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| Purpose of Disconnect Switches In HV Substation |
Types and Characteristics
Switches may be of several types, the common characteristic is that none are designed to interrupt fault currents and the insulation or insulators associated with them must coordinate with the rest of the system and a BIL (Basic insulation level) capable of withstanding voltage surges.
In general, they consist of a conducting blade, hinged at one end, and a stationary contact on the other, both terminals mounted on suitable insulators that conform to the common insulator requirements of BIL coordination.
Almost every major line or equipment in a substation has associated with it a means of completely isolating it from other energized elements as a prudent means of insuring safety by preventing accidental energization. These simple switches, called disconnects, or disconnecting switches, are usually installed on both sides of the equipment or line upon which work is to be done.
They should not be operated while the circuit in which they are connected is energized, but only after the circuit is deenergized. As a further precaution, they may be opened by means of an insulated stick that helps the operator keep a distance from the switch.
Locking devices are sometimes provided to keep the disconnects from being opened accidentally or from being blown open during periods of heavy fault currents passing through them.
Although not designed to be closed to energize the line or equipment with which they are associated, in certain circumstances they may be closed, using special care to close them firmly and rapidly.
Disconnects may be single blade units or multiple units operated together.
Air break switches have characteristics similar to disconnects, but have the stationary contacts equipped with arc suppressing devices that enable them to be opened while energized, but recognizing a limitation as to the current that may be safely interrupted.
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| Transmission line disconnect switch |
The device may be a simple arcing horn which stretches the arc that may form until it cannot sustain itself.
Another type has a flexible “whip” attached to the stationary contact that continues the contact with the moving blade until a point is reached at which the whip snaps open very rapidly extinguishing any arc that may form.
Still another has an interrupting unit mounted at the free end of the switch blade and which suppresses the arc within the unit as the switch opens; the interrupting unit may contain a vacuum chamber, or a series of grids which cause the arc to break up into smaller ones and are more readily extinguished.
Both types function similarly as larger units do in circuit breakers.
Oil switches have the blade open from its contact under oil which suppresses any arc that may form. They have higher current breaking capacity than air break switches and are particularly suited for underground systems or where moisture or pollution make air switches impractical.
However, they (oil switches) are more expensive at first cost, to operate and to maintain.
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