Lightning arrester

Abstract

 The present invention is concerned with a lightning arrester in which characteristic elements are stacked in a plurality of numbers in series, each of said characteristic elements being chiefly composed of zinc oxide and having non-linear electric resistance relative to the voltage, wherein said characteristic elements establish an electric field ofgreaterthan 2000 V/cm for a current density of 0.1 mA/cm².

Description

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a lightning arrester, and more specifically to a lightning arrester of the type of zinc oxide for protecting power systems from instantaneous abnormal voltages.

[0002] A lightning arrester is used for suppressing abnormal voltages that are generated in the power circuit. As a characteristic element for the lightning arrester, use is made of a sintered product of a metal oxide composed chiefly of zind oxide (hereinafter referred to as zinc oxide element or simply as element). The zinc oxide element has an excellent non-linearity relative to the voltage, and inhibits the flow of dynamic current from the power source after the abnormal voltage has been suppressed, obviating the need for providing the lightning arrester with series gaps.

[0003] The element is usually baked in the shpae of a disc or a cylinder having a diameter of 1 cm to 15 cm and a thickness of about 0.5 cm to about 6 cm. The elements are connected in series in a number to meet the voltage of the power system in which the arrester is installed.

[0004] Fig. 1 shows a conventional lightning arrester of the type of insulators, in which the elements 1 are accommodated in a porcelain tube 4 which has flanges 2, 3 in the upper and lower directions and which contains the dry air or nitrogen gas 5.

[0005] Generally, the element 1 exhibits electric characteristics as represented by a curve A in Fig. 2. That is, an electric field of 1000 V/cm to 2000 V/cm is established by the element 1 for a current density of 0.1 mA/cm², and an electric field which is greater by about 1.5 to 2 times than the above-mentioned electric field is established for a current density of 200 A/cm². Here, the dielectric strength of the side surface of the element 1 is roughly balanced relative to the aerial dielectric strength of the outer surface of the porcelain tube 4 when heavy currents such as lightning currents flow.

[0006] Fig. 3 illustrates a conventional lightning arrester of the type which is hermetically sealed in a metal enclosure, employing the elements 1 which have well-balanced aerial insulation and which are surrounded by a gas 6 at atmosphere presiure such as SF₆ gas having a high dielectric strength. An insulation spacer 8 is provided at the upper end of a grounded tank 7 which contains the atmosphere gas 6, and the elements 1 are connected by a lead wire 9. This lightning arrester, which is hermetically sealed in the metal enclosure, is used for miniature substations. Therefore, the size and especially the overall length must be smaller than that of the insulator-type lightning arrester which is shown in Fig. 1. For this purpose, units each consisting of a stack of several elements 1 are arranged in a zig-zag manner and are connected via the lead wire 9 as shown in Fi^g. 3.

[0007] With the zig-zag arrangement, however, a complicated mechanical construction is required for supporting the element groups and, hence, the internal construction become bulky. Further, the long lead wire 9 results in the increased residual inductance in the lightning arrester.

SUMMARY OF THE INVENTION

[0008] The present invention was accomplished in view of the above-mentioned drawbacks, and has for its object to provide a lightning arrester which features simple construction and small size, by employing elements that provide increased electric field in the electric field vs. current density characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] 

Fig. 1 is a vertical section view of a conventional insulator-type lightning arrester;

_Fig. 2 is a diagram of curves comparing the electric field vs. current density characteristics of the zinc oxide elements;

Fig. 3 is a vertical section view of a conventional lightning arrester hermetically contained in a metal enclosure; and

Fig. 4 is a vertical section view illustrating an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] An embodiment of the present invention will be described below in conjunction with the drawings. A curve B in Fig. 2 represents characteristics of the element which is used in the lightning arrester of the present invention, according to which the electric field of greater than 2000 v/cm is established for a current density of 0.1 mA/cm2. The element which establishes such a high electric field is obtained by changing the blending ratio of the compositions of metal oxides added to the zinc oxide or by changing the process of temperature change in the step of baking. The withstand capacity of the element for absorbing overvoltage remains nearly constant even for those elements for which the electric fields are set to be high, if reckoned per unit volume. Therefore, if the energy treated by the lightning arrester remains constant, the elements used for the lightning arrester of the present invention must have a large diameter.

[0011] Fig. 4 illustrates the lightning arrester of the type which is hermetically contained in the metal enclosure and which employs the above-mentioned elements, in which reference numerals 6, 7, 8 and 9 denote the same members as those of Fig. 3 and are not repeatedly described here. In Fig. 4, reference numeral 10 denotes the elements which establish an increased electric field as described above, and which are stacked in series. The elements are stacked in series because of the reason that the whole length of the stack of elements 10 which establish increased electric field is shorter than the length of the corresponding conventional elements 1. If the grounded tank 7 has a predetermined shape, the construction can be simplified as shown in Fig. 4. By arranging the elements in a zig-zag manner as shown in Fig. 3, furthermore, the lightning arrester can be realized in a further reduced size.

[0012] According to another embodiment, the invention can also be adapted to the lightning arrester of the insulator type shown in Fig. 1. In this case, the dielectric strength of the side surfaces of elements may decrease with the decrease in the length of the elements. The decrease of dielectric strength, however, can'be compensated by using an SF₆ gas having a high dielectric strength or an insulation oil in the porcelain tube. By adapting the invention to the lightning arrester of the insulator type as mentioned above, it is possible to reduce the height of lightning arresters which are installed in areas where they are not contaminated heavily and where the level of dielectric strength on the outer side of the porcelain tubes can be reduced.

[0013] According to the present invention as described above, zinc oxide elements which establish or increased electric field in the electric field vs. current density characteristics, are used. Therefore, a stack of the elements can be supported by a simplified mechanical construction, the residual inductance of the lightning arrester can be reduced, and the lightning arrester can be realized in a reduced size.

Claims

1. In a lightning arrester in which characteristic elements are stacked in a plurality of numbers in series, each of said characteristic elements being chiefly composed of zinc oxide and having non-linear electric resistance characteristics relative to the voltage applied thereto, the improvement wherein said characteristic elements establish an electric field of greater than 2000 V/cm for a current density of 0.1 mA/cm².

2. A lightning arrester according to claim 1, wherein the characteristic elements are arranged in an insulation medium which has a high dielectric strength.

Drawing