Lightning arresting insulator

Abstract: A lightening arresting insulator with a pressure-release mechanism for releasing pressure when a lightening surge flows in a power transmission line is disclosed. The arresting insulator has a plurality of slits formed in the outer wall of an insulating cylinder formed from a material having a high voltage resistance. The slits extend lengthwise to form thin wall portions in the insulating cylinder. An elastic insulator is filled in each of the slits.

Description: The present invention relates to a lightning arresting insulator, and more specifically, to a lightning arresting insulator which discharges a lightning induced surge currents in power transmission lines to the ground. The insulator suppresses or cuts off the follow current of the surge to prevent ground faults. Conventional lightning arresting insulators of this type are disclosed in Japanese Patent Publication No. 61-151913 (see Fig. 5) and in Japanese Utility Model Publication No. 1-96739 (see Fig. 6). The lightning arresting insulator 60 (Fig. 5) disclosed in Japanese Patent Publication No. 61-151913 is designed so that electrodes 51 and 52 for the ground side and applied voltage side are securely coupled to the respective ends of an insulating cylinder 50. Current-limiting varistors 53 with nonlinear voltage-current characteristics are retained in the insulating cylinder 50. The electrodes 51 and 52 are electrically connected at their ends to the respective ends of the current-limiting varistors 53. A housing 55 having a plurality of sheds 54 is formed on the outer wall of the insulating cylinder 50. A plurality of pressure-release holes 56 are made in the inner surface of the insulating cylinder 50. In forming the housing 55, an insulating material is injected through the pressure-release holes 56 to form an inner housing wall 57. In the manufacturing process for the lightening arresting insulator 60, however, pin holes may infrequently be made in the housing 55 and in the insulating materials that fill the pressure-release holes 56. At times, the pin holes will reach the current-limiting elements 53, which permits moisture from the ambient air to contact the varistor material. This deteriorates the dielectric or insulating strength of the lightening arresting insulator. The lightning arresting insulator 110 (Fig.6) disclosed in Japanese Utility Model Publication No. 1-96739 is designed so that electrodes 101 and 102 for the ground side and applied voltage side respectively, are securely coupled to the respective ends of an insulating tube 100 having a plurality of resin sheds. The insulating tube 100 contains a plurality of current-limiting varistors 105 having nonlinear voltage-current characteristics. The electrodes 101 and 102 are electrically connected at their ends to the respective ends of the current-limiting varistor stack 105. The insulating tube 100 has a plurality of recesses 103 formed in the outer wall, thereby providing thin wall portions 104 for releasing pressure. The lightning arresting insulator 110 does not require an insulating cylinder and comprises only the insulating tube 100 which has the thin pressure-releasing wall portions 104 formed on the outer surface. However, extended use of a lightening arresting insulator with this structure under severe dirt conditions will weaken the surface of the insulating tube 100 due to corrosion caused by current leaks and corona discharge. Accordingly, cracks or pin holes will develop, which may extend entirely through the thin wall portions. In such a situation, humidity will enter inside through the cracks or the pin holes, deteriorating the dielectric or insulating strength of this lightening arresting insulator 110. Further, this lightning arresting insulator 110 has the thin wall portions exposed through the outer surface of the insulating tube 100. In the manufacturing process, therefore, workers may accidentally hit the thin wall portions will tools or the like, breaking the insulating tube 100. Or, while the lightening arresting insulator is in use, birds may peck at the thin wall portions, damaging the insulating tube 100. Furthermore, dirt or the like may be deposited in the recesses 103 that are forming the thin wall portions 104, thereby deteriorating the dielectric or insulating strength of the lightening arresting insulator. Document EP 393 854 discloses a lightening arresting insulator corresponding to the introductory position of Claim 1, the insulator being provided with an insulating cylinder and a housing on the outer wall of the cylinder longitudinally extending grooves being featured in the outer surface of the cylinder. It is therefore an object of the present invention to provide an improved airtight lightning arresting insulator which prevents humidity in the air from entering therein. It is another object of the present invention to provide a lightning arresting insulator whose dielectric or insulating strength is prevented from substantially decreasing while in use. It is a still further object of the present invention to provide a lightning arresting insulator whose pressure-releasing portions are not likely to be damaged during manufacturing or usage. According to the present invention, there is provided a lightning arresting insulator as set out in claim 1. The invention, together with the objects and advantages thereof, may be best understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: Fig. 1 is a cross sectional view of a portion of a lightning arresting insulator according to one embodiment of the present invention; Fig. 2 is a partial cross section view showing the whole lightning arresting insulator; Fig. 3 is a partially cutaway perspective view of the insulating body of the lightning arresting insulator; Fig. 4 is a cross sectional view of the lightning arresting insulator taken along the line A-A in Fig. 2; Fig. 5 is a cross sectional view showing a conventional lightning arresting insulator; and Fig. 6 is a partial cross sectional view of another conventional lightning arresting insulator. One preferred embodiment of the present invention will now be described referring to Figs. 1 to 4. As shown in Fig. 2, an insulating cylinder 1 is formed of a durable reinforced resin having excellent weathering resistance. One suitable resin is epoxy resin-impregnated FRP (fiber reinforced plastics). An electrode 2 on the ground side is securely fitted over the top end of the insulating cylinder 1, and a flange 3 is integrally formed on the outer wall of the electrode 2. Through the flange 3, the lightning arresting insulator is attached to an adaptor provided on the cross arm of a transmission tower (not shown). An electrode 4 having a cylindrical bottom on the applied voltage side is securely fitted into the bottom end of the insulating cylinder 1. A bracket 5 is integrally and protrusively formed on the bottom surface. An arcing horn (not shown) on the ground side is attached to the bracket 5 in such a way that the arcing horn faces an arcing horn on the applied voltage side, with a given air gap therebetween. A rim 6 is formed on the inner top surface of the electrode 4. A conductor 7 on the applied voltage side is fitted into the rim 6. Current-limiting varistors 8 essentially made of zinc oxide having a nonlinear voltage-current characteristic are stacked in series on the upper surface of the conductor 7 on the applied voltage side. A conductor 9 on the ground side is provided on the upper surface of the topmost current-limiting varistor 8, and also serves as a spring receiver. A ring 10 is screwed into a threaded portion 11 on the inner wall of the upper end of the electrode 2 on the ground side. A coil spring 14 having a conductive foil 13 is inserted between a conductor 12 fitted into the ring 10 and the conductor 9 on the ground side. The coil spring 14 urges the current-limiting varistors 8 to be securely pressed between the conductors 7 and 9 respectively on the applied voltage side and the ground side. An insulating cap 15, is fitted over the top end of the electrode 2 on the ground side to seal the upper opening of the electrode 2. An insulating layer 16 is formed in the sealed space between the insulating cylinder 1 and the current-limiting varistors 8. According to this embodiment, room-temperature thermosetting silicone rubber is filled in that space to form the insulating layer 16. As shown in Figs. 1 to 4, a plurality of slits 17, (two sets of three slits in this case), are provided on the outer wall of the insulating cylinder 1 such that they extend in the axial direction of the insulating cylinder 1. The reason why the slits are provided at two locations is that this is the minimum number of slits required in view of the operating specifications of the arrestor. Specifically, the accident current and the current flowing time permitted to discharge the voltage. This slit arrangement may differ depending on the above conditions. With the insulating cylinder 1 having the same thickness, as the number of slit locations on the outer surface increases, pressure will be smoothly released at the price of decreased strength of the insulating cylinder 1. If the gaps between the slits 17 are narrowed to increase the number of slits in the axial direction, the insulating cylinder 1 will be easily torn and less durable. In other words, the number of the locations of the slits on the outer wall of the insulating cylinder 1, and the number of the slits aligned in the axial direction depend on the pressure-releasing conditions. It is therefore important not to make the slit gaps too narrow. The slits 17 are thus made to provide thin wall portions 18 for releasing pressure on the surface of the insulating cylinder 1. In this embodiment, an elastic insulator 19 in the form of a room-temperature thermosetting rubber, is filled in each slit 17. A housing 20, formed from a high temperature thermosetting silicone rubber, is coated on the outer surface of the insulating body 1. With the housing 20 made of the high temperature thermosetting silicone rubber, the lightening arresting insulator can keep the water-repellent property while in use outside for a long period of time. Compared with other cover materials, such as of EPDM or resins, the dirt withstanding characteristics of the lightening arresting insulator can be improved without changing the length of the housing 20 (which corresponds to the surface leak distance). The housing 20 has a plurality of sheds 21 formed on the outer wall. The upper and lower ends of the housing 20 extend to the outer surfaces of the electrodes 2 and 4 on the ground side and applied voltage side, respectively, thus ensuring the airtight condition between the housing 20 and the electrodes 2 and 4. The action of the thus structured lightning arresting insulator will now be described. A lightning surge current flows through a power-transmission line with the lightning arresting insulator attached to the cross arm of a transmission tower. The current starts to be discharged (flash-over) from arcing horns (not shown) on the power-transmission line to arcing horns or series gaps (also not shown) attached to the lower end of the lightening arresting insulator. The current runs through the electrode 4, the conductive fixture 7, the current-limiting varistors 8, the conductor 9, the foil 13, the conductor 12, the ring 10 to the electrode 2. The lightning surge current finally runs to the transmission tower through the adaptor (not shown) to be discharged to the ground. A follow current which follows is limited and then interrupted by the series gaps and the current-limiting varistors elements 8. If an unexpectedly large amount of lightning surge current flows through the lightning arresting insulator, the current-limiting elements 8 are rendered conductive, and then a current flows through the lightning arresting insulator in response to a drive voltage, thus generating a high-temperature and high-pressure gas arc inside the lightning arresting insulator. The gas arc is transmitted to the thin wall portions 18 through the insulating layer 16 in the sealed space between the insulating cylinder 1 and the current-limiting elements 8. The gas arc is then rapidly released outside through the thin-wall portions 18 and part of the housing 20. The lightening arresting insulator will not therefore be damaged as a whole. In the lightning arresting insulator according to the embodiment as described above, a plurality of slits 17 are provided in the outer surface of the insulating cylinder 1 to form the pressure-releasing thin wall portions 18. With this structure, unlike conventional structures having pressure-release holes formed in the outer surface of the insulating cylinder, the thin wall portions 18 can suppress water from passing therethrough so that the airtight condition inside the insulating cylinder 1 can be kept. The lightning arresting insulator according to this embodiment has a rubbery elastic insulator 19 in the slits 17 on the outer wall of the insulating cylinder 1. Because of the elastic insulator 19, the thin wall portions 18 are not exposed directly through the outer wall of the insulating cylinder 1. The lightning arresting insulator in this embodiment differs from the conventional from arresting insulators which do not have an insulating cylinder but rather have thin wall portions formed on the outer surface of the insulating tube of a polymer material. More specifically, the thin wall portions 18 of the lightning arresting insulator according to this embodiment will not be hit by tools or the like in the manufacturing process so are unlikely to be damaged or broken by such an accident. Further, it is possible to prevent the lightning arresting insulator from being damaged by birds' pecking. In the lightning arresting insulator according to this embodiment, the slits 17 can be easily formed in the outer wall of the insulating cylinder 1 through post fabrication working. The present invention is not limited to the structure of the above-described embodiment, but for example can be modified and worked as follows. (1) Before the housing 20 is coated on the outer wall of the insulating cylinder 1, the elastic insulator 19 is formed integrally with the slits 17 on the outer wall of the insulating cylinder 1. This can eliminate the need for filling an insulator in each slit. (2) Gas with a high insulating property, such as sulfur hexafluoride (SF6), is sealed into the airtight space between the insulating cylinder 1 and the current-limiting elements 8. Since sulfur hexafluoride (SF6) is highly heat resistant and non-corrosive, it is thermally and chemically stable and will not react on the current-limiting elements in the lightening arresting insulator. (3) The housing 20 is formed of ethylene propylene rubber (EPDM). This housing 20 has excellent water resistance, oxidation resistance, and ozone resistance, as well as good electric characteristics as an insulator. 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: A lightning arresting insulator with a pressure-release mechanism for releasing pressure when a lightning surge flows in a power transmission line, comprising: an insulating cylinder (1) having a plurality of longitudinally extending grooves (17) in its outer surface; non-linear resistors (8) inside said insulating cylinder (1) and electrodes (2,4) at the ends of the insulating cylinder (1) and electrically connected to the non-linear resistors (8); and a housing (20) on the outer wall of the insulating cylinder (1) characterized in that said longitudinally extending grooves (17) are aligned end-to-end and spaced apart longitudinally; and an elastic insulator material (19) is filled in each of the plurality of grooves (17). A lightning arresting insulator according to claim 1, wherein the housing (20) and the elastic insulator material (19) that fills the grooves (17) are integrally formed. A lightning arresting insulator according to claim 1 or claim 2, wherein an insulating gas is sealed in an airtight space between the insulating cylinder (1) and the current-limiting elements (8). A lightning arresting insulator according to claim 3, wherein the insulating gas is sulfur hexafluoride. A lightning arresting insulator according to any one of claims 1 to 4, wherein upper and lower end portions of the housing (20) extend to outer surfaces of the electrodes (2,4) on the ground side and applied voltage side. A lightning arresting insulator according to any one of claims 1 to 5, wherein the housing (20) is made of a high temperature thermosetting rubber. A lightning arresting insulator according to any one of claims 1 to 5, wherein the housing (20) is made of ethylene propylene rubber (EPDM). A lightning arresting insulator according to any one of claims 1 to 7, wherein one of said electrodes (2) is a ground side electrode and includes an integrally formed flange (3) and the other electrode (4) which is on the applied voltage side includes an integrally formed bracket (5); a cylindrical conductor (9) is disposed between a ground side end of the current-limiting element stack (8) and serves as a spring receiver; a coil spring (14) is disposed between the cylindrical conductor and said ground side electrode (2); and the housing (20) is formed of a room-temperature thermosetting silicone rubber.

Inventors: NAKAYAMA TETSUYA, NAKAYAMA, TETSUYA
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1991
Main classification: H01C7
IPC codes: H01T1, H01B17, H01C7