GB2258352A

GB2258352A - Overvoltage arrester

Info

Publication number: GB2258352A
Application number: GB9216184A
Authority: GB
Inventors: Tetsuya Nakayama; Daisaku Goto; Shigeo Ishino; Yuji Kudo
Original assignee: NGK Insulators Ltd
Current assignee: NGK Insulators Ltd
Priority date: 1991-08-02
Filing date: 1992-07-30
Publication date: 1993-02-03
Anticipated expiration: 2012-07-30
Also published as: FR2680054A1; FR2680054B1; JP2837978B2; JPH0541125A; GB9216184D0; GB2258352B

Abstract

The arrester includes conductive end caps 9, 10 having caulking portions 11, 12 which are pressed, (Figs 3, 6) against end portions of an inner tube 1 and an outer bushing 2 to secure the end caps. The outer bushing 2 of silicone rubber or EPDM polymer is first moulded onto an inner tube 1 of fibreglass reinforced plastics. The lower end cap 12 is then pressed on to tube 1 and bushing 2 at lower ends thereof wherein a metal spacer 8 is positioned. Springs 5, 6 and varistors 4 are assembled into tube 1, silicone rubber filler 14 is injected into the peripheral space between varistors 4 and tube 1, a further metal spacer 7 is positioned in the upper end of tube 1, and then the upper end cap 9 is pressed onto tube 1 and bushing 2. Adhesive 13 is applied between end caps 9, 10 and outer surfaces of tube 1 and bushings 2 prior to the pressing steps. The spacers 7, 8 may be integral with the end caps 9, 10 respectively, (Fig 7). <IMAGE>

Description

ARRESTER AND MANUFACTURING METHOD THEREOF The present invention generally relates to an arrester and a method for manufacturing the same. More particularly, the present invention relates to an arrester for power transmission lines, which discharges a surge current originating from lightning, and which limits current flow which succeeds the surge current, thereby preventing a ground fault.

A conventional arrester generally includes an insulating inner tube which has a pressure resisting characteristic, a plurality of resistor elements which are housed in series within the inner tube and have non-linear voltage-current characteristics, metal cap electrodes which are attached to the upper and lower ends of the inner tube, respectively, and a rubber insulating outer bushing which is formed around an outer periphery of the inner tube. The applicant of the present invention has already proposed several types of similarly constructed arresters, which are described hereinafter.

The Japanese Unexamined Patent Publication No. 1-163925 discloses an arrester which has cap electrodes, connected to the upper and lower ends of the inner tube by means of adhesive. The arrester includes resistor elements housed in series therein. After the cap electrodes are secured, an outer bushing having a plurality of sheds are integrally molded around the outer periphery of the inner tube.

However, it generally takes a relatively long time to apply the adhesive between both the upper and lower ends of the inner tube, and the upper and lower cap electrodes completely cured, to have sufficient adhesive strength. Consequently, the arrester would require longer manufacturing time and higher cost.

In addition, the coupling surfaces of the inner tube and the cap electrodes require a special treatment to substantially increase the mechanical coupling strength therebetween.

Furthermore, before molding the outer bushing around the inner tube, the inner tube which houses the resistor elements and which includes the cap electrodes, is disposed within a predetermined mold. Then, molten rubber heated at a predetermined temperature is injected into the mold cavity, to integrally form the outer bushing and the inner tube.

Therefore, during the injection molding process, the resistor elements and the cap electrodes as well as the inner tube must be heated to maintain the constant flow property of molten rubber. More energy is needed as number of parts to be heated increases. Thus, longer time is required for molding, which increases the cost for manufacturing the arrester.

The Japanese Unexamined Patent Publications Nos. 61-151913 and 61-200620 disclose arresters having cap electrodes which are securely attached to the tube, by screwing the upper and lower external threads formed at the upper and lower end portions of the tube, with the upper and lower internal threads formed at the upper and lower cap electrodes, respectively. After the upper and lower cap electrodes are secured to the tube, a rubber bushing is integrally molded around the outer periphery of the tube by means of rubber molding process.

According to this second arrester type , it is required to form the external and internal threads on the inner tube and the cap electrodes, respectively. Such a screw forming step causes rather complicate manufacturing processes.

Furthermore, adhesive is required to be injected into clearances between the tube and the upper and lower cap electrodes, in order to prevent the cap electrodes from swaying.

The Japanese Unexamined Patent Publication No. 3-257721 discloses an arrester which includes springs disposed between the resistor elements housed within an inner tube, an upper cap electrode, a lower cap electrode, and a plurality of elongated insulating support members provided around the outer periphery of the inner tube at equal arcuate intervals. Each of support members has holes for inserting bolts, and is positioned at the upper and lower end portions.

Upon assembling this arrester, the upper and lower cap electrodes are thrust in reverse directions to approach each other, by means of upper and lower thrusting rods, respectively. While both the cap electrodes are being thrust, bolts are screwed into the corresponding threaded holes formed in the outer wall of the cap electrodes, via the support member. As a result, the cap electrodes and the support members are attached to the inner tube. After the abovedescribed processes are completed, an outer bushing is molded outside of the support members, in a similar manner to the first conventional type described above.

However, in this third type, a relatively large number of parts are required, and its assembly becomes rather complicated. In addition, all the foregoing arresters require significant energy expenditure for molding the bushing, since the steps of assembling the resistor elements and the cap electrodes proceed the molding step.

Accordingly, it is a primary object of the present invention to provide an economical arrester, which can reduce the number of parts required to simplify the manufacture process.

It is another object of the present invention to provide a manufacturing method of the arrester, which can reduce the energy required for molding the arrester, thereby reducing the manufacturing cost.

The arrester and its manufacturing method of the invention are as follows.

The arrester includes an insulating inner tube, an outer bushing, a plurality of resistor elements, a conductive spacer and a conductive cap. The inner tube has an opening in at least one of an upper and lower end portions thereof. The outer bushing is provided around the inner tube and has an insulating characteristic. Each of the resistor elements is housed within the inner tube, and has an non-linear voltagecurrent characteristic. The conductive spacer is fitted into the above-described opening for sealing it. When the conductive cap electrode is attached to an opening-side end portion of the inner tube, the cap electrode is fitted over the outer periphery of the end portion. Further, the cap electrode has a caulking portion which is caulked against the outer periphery of the end portion of the inner tube.By caulking it is meant fitting the cap electrode over the outer periphery, and deforming it by pressing if to prevent it from becoming disconnected.

The method for manufacturing the arrester includes the following steps: A. molding the outer bushing around the inner tube; B. fitting the spacer into the opening at the end portion of the inner tube produced by step (A); C. attaching the conductive cap at the opening-side end portion of the inner tube produced by step (B); D. upon completion of step (C), caulking the caulking portion of the cap electrode against the inner tube, for securing the spacer and the cap electrode to the inner tube; and E. housing the resistor elements within the inner tube at any stage following step (A).

The invention, and preferred objects and advantages thereof, may best be understood by reference to the following description of the certain exemplifying embodiments together with the accompanying drawings in which: Figs. 1 through 6 illustrate one embodiment of an arrester according to the present invention, wherein: Fig. 1 is a partially cut away cross sectional view of the arrester; Fig. 2 is a cross sectional view illustrating the inner tube and the outer bushing of the arrester of Fig. 1; Fig. 3 is a cross sectional view illustrating the caulking process of the lower cap; Fig. 4 is a cross sectional view taken along line A-A in Fig. 3; Fig. 5 is a cross sectional view illustrating the process of injecting the insulating filler into a clearance between the inner surface of the inner tube and the outer periphery of the resister elements; and Fig. 6 is a sectional view illustrating the caulking process of the upper; and Fig. 7 is a sectional view of another modification of the according to the present invention.

The preferred embodiment of the present invention will now be described with reference to Figures 1 through 6.

As shown in Fig. 1, an arrester according to the present invention includes an insulating inner tube 1 and an insulating outer bushing 2. The inner tube 1 is preferably made of fiberglass reinforced plastics (FRP), and has pressure resistance characteristic. Further, the inner tube 1 has a plurality of pressure release holes 15 which are formed in a cylindrical wall of the inner tube 1, for releasing internal pressure generated within the inner tube 1 to the outside of the inner tube 1. The outer bushing 2 has a plurality of skirts or sheds 3 that are integrally formed on the outer periphery. The body of the outer bushing 2, excluding both the upper and lower end portions, is made of polymer, such as ethylene propylene diene terpolymer (EPDM) or silicone rubber.

A plurality of resistor elements 4 are made of zinc oxide or similar material, have non-linear voltage-current characteristics, and are stacked and housed inside the inner tube 1. An upper spacer 7 and a lower spacer 8 are inserted into the upper and lower ends of the inner tube 1, respectively. Both spacers 7 and 8 are made of metal, such as aluminum, copper or similar alloy, and have a cylindrical shape having an outer diameter substantially equal to the inner diameter of the inner tube 1. The resistor elements 4 are securely supported by the upper and lower spacers 7 and 8, inside the inner tube 1, via an upper and lower disc springs 5 and 6, respectively.

A metal upper cap electrode 9 and a metal lower cap electrode 10 are fitted over both the upper and lower ends of the inner tube 1, and over the outer bushing 2. Each one of the cap electrodes 9 and 10 has an upper and lower caulking portions 11 and 12, respectively. Further, the caulking portions 11 and 12 have first inner peripheral surfaces 11a and 12a, and second inner peripheral surfaces 11b and 12b, respectively.

Each one of the second inner peripheral surfaces lib and 12b has a larger inner diameter than that of the first inner peripheral surface ila and 12a, respectively.

When the upper and lower caulking portions 11 and 12 are caulked against the upper and lower spacers 7 and 8, the first inner peripheral surfaces 11a and 12a are pressed against the outer periphery of the inner tube 1. Similarly, the second inner peripheral surfaces 11b and 12b are pressed against the outer periphery of the outer bushing 2. Consequently, the upper and lower cap electrodes 9 and 10, and the upper and lower spacers 7 and 8 are secured to the upper and lower ends of the inner tube 1, respectively.

Adhesive 13 is applied between the outer periphery of the inner tube 1 and the first inner peripheral surfaces ila and 12a of the cap electrodes 9 and 10, and between the outer periphery of the outer bushing 2 and the second inner peripheral surfaces lib and 12b. Furthermore, an insulating filler 14, such as silicone rubber, is injected into the clearance between the inner periphery of the inner tube 1 and each outer periphery of the resistor elements 14, to provide a more sufficient insulation therebetween as it will described later in greater detail.

In the event the resistor elements 4 are damaged by overconduction caused by an excessive current surge, similar to that generated by a lightning strike, gases may be generated by the inner arc sparking within the inner tube 1.

These gases are at significant high temperature and pressure.

The gases then burst through the outer bushing 2, and are discharged through pressure release holes 15 to the outside.

Therefore, damage to the inner tube 1 can be avoided.

Each of the upper and lower cap electrodes 9 and 10 has a linking portion 17 with a tapped hole disposed therein. In order to install the arrester on a supporting arm of a high transmission tower, an external thread, which is formed on a hanger (not illustrated) on the supporting arm, is screwed into the tapped hole 16 of the upper cap electrode 9. The linking portion 17 of the lower cap electrode 10 is used in order to support a discharge electrode (not illustrated).

The manufacturing method of the foregoing arrester will now be described with reference to Figs. 2 through 6.

The inner tube 1 has a generally cylindrical form, and is housed into a mold (not shown) for molding an outer bushing 2 around the outer periphery of the inner tube 1. Molten rubber is injected into the cavity of the mold, and is cured to form the outer bushing 2. The outer bushing 2 is therefore adhered to the outer periphery of the inner tube 1 by a vulcanization process.

Next, a lower cap electrode 10 is fitted to the lower end of the inner tube 1 and is supported by a stand holder 21, as shown in Fig. 3. Eight caulker cylinders 23 (Fig. 4) are radially provided at equal interval, around the lower cap electrode 10. The arrester is supported by a stand holder 21, as shown in Figs. 3 and 4. Each one of the cylinders 23 has a thrust plate 25 which is disposed at the distal end of a corresponding piston rod 24.

A lower spacer 8 is fitted into the inner tube 1, while the top surface of the lower spacer 8 is pressed or thrust by means of a first thrust rod 22, as shown in Fig. 3. The outer periphery of the lower caulking portion 12 of the lower cap electrode 10, is thrust toward the center of the lower spacer 8, by means of piston plates 25, as shown in Fig. 4.

The lower cap electrode 10 and the lower spacer 8 are secured to the inner tube 1 and the outer bushing 2. Prior to the pressing the lower cap electrode, adhesive 13 is applied between the inner tube 1 and the lower cap electrode 10.

As shown in Fig. 5, the lower disc springs 6, the resistor elements 4 and the upper disc springs 5 are accommodated inside the inner tube 1. The top surface of the upmost disc spring 5 is pressed downward by a filler injector 26. The filler injector 26 includes an injection passage 27 and an air vent passage 28. An insulating filler 14, such as silicone resin, is injected through the injection passage 27 into the clearance between the inner periphery of the inner tube 1 and the outer peripheries of the resistor elements 4. Air is then discharged from the inner tube 1 to the outside, through the air vent passage 28, so that the silicone rubber is uniformly filled throughout the clearance. After the filling is completed, the filler 14 starts curing.

The filler injector 26 is then removed from the inner tube 1.

As shown in Fig. 6, the upper spacer 7 is inserted into the upper end of the inner tube 1. The upper cap electrode 9 is fitted over the upper end. The top surface of the upper cap electrode 9 is thrust downward by means of a second thrust rod 29.

The upper caulking portion 11 is caulked by means of thrust plates 32 which are disposed at the distal ends of corresponding piston rods 31 of caulker cylinders 30, in a similar manner to that described above in relation to the lower cap electrode 10. As a result, the upper cap electrode 9 and the upper spacer 7 are secured to the inner tube 1 and the outer bushing 2. Prior to thrusting the upper cap electrode 9, adhesive 13 is applied between the inner tube 1 and the upper cap electrode 9.

The arrester constructed in above manner is employed for suspending a power transmission line. A ground discharge electrode (not illustrated) is connected to the lower cap electrode 10. An aerial discharge gap is formed between the ground discharge electrode and a line discharge electrode (not illustrated) disposed on the power line.

A currant surge in the power line generates flashovers in the aerial discharge gap from the line to the ground discharge electrodes. The current surge flows through the lower cap electrode 10, the lower spacer 8, the lower disc springs 6, the resistor elements 4, the upper disc springs 5, the upper spacer 7, the upper cap electrode 9 and the support arm of the tower, and then discharges into the ground. The follow current, which follows the current surge in the power line, is limited by the substantial function of the aerial discharge gap and the restored resistance of the resistor elements 4, thus preventing a ground fault in the power transmission system.

In this embodiment, the upper and lower cap electrodes 9 and 10 are caulked against the inner tube 1 and the outer bushing 2. Therefore, the manufacturing and installation of the arrester are completed in a faster and simpler manner than conventional arresters. One such conventional arresters has a screw and a threaded hole formed in the inner tube and the cap electrode. Another conventional arrester has its inner tube connected to the cap electrode by means of adhesive. As a result, the manufacturing cost of the present arrester is significantly reduced.

Furthermore, according to this embodiment, the outer bushing 2 is molded integrally with the inner tube 1, prior to assembling the resistor elements 4, the spacers 7 and 8, and the cap electrodes 9 and 10 to the inner tube 1. Therefore, the energy required for pre-heating during the molding process is significantly reduced. Consequently, the time required for molding is shortened and the manufacturing cost is reduced.

Although only one embodiment of the present invention has been described herein, it should be apparent to those skilled in the art that the present invention may be embodied in many other forms. Particularly, it should be understood that a spacer 7 (or 8) may be formed integrally with the cap electrode 9 (or 10), as shown in Fig. 7. An advantage for employing the cap electrodes 9 (or 10) as shown in Fig. 7, is that the number of parts required in an arrester and the number of assembling steps are reduced further.

Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein.

Claims

1. An arrester including an open ended insulating tube having upper and lower end portions, an insulating bushing disposed around the tube and a plurality of resistor elements having non-liner voltage current characteristics and housed within the tube, there being at at least one of said upper and lower end portions of the tube a conductive cap which covers the end portion and a conductive spacer within the end portion, the conductive cap being caulked against the outer periphery of the end portion of the tube.

2. The arrester according to claim 1 wherein said conductive cap has a caulking portion including a first inner peripheral surface which is pressed against said outer periphery of the end portion.

3. The arrester according to claim 2 further including adhesive material applied between said first inner peripheral surface and said outer periphery of the end portion.

4. The arrester according to any one of claims 1 to 3 wherein said bushing includes an end portion inserted between the tube and the conductive cap, the cap having an inner peripheral surface which is pressed against the outer peripheral surface of the bushing.

5. The arrester according to claim 4 further including adhesive material applied between the inner peripheral surface of the cap and the outer peripheral surface of the bushing.

6. The arrester according to any one of claims 1 to 5 wherein said conductive spacer is integrally formed with said conductive cap.

7. The arrester according to any one of claims 1 to 6, wherein said tube includes at least one pressure release hole for releasing high pressure gas generated within said tube.

8. The arrester according to any one of claims 1 to 7 further comprising an insulating filler filled inside a cavity formed between said tube and said resistor elements.

9. The arrester according to any one of claims 1 to 8 having at each said end portion of said tube, a said conductive spacer fitted into the end portion and a said conductive cap secured to the respective end portion.

10. A method of manufacturing an arrester including an open ended tube having an upper and lower end portions, a bushing molded around the tube, a plurality of resistor elements, a conductive spacer fitted into at least one of the upper and lower end portions, and a conductive cap secured to at least one of the upper and lower end portions, the method including the steps of: A. fitting the spacer into one of said end portions; B. mounting the conductive cap to the tube, so that a peripheral portion of the cap covers the outer periphery of the end portion; C. pressing said peripheral portion of the cap against the tube to secure the spacer and the cap thereto; and D. housing the plurality of resistor elements within the tube.

11. The method according to claim 10 further comprising the step of: applying adhesive to the outer periphery of the end portion of the tube before the cap is secured to it.

12. The method according to claim 10 or claim 11 further including the steps of: fitting a further conductive spacer into the other of the end portions; and fitting a further conductive cap to the other of the end portions.

13. The method according to any one of claims 10 to 12 wherein said conductive spacer is integral with said conductive cap.

14. An arrestor substantially as herein described with reference to and as shown in Figs. 1 to 6 or modified as shown in Fig. 7 of the accompanying drawings.

15. A method of manufacturing an arrester substantially as herein described with reference to the accompanying drawings.