Electricity: electrical systems and devices – Safety and protection of systems and devices – High voltage dissipation
Patent
1998-11-20
1999-12-28
Jackson, Stephen W
Electricity: electrical systems and devices
Safety and protection of systems and devices
High voltage dissipation
361 56, 361111, 361127, H02H 100
Patent
active
06008975&
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates generally to electrical power distribution equipment. More particularly, the invention relates to sub-assemblies or modules that contain discrete electrical components and that are employed in protective devices such as surge arresters. Still more particularly, the invention relates to apparatus and methods for applying an axially-compressive force to an array of electrical components and retaining those components under compression in end-to-end relationship within the module.
Under normal operating conditions, electrical transmission and distribution equipment is subject to voltages within a fairly narrow range. Due to lightning strikes, switching surges or other system disturbances, portions of the electrical network may experience momentary or transient voltage levels that greatly exceed the levels experienced by the equipment during normal operating conditions. Left unprotected, critical and costly equipment such as transformers, switching apparatus, computer equipment, and electrical machinery may be damaged or destroyed by such over-voltages and the resultant current surges. Accordingly, it is routine practice within the electrical industry to protect such apparatus from dangerous over-voltages through the use of surge arresters.
A surge arrester is a protective device that is commonly connected in parallel with a comparatively expensive piece of electrical equipment so as to shunt or divert the over-voltage-induced current surges safely around the equipment, thereby protecting the equipment and its internal circuitry from damage. When caused to operate, a surge arrester forms a current path to ground having a very low impedance relative to the impedance of the equipment that it is protecting. In this way, current surges which would otherwise be conducted through the equipment are instead diverted through the arrester to ground. Once the transient condition has passed, the arrester operates to open the recently-formed current path to ground, thereby again isolating the distribution or transmission circuit in order to prevent the non-transient current of the system frequency from "following" the surge current to ground, such system frequency current being known as "power follow current."
Conventional surge arresters typically include an elongate outer enclosure or housing made of an electrically insulating material, a pair of electrical terminals at opposite ends of the enclosure for connecting the arrester between a line-potential conductor and ground, and an array of other electrical components forming a series path between the terminals. These components typically include a stack of voltage-dependent, nonlinear resistive elements. These nonlinear resistors or "varistors" are characterized by having a relatively high resistance at the normal steady-state voltage and a much lower resistance when the arrester is subjected to transient over-voltages. Depending on the type of arrester, it may also include one or more spark gap assemblies housed within the insulative enclosure and electrically connected in series with the varistors. Some present-day arresters also include electrically conducting spacer elements coaxially aligned with the varistors and gap assemblies. Electrodes of a variety of types and configurations may also be included in the component array in conventional arresters.
For an arrester to function properly, it is important that contact be maintained between the ends of the various surge arrester components in the array. To accomplish this, an axial load is placed on the elements in the array. Such loading is typically applied by employing springs within the housing to urge the stacked elements into engagement with one another. Good axial contact is important to ensure a relatively low contact resistance between the adjacent faces of the components, to ensure a relatively uniform current distribution through the elements, and to provide good heat transfer between the arrester elements in the array and the end terminals.
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Bailey David P.
Daley Charles W.
Hoover Todd R.
Kester Jeffrey Joseph
Raimondi Tomon K.
Jackson Stephen W
McGraw-Edison Company
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