If the trouble has been cleared, service will be restored. Should the second fuse also blow, a third is also automatically connected in the circuit; when the third fuse blows, the portion of the circuit is finally de-energized. Repeated fuses hold down to a minimum interruptions in service caused by temporary faults. These faults may arise from wires swinging together when improperly sagged, or from tree branches or animals making momentary contact with the line, or from lightning surges causing temporary flashover at an insulator.

Switches

Switches are used to interrupt the continuity of a circuit. They fall into two broad classifications for distribution systems: air switches and oil switches. See also: Transmission Systems: Circuit Switchers.

Classification of Distribution Switches As their name implies: Air Switches are those whose contacts are opened in air. Air switches are further classified as air-break switches and disconnect switches. The air-break switch has both the blade and contact equipped with arcing horns, a piece of metal between which the arc resulting from opening a circuit carrying current is allowed to form. As the switch opens, these horns are spread farther and farther apart and the arc is lengthened until it finally breaks. A modern development of the air-break switch is the load-break switch which breaks the arc inside a fiber tube. The tube produces a gas which helps to confine the arc and blow it out. The disconnect switch is not equipped with arc-quenching devices and therefore should not be used to open circuits carrying currents. It isolates one portion of the circuit from another and is not intended to be opened while current is flowing. Air-break switches maybe opened under load, but disconnect switches must not be opened until the circuit is interrupted by some other means.

Oil Switches are those whose contacts are opened under oil. Oil switches are usually necessary only in very high-voltage, high-current circuits. This switch has both the blade and the contact mounted in a tank filled with oil. The switch is usually operated from a handle on the outside of the case. As the switch opens, the arc formed between the blade and contact is quenched by the oil. Oil switches maybe remote-controlled as well as manually operated. They are used for capacitor switching, street lighting control and automatic disconnect in case of power failure.

Reclosers

Reclosers consist essentially of an oil switch or breaker actuated by relays which cause it to open when predetermined current values flow through it. The recloser resembles the repeater fuse cutout. They are usually connected to protect portions of primary circuits and may take the place of line fuses. The switch or breaker is arranged to reclose after a short interval of time and re-open again should the fault or overload which caused the excess current-flow persist. Also, like the repeater fuse cutout, the recloser can be set for 3 or 4 operations before it locks itself open for manual operation.

Distribution Substation Design