Lightning Arrester

This invention relates to a lightning arrester, and more particularly to a lightning arrester having no series gap and utilizing, as characteristic elements, nonlinear resistance elements containing, as a main component, zinc oxide.

The lightning arrester is known as a protective device for an electric power system, and now a lightning arrester with no gap, or a so-called gapless lightning arrester is widely used. The lightning arrester of this kind, as disclosed, for example, in U.S. Pat. No. 4,262,318, is formed of a plurality of stacked nonlinear sheet resistance elements as its characteristic elements. Thus, for a high-voltage poer system, a large number of stacked nonlinear sheet resistance elements must be used, resulting in a size of great height.

To avoid this, a system is employed, as disclosed in Japanese patent pre-examination publications KOKAI Nos. 91360/78, 115279/80 and 164502/81, in which a plurality of blocks of stacked nonlinear resistance elements are disposed in parallel and the resistance elements are electrically connected in series in spiral shape by jumper conductors.

In this system, the total height of the arrester can be reduced by properly selecting the number of blocks.

On the other hand, in order to permit the electrical connection mentioned above, it is necessary to provide insulating spacers at selected positions in each block. This insulating spacer is made of epoxy resin. Since each insulating spacer has a considerable thickness in the direction in which the elements are stacked, the spacers affect adversely the attempt to reduce the height of the arrester. Thus, it is desired to overcome this problem.

An object of this invention is to provide a lightning arrester of small size capable of absorbing a large amount of energy.

According to this invention, there is provided a lightning arrester in which the insulating spacers used for providing electrical connection between the blocks are formed of nonlinear resistance elements having large thermal conductivity, thermal capacity and dielectric constant. These nonlinear resistance elements are made of a sintered substance containing, as a main component, zinc oxide similar to the characteristic elements.

According to a preferred embodiment of this invention, the voltage-current characteristics of the resistance element used for the insulating spacer and the characteristic element are so selected that the specific resistance of the element of the insulating spacer is larger than that of the characteristic element and the discharge voltage of the former element is higher than that of the latter element. Therefore, the energy due to a switching surge can be absorbed not only by the characteristic elements but also by the elements of the insulating spacers, the lightning arrester being capable of absorbing a large amount of energy.

The invention will be well understood from the following description with reference to the accompanying draiwngs, in which:

FIG. 1 is a development showing an arrangement of a main portion of the characteristic elements of a lightning arrester of the invention;

FIGS. 2 and 3 are equivalent circuit diagrams of the arrangement of FIG. 1; and

FIG. 4 shows voltage-current characteristic curves of two types of nonlinear resistance elements used in the embodiment of FIG. 1.

With reference to FIG. 1, there is shown an arrangement of three column-like blocks or stacks of characteristic elements in a view of development. For convenience of explanation, one block or stack 1 is repeatedly shown on both sides in FIG. 1. The block 1 is formed of stacked groups or resistor units 4a, 4b and 4c of nonlinear resistance elements each made of a sintered substance containing, as a main component, zinc oxide, and spacers 7a and 7b disposed between the adjacent groups or resistor units. Each group of elements is formed of three stacked nonlinear resistance elements.

The blocks 2 and 3 are formed in the same way as the block 1. The lower end of the element group 5a is connected to the upper end of the element group 4a by a jumper conductor 10, and the lower end of the element group 4a to the upper end of the element group 6a by a jumper conductor 11. Moreover, the lower end of the element group 6a is connected to the upper end of the element group 5b by a jumper conductor 12, and the lower end of the element group 5b to the upper end of the element group 4b by a jumper conductor 13. The other jumper conductors 14 to 17 connect other groups similarly.

In this way, the element groups of the blocks are electrically connected in series so as to provide a predetermined resistance characteristic.

The spacers 8a, 8b and 8c of the block 2 and spacers 9a, 9b and 9c of the block 3 are made of the same material as the spacers 7a and 7b of the block 1, to provide nonlinear resistance elements with large thermal conductivity, thermal capacity and dielectric constant preferably in the order of 0.01-0.5 Watt/cm.multidot..degree.C., 1-5 Joul/.degree.C..multidot.cm.sup.3 and 1000-5000, respectively. Such a nonlinear resistance element can be made of a sintered substance containing, as a main component for example, zinc oxide. The nonlinear resistances of the spacers are hereinafter termed added nonlinear resistances.

The difference between the characteristic element and the added nonlinear resistance will be described with respect to the spacer 7a as a typical example. The series connection of element groups 5b and 6a is electrically connected in parallel with the spacer 7a. The thickness of the spacer 7a is smaller than the total thickness of the element groups 5b and 6a. The maximum energy which the spacer 7a can absorb is smaller than the maximum total energy which both the element groups 5b and 6a can absorb. The specific resistance of the spacer 7a is larger than the resultant specific resistance of groups 5b and 6a. The voltage-current characteristics of the spacer and element groups are shown in FIG. 4. The discharge voltage of the spacer 7a as shown by curve Q is so selected as to be about 10% higher than the total discharge voltage of a series circuit of resistor units or element groups 5b and 6a as shown by curve P.

The equivalent circuit of the zinc-oxide type lightning arrester shown in FIG. 2 can be further rewritten, for easy of understanding, into another equivalent circuit in FIG. 3.

From FIG. 3 it will be seen that the equivalent nonlinear resistances R.sub.7a, R.sub.7b, R.sub.8a, R.sub.8b, R.sub.8c, R.sub.9a, R.sub.9b and R.sub.9c of the spacers 7a, 7b, 8a, 8b, 8c, 9a, 9b and 9c, which were not used so far, are added in parallel to the equivalent nonlinear resistances R.sub.4a, R.sub.4b, R.sub.4c, R.sub.5a, R.sub.5b, R.sub.5c, R.sub.6a, R.sub.6b and R.sub.6c of the resistor units or element groups 4a, 4b, 4c, 5a, 5b, 5c, 6a, 6b and 6c. Therefore, this lightning arrester of the same size as that of the conventional one is able to absorb larger energy than the conventional one by an amount absorbed by the added nonlinear resistance thereby to decrease the discharge voltage at a nominal discharge current.

In the normal state in which a rated voltage V.sub.1 is applied, the current i.sub.1Q flowing through the added nonlinear resistance is much smaller than the current i.sub.p flowing through the characteristic element. When a switching surge where a higher voltage V.sub.2 is applied occurs and a large energy must be absorbed, the currents flowing through the added nonlinear resistance and characteristic element are respectively shifted to i.sub.2Q and i.sub.2P. Therefore, this arrester is able to absorb a larger energy than the conventional one by an amount corresponding to the current thereby to decrease the discharge voltage at a nominal discharge current.

When a large energy is absorbed, it is desired, in view of service life and tolerable amount of energy that the ratio between the currents i.sub.2P flowing through the characteristic element and the current i.sub.2Q flowing through the added nonlinear resistance be almost approximately equal to the ratio between their volumes, or the ratio between their thicknesses and that the energy per unit volume absorbed by the characteristic element is the same as that by the added nonlinear resistance.

Also, since the spacers 7a, 7b and so on have large thermal conductivity and thermal capacity as compared with the conventional insulating spacers, the arrester of the invention has, as a whole, large thermal conductivity and thermal capacity resulting in small size. In addition, the spacers have large dielectric constant and hence large capacitance, which is effective to provide uniform potential distribution among the element groups connected in series.

While in the above embodiment three cylindrical blocks are disposed in parallel, this invention can use two, four or more blocks in parallel. Moreover, the nonlinear resistance elements forming spacers are not limited to the above zinc oxide elements, but may be elements of other materials having large thermal conductivity, thermal capacity and dielectric constant.