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Transmission disconnect switches

Transmission disconnect switches are key components of power transmission and distribution (T&D) systems. These switches are used to control the flow of electricity between substation equipment and to isolate substation equipment for maintenance. Figure 1.1 shows an elevated three-phase (pole) vertical-break disconnect switch. At each terminal, the switch is connected to stiff aluminum bus tubes that are attached to bus supports consisting of insulators and steel tube structures. The aluminum bus tubes and their supports can also be seen in the figure.

Transmission disconnect switches

Typically, disconnect switches consist of three poles (or phases), each consisting of two or three insulator posts. The insulators are either porcelain or composite polymers. Cast or extruded aluminum is used for most of the live (current-carrying) parts. Base and operation mechanism hardware is generally manufactured from structural or alloy steel or ductile iron.
Disconnect switches are typically mounted on support structures to provide sufficient clearance of the ground, and to integrate them into the design of the substation. For the single-pole switch tested as part of this study, the pole consists of three insulator posts that are mounted on the switch base, which is in turn attached to 14-ft-tall tapered steel supports. The pole’s main blade mounted on top of the posts provides control of the electrical connection. In addition to the main blade, each pole of a grounding switch is grounded by a grounding blade during maintenance procedures. A crank and interpole linkages operate the main and grounding blades and synchronize the operation of the three poles. Typically, the mechanical operation of the switch is achieved by means of a motor mechanism that provides remote-control-powered operation of disconnect switches in large switchyards. Gang-operated, manually controlled switches are also used by some utilities. The main blade of the pole tested as part of this study opens in-plane of the pole, so the switch is termed a “vertical-break line switch.”

There are limitations on the size of a test specimen intended for seismic qualification testing on an earthquake simulator. In order to accommodate these limitations, only one pole of the switch is used for the experimental study and is referenced further in the discussion simply as “the switch.”

Porcelain disconnect switches have frequently suffered two types of failures in past earthquakes: structural damage (fracture of brittle components) and loss of functionality (blades not operating correctly), although numerous other types of failures have also been observed. Additional information on the seismic performance of disconnect switches may be found in Schiff, 1999. Since disconnect switches form an important part of power T&D 3 systems, their structural and electrical integrity are critical to maintaining operation of the electrical power grid after a major earthquake.

Transmission disconnect switches are designed and manufactured in accordance with IEEE, ANSI and IEC standards or other specifications.


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