This application relates to a method of joining materials that may be used in surge arresters and other types of electrical power distribution equipment.
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 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.
When the over-voltage condition has passed, the surge arrester returns to operation in the high impedance mode. This high impedance mode prevents normal current at the system frequency from flowing through the surge arrester to ground.
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 impedance when exposed to a normal system frequency 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 electrically connected in series or parallel with one or more of 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 components of the stack. 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 components, and to provide good heat transfer between the components and the end terminals.
One way to apply this load is to employ springs within the housing to assure the stacked components engage with one another. Another way to apply the load is to wrap the stack of arrester components with glass fibers to axially-compress the components within the stack.
In one general aspect, a surface of a first ceramic component is joined to a surface of a second ceramic component using a silver-based composition. The silver-based composition is a mixture of silver metal and a metal oxide and the metal in the metal oxide is a metal other than silver. The silver-based composition is applied to the surface of the first ceramic component and to the surface of the second ceramic component. The silver-based composition applied to the first ceramic component is contacted to the silver-based composition applied to the second ceramic component. The surfaces of the first and second ceramic components are heated to melt the applied silver-based compositions. The surfaces of the first and second ceramic components are cooled to form a bond between the first and second ceramic components.
Implementations may include one or more of the following features. The first ceramic component may include a varistor or, more particularly, a metal oxide varistor. The second ceramic component may include a varistor or, more particularly, a metal oxide varistor.
The silver-based composition is applied to the surface of the first ceramic component by preparing a powder of the metal oxide and then mixing the prepared metal oxide powder to form a metal oxide paste. A foil of silver metal is applied to the surface of the first ceramic component and the metal oxide paste is spread onto the applied silver foil to obtain the silver-based composition.
The silver-based composition may melt at a temperature less than melting points of the first and second ceramic components. In particular, the silver-based composition may melt between around 900xc2x0 Celsius and 1000xc2x0 Celsius.
The silver-based composition may be a mixture of silver metal and vanadium oxide. In this case, the mixture may include between around 0.1 to around 10% vanadium oxide by weight.
The first and second ceramic components may be compressed together before heating the surfaces of the ceramic components.
Application of the silver-based composition to the surface of the first ceramic component may include preparing the metal oxide and the silver metal, mixing the prepared metal oxide and the prepared silver metal to form a silver-based composition paste, and then spreading the silver-based composition paste on the first ceramic component to obtain the silver-based composition.
In another general aspect, a surface of a first ceramic component is joined to a surface of a second ceramic component using a silver-based composition. The silver-based composition is a mixture of silver metal and a metal oxide and the metal in the metal oxide is a metal other than silver. The silver-based composition is applied to the surface of the first ceramic component. The silver-based composition applied to the first ceramic component is contacted to the surface of the second ceramic component. The surfaces of the first and second ceramic components are heated to melt the applied silver-based composition. The surfaces of the first and second ceramic components are cooled to form a bond between the first and second ceramic components.
In another general aspect, a bonded component stack for use in surge arrester includes a first ceramic component having a surface, a second ceramic component having a surface, and a silver-based composition. The silver-based composition is a mixture of silver metal and a metal oxide and the metal in the metal oxide is a metal other than silver. The silver-based composition is brazed to the surfaces of the first and second ceramic components to bond the surface of the first ceramic component to the surface of the second ceramic component.
Implementations may include one or more of the following features. The first ceramic component may include a varistor or, more particularly, a metal oxide varistor. The second ceramic component may include a varistor or, more particularly, a metal oxide varistor.
The silver-based composition may be brazed between the surfaces of the first and second ceramic components by applying the silver-based composition to the surface of the first ceramic component; contacting the silver-based composition applied to the first ceramic component to the surface of the second ceramic component; heating the surfaces of the first and second ceramic components to melt the applied silver-based composition; and cooling the surfaces of the first and second ceramic components to form a bond between the first and second ceramic components.
The silver-based composition may be brazed between the surfaces of the first and second ceramic components by compressing the first and second ceramic components together before heating the surfaces of the ceramic components.
The silver-based composition may be brazed between the surfaces of the first and second ceramic components by applying the silver-based composition to the surface of the first ceramic component and to the surface of the second ceramic component; contacting the silver-based composition applied to the first ceramic component to the silver-based composition applied to the second ceramic component; heating the surfaces of the first and second ceramic components to melt the applied silver-based compositions; and cooling the surfaces of the first and second ceramic components to form a bond between the first and second ceramic components.
The silver-based composition may melt at a temperature less than melting points of the first and second ceramic components. The silver-based composition may melt between around 900xc2x0 Celsius and 1000xc2x0 Celsius.
The silver-based composition may be a mixture of silver metal and vanadium oxide. In that case, the mixture may include between around 0.1 to around 10% vanadium oxide by weight.
The bonded component stack and the method of making the bonded component stack provides the following advantages. The surfaces may be joined in an air atmosphere, which reduces production and manufacturing costs. The surfaces joined without the use of aggressive fluxes or a secondary heat treatment. The resulting bonded component stack has a relatively low contact resistance between the adjacent faces of the components, a relatively uniform current distribution through the components, and good heat transfer between the components. For these reasons, the need for additional mechanical reinforcement, such as axially-loaded springs, is eliminated.
Additionally, the method of joining eliminates the need for a metallized layer on the surface of the ceramic components because the silver-based composition is bonded directly to the surface. The method of joining also may be used without the need for additional heat treatment steps to recover varistor properties, which otherwise may be lost during the joining process.
Other features and advantages will be apparent from the description, the drawings, and the claims.