Electricity: electrical systems and devices – Safety and protection of systems and devices – High voltage dissipation
Reexamination Certificate
2000-05-25
2002-11-19
Jackson, Stephen W. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
High voltage dissipation
C361S056000, C361S058000, C361S117000, C361S127000
Reexamination Certificate
active
06483685
ABSTRACT:
TECHNICAL FIELD
The invention relates to surge arresters and other types of electrical power distribution equipment.
BACKGROUND
Electrical transmission and distribution equipment is subject to voltages within a fairly narrow range under normal operating conditions. However, system disturbances, such as lightning strikes and switching surges, may produce momentary or extended voltage levels that greatly exceed the levels experienced by the equipment during normal operating conditions. These voltage variations often are referred to as over-voltage conditions.
If not protected from over-voltage conditions, critical and expensive equipment, such as transformers, switching devices, computer equipment, and electrical machinery, may be damaged or destroyed by over-voltage conditions and associated current surges. Accordingly, it is routine practice for system designers to use surge arresters to protect system components from dangerous over-voltage conditions.
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 over-voltage-induced current surges safely around the equipment, thereby protecting the equipment and its internal circuitry from damage. When exposed to an over-voltage condition, the surge arrester operates in a low impedance mode that provides a current path to electrical ground having a relatively low impedance. The surge arrester otherwise operates in a high impedance mode that provides a current path to ground having a relatively high impedance. The impedance of the current path is substantially lower than the impedance of the equipment being protected by the surge arrester when the surge arrester is operating in the low-impedance mode, and is otherwise substantially higher than the impedance of the protected equipment.
Upon completion of the over-voltage condition, the surge arrester returns to operation in the high impedance mode. This prevents normal current at the system frequency from following the surge current to ground along the current path through the surge arrester.
Conventional surge arresters typically include an elongated 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 electrical ground, and an array of other electrical components that form a series electrical path between the terminals. These components typically include a stack of voltage-dependent, nonlinear resistive elements, referred to as varistors. A varistor is characterized by having a relatively high resistance when exposed to a normal operating voltage, and a much lower resistance when exposed to a larger voltage, such as is associated with over-voltage conditions. In addition to varistors, a surge arrester also may include one or more spark gap assemblies housed within the insulative enclosure and electrically connected in series with the varistors. Some arresters also include electrically-conductive spacer elements coaxially aligned with the varistors and gap assemblies.
For proper arrester operation, contact must be maintained between the components of the stack. To accomplish this, it is known to apply an axial load to the elements of the stack. Good axial contact is important to ensure a relatively low contact resistance between the adjacent faces of the elements, to ensure a relatively uniform current distribution through the elements, and to provide good heat transfer between the elements and the end terminals.
One way to apply this load is to employ springs within the housing to urge the stacked elements into engagement with one another. Another way to apply the load is to wrap the stack of arrester elements with glass fibers so as to axially-compress the elements within the stack.
SUMMARY
In one general aspect, an electrically-conductive and mechanically-compliant joint is formed between a pair of electrical components. The joint is positioned between a lower face of a first electrical component and an upper face of a second electrical component. The Young's modulus of the joint is less than approximately half that of the Young's modulus of the electrical components.
Embodiments of the joint may include one or more of the following features. For example, the Young's modulus of the joint may be approximately one-eightieth to one-tenth of the Young's modulus of the electrical components. More particularly, the Young's modulus of the joint may be approximately one fortieth of the Young's modulus of the electrical components. The Young's modulus of the joint may be between approximately 200,000 psi and 1,600,000 psi and the Young's modulus of the electrical components may be between approximately 13,000,000 psi and 18,000,000 psi. More particularly, the Young's modulus of the joint may be between approximately 300,000 psi and 500,000 psi and the Young's modulus of the electrical components may be between approximately 14,000,000 psi and 17,000,000 psi. Even more particularly, the Young's modulus of the joint may be approximately 400,000 psi and the Young's modulus of the electrical components may be approximately 15,000,000 psi.
The joint creates a region between the electrical components that is mechanically more compliant than the components themselves. One reason for the greater compliance within the joint is a Young's modulus which can be less than half of that of the electrical components. The lower modulus of the joint serves to attenuate or dampen the thermo-mechanical forces generated within the electrical components during operation of, for example, a surge arrester.
The joint may further include an electrically conductive polymer that provides mechanical compliance. The joint also may further include an electrically-conductive, mechanically-compliant metal alloy.
The joint may be between approximately one-sixteenth of an inch thick and one-half of an inch thick. More particularly, the joint may be between approximately one-eighth to three-eighths of an inch thick. Even more particularly, the joint may be approximately one-fourth of an inch thick. The joint may be incorporated in an electrical device.
In another general aspect, an electrically-conductive and mechanically-compliant joint is formed between a pair of electrical components. The joint is positioned between a lower face of a first electrical component and an upper face of a second electrical component. Electrical conductivity is provided by a first layer of an electrically-conductive adhesive adhered to the lower face of the first electrical component and a second layer of the electrically-conductive adhesive adhered to the upper face of the second electrical component. Mechanical compliance is provided by the two layers of electrically-conductive adhesive and by a polymer composite layer that is between the two layers.
Embodiments of the joint may include one or more of the following features. For example, the joint may further include a conductive shunt having a first end, a second end, and a middle section connecting the first end and the second end. The first end is positioned in the first layer of electrically-conductive adhesive, the second end is positioned in the second layer of electrically-conductive adhesive, and the middle section passes through the polymer composite layer.
The polymer composite layer may include a first surface in contact with the first layer of adhesive, a second surface in contact with the second layer of adhesive, a first opening on the first surface, a second opening on the second surface, and a channel passing between the first and second openings. The conductive adhesive of the first and second layers also is in the channel so that it provides an electrically-conductive path between the first and second layers. The polymer composite layer may further include multiple channels passing between multiple first and second openings, with the conductive adhesive in
Bailey David P.
Hartman Thomas C.
Perkins Roger S.
Ramarge Michael M.
Scharrer Michael G.
Fish & Richardson P.C.
Jackson Stephen W.
McGraw -Edison Company
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