Electrical resistors – Resistance value responsive to a condition – Current and/or voltage
Reexamination Certificate
2001-08-30
2003-01-14
Easthom, Karl D. (Department: 2832)
Electrical resistors
Resistance value responsive to a condition
Current and/or voltage
C338S020000
Reexamination Certificate
active
06507269
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2000-262949, filed Aug. 31, 2000; and No. 2001-185285, filed Jun. 19, 2001, the entire contents of both of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a voltage nonlinear resistor useful in an overvoltage protection device for protecting, e.g., an electric power system, and to a method of manufacturing the voltage nonlinear resistor. More specifically, the present invention relates to a voltage nonlinear resistor having a high-resistance layer on the side surface of a voltage nonlinear resistive body, and also to a method of manufacturing the voltage nonlinear resistor.
2. Description of the Related Art
In an ordinary electric power system, an overvoltage protection device such as a surge arrestor or a surge absorber is employed to protect the power system by eliminating any overvoltage superimposed on the normal voltage. This overvoltage protection device is generally provided with a voltage nonlinear resistor. The voltage nonlinear resistor is a resistor that exhibits substantially insulating properties under the normal voltage, but exhibits a relatively low resistance when an overvoltage is applied to the resistor. Such a voltage nonlinear resistor has a sintered body (voltage nonlinear resistive body) that is formed from a composition containing ZnO (zinc oxide) as a main component. The composition further contains one or more metal oxides for obtaining voltage nonlinear resistance properties. The composition is mixed well, granulated, molded and sintered to provide the sintered body. On the side face of the sintered body, there is provided a high-resistance layer (side face high-resistance layer) formed of an electrically insulating material. The side face high-resistance layer serves to prevent the generation of flashover from the side face upon surge absorption.
Japanese Patent Unexamined Publication H8-172002 discloses a side face high-resistance layer mainly formed of an organic polymeric resin such as an epoxy resin. Japanese Patent Unexamined Publication H3-30301 discloses a side face high-resistance layer which is formed from zinc silicate (ZnSiO
4
) or zinc antimony spinel (Zn
7
Si
2
O
12
). Japanese Patent Unexamined Publication H10-312908 discloses a side face high-resistance layer formed from a crystalline inorganic substance including Zn—Sb—Si and Fe—Mn—Bi—Si. Japanese Patent Unexamined Publication H8-306506 discloses a side face high-resistance layer including zinc silicate (ZnSiO
4
), as a main component, and zinc antimony spinel (Zn
7
Si
2
O
12
) in which Fe is solid-solutioned, as a sub-component. Japanese Patent Unexamined Publication H5-205907 discloses a side face high-resistance layer formed of glass containing lead as a main component. Japanese Patent No. 2516531 discloses a side face high-resistance layer formed of bismuth borosilicate glass (B—Si—Bi—Zn—O).
In view of the increasing demand for electric power supply and the remarkable development of highly information-oriented society in recent years, a stable and cheap power supply is now strongly demanded. Under such circumstances, there is also an increasing demand to develop an overvoltage protection device that is highly reliable, smaller is size and more economical. To meet such a demand, it is now tried to miniaturize the overvoltage protection device through the reduction in thickness of the nonlinear resistor by increasing the magnitude of voltage per unit thickness of the voltage nonlinear resistor, and/or through the reduction in diameter of the voltage nonlinear resistor by enhancing the energy absorption capability of the nonlinear resistor.
However, the prior art side face high-resistance layer mainly composed of an organic polymeric resin such as an epoxy resin as proposed by Japanese Patent Unexamined Publication H8-172002 tends to be peeled away from the sintered body due to a large difference in thermal expansion coefficient between the sintered body and the high-resistance layer as heat is generated when a surge is imposed on the voltage nonlinear resistor. As a result, the voltage nonlinear resistor becomes impossible to exhibit an excellent energy absorption capability, making it impossible to miniaturize the nonlinear resistor beyond a certain extent. Namely, this voltage nonlinear resistor cannot have a sufficient protecting capability to resist against a surge such as thunderbolt impulse or overvoltage, which is required when the voltage nonlinear resistor is miniaturized through an increase in magnitude of voltage per unit thickness thereof or through a reduction in its diameter.
Incidentally, an overvoltage protection device whose container is made of a porcelain insulator has been traditionally employed. However, if the overvoltage protection device is exploded in the worst case due to an excessive surge, the porcelain insulator would be scattered around, endangering the surroundings. In view of this problem, an overvoltage protection device whose container is made of a polymeric rubber or resin is increasingly employed in a power distribution system in recent years.
However, the prior art side face high-resistance layer made from a crystalline inorganic substance including Zn—Sb—Si—O or Zn—Si—O as proposed in Japanese Patent Unexamined Publication H3-30301 has a poor water resistance. Thus, the electrical properties may be deteriorated by a little quantity of water permeated through the polymeric container wall. Therefore, the voltage nonlinear resistor provided with a side face high-resistance layer formed of the crystalline inorganic substance noted above would be not be suitably applied to an overvoltage protection device whose container is made of a polymer rubber or resin. If such resistor is applied to overvoltage protection device whose container is made of a polymer rubber or resin, it would be necessary to take a suitable measure to prevent the permeation of water, which is economically disadvantageous.
The side face high-resistance layer formed of a glass compound mainly composed of lead as proposed in Japanese Patent Unexamined Publication H5-205907 can overcome both of the aforementioned problems. However, since lead is a poisonous material, an enormous cost would be required for the prevention of intoxication in the manufacturing process, or for the disposition of residual materials, waste matters or waste liquid produced in the manufacturing process. At the same time, environmental contamination may occur depending upon a way in which used overvoltage protection devices are disposed.
In view of the above, a side face high-resistance layer which is characterized by being lead-free and is formed of bismuth borosilicate glass (B—Si—Bi—Zn—O) as taught in Japanese Patent No. 2516531 is increasingly employed in the voltage nonlinear resistor. However, the bismuth borosilicate glass composition as taught in Japanese Patent No. 2516531 has a region at which the composition can not be vitrified by baking at a temperature of 550° C. or less. Thus, this region of the composition requires baking at a temperature of above 550° C. or more. As a result, the glass component or components may be dissolved into the sintered body at the baking step, or the crystal structure of the sintered body may be transformed, thereby deteriorating the current-voltage characteristics as well as applied-voltage life characteristics of the voltage nonlinear resistor.
Additionally, the bismuth borosilicate glass composition as taught in Japanese Patent No. 2516531 has a region at which the glass exhibits a thermal expansion coefficient that greatly differs from that of the sintered body. As a result, the side face high-resistance layer may be peeled away from the sintered body, or cracks may be generated in the side face high-resistance layer when a surge is imposed on the voltage nonlinear resistor. Thus, it becomes impossible for the voltage nonlinear res
Imai Toshiya
Kasuga Yasunori
Udagawa Takeshi
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