Abstract:
An insulator for use in a suspension-type line arrester that supports a transmission line is disclosed. Such line arresters have a plurality of insulators connected in a string. The insulators have linking members configured to prevent relative rotation between adjacent insulators in the line arrester&#39;s insulator string.

Description:
This application claims the priority of Japanese Patent Application No. 2-318342 filed on Nov. 21, 1990 which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a suspension-type line arrester. More particularly, this invention pertains to a suspension-type line arrester which grounds the surge current generated by lightning strike in transmission lines. The line arrester also promptly cuts off the follow current to prevent ground faults. 
     2. Description of the Related Art 
     Generally, surge absorbing devices are provided on transmission lines to absorb any switching surge currents generated by opening or closing of the circuit breaker and any lightning surge currents generated by a lightning strike. Surge absorbing devices prevent ground faults. Conventional arresters do not make suitable surge absorbing devices for transmission lines, because their structure have limited endurance for the line load. 
     If a conventional arrester is provided on transmission lines, it influences the complex design of the transmission tower and the insulating devices which support the transmission lines. Therefore, a suspension-type line arrester has been proposed for arresting, insulating and supporting transmission lines. 
     The body of the proposed line arrester has an integrally formed head and shed structure. The shed is formed about the head. A metal pin within the head. A metal cap is installed at the top of the head. The pin and the cap are aligned along the center axis of the insulator body. They are insulated from each other by the insulator body. Bore holes are constructed within the shed. Several resistors having nonlinear varistor voltage-current characteristics are accommodated in each bore hole. 
     A conductive seal is provided at each vertical end of the bore hole. The resistors are supported within each bore hole by the top and bottom seals. Each top seal is connected to the cap by a connector, and each bottom seal is connected to the pin by a wire. 
     The pin is a ball type pin. Therefore, the pin base has a larger diameter than that of the pin body. The cap is a socket type cap with an opening formed on the top portion of the cap. The opening opens to the side and therefore, the pin base of the adjoining insulator can be horizontally inserted into the opening. 
     Once the pin base of a first insulator is engaged with the opening of the cap of a second insulator, the two insulators can not be vertically separated from each other. An insulator string is constructed by linking several insulators. The transmission lines are suspended from the tower by the insulator strings. 
     In the event of a lightning strike, a large surge current flows from a tower to the transmission lines or from a transmission line to a tower by way of insulator strings. In each insulator, the surge current flows through the route sequentially set by a pin, a wire, a bottom seal, resistors, a top seal, a connector and a cap. 
     Conventional insulators can rotate relative to their adjacent (upper and lower) insulators around the center axis of the insulator string. Accordingly, even though each insulator is mounted so that resistors of the vertically linked insulators are aligned in a vertical direction, swaying in the transmission lines can cause misalignments between the resistors of the upper insulator and the resistors of the lower insulator. 
     When the energy of the surge is over the capacity of the resistor, the resistors break down due to the excess surge current and become conductive. Then, an arc discharge will occur directly between the resistors of the upper insulator and the resistors of the lower insulator. If the resistors are misaligned, the course of the arc discharges are in a zigzag manner along the insulator string. Such an arc discharge pattern damages the surface of the sheds of the insulators. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a primary objective of the present invention to provide a suspension-type line arrester which can be prevented from rotating relatively to an adjoining insulator to maintain the alignment of the resistors. 
     To achieve the foregoing and other objects and in accordance with the purpose of the present invention, an improved insulator is provided for use in a suspension-type line arrester that supports a transmission line. Such line arresters have a plurality of insulators connected in a string. The insulator includes a body, a resistor and a connecting apparatus for connecting the insulators with each other. The body has a head and a shed formed integrally with the head. The shed has at least one bore hole in which resistors are accommodated. 
     The connecting apparatus has a first linking member provided substantially at the center of the shed and projecting downwards and a second linking member provided at the top of the head. When a plurality of insulators are connected in a string, the first link in of a first one of the connected insulators is arranged to engage the second linking member of an adjacent (second) one of said connected insulators. The linking members are designed to prevent relative rotation between the first and second insulators. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention, together with the objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
     FIG. 1 is a longitudinal half cross sectional view of a suspension-type line arrester formed in accordance with the present invention. 
     FIG. 2 is a cross sectional view taken along 2--2 line in FIG. 1. 
     FIG. 3 is an enlarged side view of a metal pin. 
     FIG. 4 is a bottom view of the insulator shown in FIG. 1. 
     FIG. 5 shows an insulator string in use. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiment of the present invention will now be described referring to the accompanying drawings. 
     As shown in FIG. 1, a metal cap 2 is securely fixed on a head 1a of an insulator body 1 by cement 3. A pin head 4d is fixed within the head la of the body 1 by cement 5. The pin head 4d and the pin base 4a are larger in diameter than the pin body. 
     As shown in Figs. 1 and 2, the upper section of the cap 2 has an opening 2a, which receives a pin base 4a of an upper adjoining insulator. For that purpose, the opening 2a has an enlarged recess 2b, which horizontally receives the pin base 4a. The opening 2a has a mouth 2c in the upper portion of the opening 2a, which the body of the pin 4 can pass through. The width of the pin mouth 2c is set narrower than that of the enlarged recess 2b. 
     As shown in FIGS. 2 and 3, an arm 4b is formed integrally with 11, the pin 4. The arm 4b of the pin 4 engages the mouth 2c to prevent relative rotation between the cap 2 and the pin 4. The head segment of the arm 4b has a hole 4c. 
     As shown in FIG. 1, the insulator body 1 has a shed 1b. A pair of bore holes 1c are formed in the shed 1b. Bore holes 1c angularly spaced by 180 degrees. The opening direction of the enlarged recess 2b is perpendicular to the line that connects the bore holes 1c. 
     Several resistors 6 are accommodated in each bore hole 1c. The resistors 6 are made of sintered material consisting essentially of zinc oxide and other metal oxides. The resistors 6 have non-linear varistor voltage-current characteristics. 
     An upper inner seal 7 and a lower inner seal 8 are respectively provided on the upper and lower sections of each bore hole 1c. The resistors 6 are sealed in each bore hole 1c by the upper inner seal 7 and the lower inner seal 8. An upper outer seal 9 is attached at the upper portion of each bore hole 1c. A lower outer seal 10 is attached at the lower portion of each bore hole 1c. Upper inner seal 7, lower inner seal 8, upper outer seal 9 and lower outer seal 10 have a function of being electrodes. 
     A conductive coupler 11 bridges the upper inner seal 7 and the upper outer seal 9, and also a conductive coupler 12 bridges the lower inner seal 8 and the lower outer seal 10. Each conductive coupler 11,12 electrically couples its corresponding inner and outer seal portions. 
     The upper outer seal 9 is connected to the cap 2 by a connector 13. One end of the connector 13 is fastened to the upper outer seal 9 by a coupling pin 14, and the opposite end of the connector 13 is fastened to the cap 2 by a screw 15. 
     A wire 16 couples an underside of the lower outer seal 10 and the pin 4. Both ends of the wire 16 have a connector 17,18 clamped thereon. The connector 17 is fixed to the lower outer seal 10 by a coupling pin 19, to connect the wire 16 to the lower outer seal 10. The second end of wire 16 is secured to the pin 4 by a screw 20, which also screws down the connector 18 through the hole 4c of the arm 4b. 
     Arc guides 21 are fastened to the cap 2 such that they are parallel with the upper outer seals 9. In the event that an arc discharge occurs through a resistors 6, the direction of arc discharge is altered outward by the corresponding arc guide 21. 
     As shown in FIG. 5, an insulator string consisting of linked insulators is suspended vertically from the tower arm 22. For A each adjacent pair of insulators, the pin base 4a of the upper insulator engages the opening 2a of the cap 2 of the lower insulator. The insulators are arranged such that their resistors 6 are vertically aligned. 
     A transmission line 25 is suspended from a tower arm 22 by the insulator string. Arcing horns 23,24 are connected at the upper and lower end of an insulator string. The arcing horns 23,24 prevent an arc from discharging along the side surface of the insulator string. 
     The operation of the line arrester will be described below. For the purposes of this description, it will be assumed that the line arrester is connected as shown in FIG. 5, and a lightning surge in transmission line 25 is received by the most bottom pin 4. 
     The lightning surge passes through the wire 16, the lower outer seal 10, the conductive coupler 11, the lower inner seal 8, the resistors 6, the upper inner seal 7, the conductive coupler 11, the upper outer seal 9, the connector 13 and the cap 2 in the bottom insulator. The lightning surge then passes to the next insulator connected directly above the bottom insulator through the pin 4. By following the same path in each insulator, the surge current is guided to the cap 2 of the upper most insulator. The surge is then grounded through the tower arm 22, the tower and to the ground. After discharging, the resistors 6 cut off the follow current generated in the insulator string to prevent ground faults. At times, it is possible that the insulators will be subjected to an unexpectedly large surge that is greater than the capacity of the insulator. This may be due to a wide variety of factors. For example, the resistors sometimes decay as a result of long term use. The resistors 6 and seals 7,8,9 and 10 are heated up exceedingly by the unsuspected large surge. In a worst case, the seals 7,8,9 and 10 might be blown off due to the high internal pressure of the bore hole 1c generated by over heating. 
     In each of the above cases, the arc passing through each bore hole 1c is directed toward the side of the insulator bodies 1 by the arc guide 21 and is led to a region between the arcing horns 23,24. Because of this operation, the unexpected arcing surge is grounded by passing between arcing horns 23,24 and then to the tower and ground. 
     According to this embodiment, the arm 4b of the pin 4 situated on the upper one of two adjacent insulators engages the pin mouth 2c of the cap 2 situated on the lower insulator. Because the insulators are connected in this manner, the relative rotation of each insulator around the pin 4 is prevented. Therefore, the resistors 6 in adjacent insulators remain aligned at all times. 
     Therefore, even if the resistors of the upper and the lower insulator are broken down by the unexpected large lightning surge, the follow current is quickly transferred to between the arcing horns 23,24. Accordingly, discharging along the side surface of the insulator string is prevented. This prevents the shed 1b from fracturing due to arcing along the edge of the insulator. 
     In conventional line arresters, the metal pin and the lower outer seal are normally connected by a wire. Three different methods for connecting the wire with the pin are well known. In a first method, one end of the wire is directly fastened to the pin body by a screw. A second method contemplates connecting one end of a wire with a projection formed on the pin body. A third method contemplates directly welding one end of a wire to the pin surface. 
     According to this embodiment, a hole 4c is formed on the arm 4b of the pin 4. Thus, the wire 16 can be attached to the arm 4b. Therefore, in contrast to conventional type insulators, the suspension-type line arrester of the present invention does not require the fabrication of a special part to attach the wire 16. Rather, the wire 16 may be easily connected with the arm 4b. Such a simple structure contributes to lower the production cost for the line arrester. 
     Although only 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 specific forms without departing from the spirit or scope of the invention. 
     For instance, the arm 4b of a metal pin 4 may be altered such that a bolt for attaching the wire is built into the pin 4. Alternatively, the hole 4c of the arm 4b can be altered such that it forms a hole in which the distal end of the wire 16 may be clamped. 
     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, but may be modified within the scope of the appended claims.