IMPROVEMENTS IN OVERHEAD ELECTRIC TRANSMISSION LINES

Abstract: An overhead electric transmission line in which each group of at least two mutually spaced insulators or string of insulators (3) supporting a single conductor (2) or the conductors of a group of transversely spaced conductors carries means (5) for dissipating the energy arising from the fracture of an insulator or string of insulators of the group as the resultant additional load is applied to the or each sound insulator or sound string of insulators of the group, which energy dissipating means is of such a form that as the load on the or each sound insulator or sound string of insulators in­ creases to the maximum safe working load, the energy dissipating means is not permanently extensible in a rectilinear direction substantially parallel to the longitu­ dinal axis of the insulator or string of insulators and, as the load on the or each sound insulator or sound string of insulators exceeds the maximum safe working load and increases towards the load at which the in­ sulator or string of insulators will fracture, the energy dissipating means smoothly and continuously extends permanently in said rectilinear direction to reduce sub­ stantially risk of fracture of the insulator or string of insulators.

Description: IMPROVEMEiNTS IN OVERHEAD ELECTRIC TRANSMISSION LINES This invention relates to overhead electric transmission lines of the kind in which an electric conductor or a plurality of electric conductors is or are suspended in long lengths between pylons or other upstanding supporting structures at spaced positions along the length of the line, all such supporting structures hereinafter, for convenience, being included in the generic term "pylon". The invention is particularly concerned with overhead electric transmission lines of this kind in which single electric conductors or separate groups of electric conductors are suspended from a transversely extending crossarm or transversely extending crossarms of each of a number of spaced pylons by insulators or strings of insulators, each single conductor being suspended from a crossarm of each pylon by at least two mutually spaced insulators or strings of insulators or the conductors of each group being transversely spaced apart and each conductor of the group being suspended from a crossarm of each pylon by a separate insulator or by a separate string of insulators, in both cases the insulators or strings of insulators being interconnected at their lower ends by at least one link. An overhead electric transmission line of this particular kind will hereinafter be referred to as "of the kind described". Usually, but not necessarily, each conductor or each separate group of conductors is supported by two mutually spaced insulators or strings of insulators and, where the context permits, the invention will be described with respect to overhead electric transmission lines of the kind described in which each conductor or each group of conductors is supported by two mutually spaced insulators or strings of insulators, but it is to be clearly understood that the invention is also applicable to the case where each conductor or each group of conductors of an overhead line is supported by three or more than three mutually spaced insulators or strings of insulators. It has been found that where an insulator or a string of insulators of two mutually spaced insulators or strings of insulators supporting a conductor or a group of transversely spaced conductors of an overhead electric transmission line of the kind described is damaged to such an extent that the insulator or string of insulators is fractured, the sound insulator or sound string of insulators is subjected to stresses caused by application to the sound insulator or sound string of insulators of the load hitherto supported by the fractured insulator or fractured string of insulators. There is a substantial risk that, as a result of these stresses, the sound insulator or sound string of insulators will also fracture and cause complete breakdown of the line and possible consequential damage to one or more of the pylons. There is an additional risk that, due to transitory effects caused in the parts of the fractured insulator or fractured string of insulators and in the sound insulator or sound string of insulators due to fracture of the insulator or string of insulators, collision of the insulators or strings of insulators may occur, damage to the hitherto sound insulator or sound string of insulators may result, and consequential damage to one or more pylons may follow. Several proposals have been made for reducing the risk of fracture of a sound insulator or sound string of insulators of a group of at least two mutually spaced insulators or strings of insulators arising from fracture of the other or another insulator or string of insulators of the group but, for various reasons, none of these proposals has proved satisfactory in a sufficiently wide range of conditions likely to be met in service and the problem of protecting a sound insulator or sound string of insulators of such a group from stresses arising from fracture of the other or another insulator or strings of insulators of the group is still therefore not completely resolved. It is an object of the present invention to provide an improved solution to this problem. According to the invention, in an overhead electric transmission line of the kind described, at at least one and preferably each pylon, each group of at least two mutually spaced insulators or strings of insulators supporting a single conductor or the conductors of a group of transversely spaced conductors carries means for dissipating the energy arising from fracture of an insulator or string of insulators of the group as the resultant additional load is applied to the or each sound insulator or sound string of insulators of the group, which energy dissipating means is of such a form that, as the load on the or each sound insulator or sound string of insulators increases to the maximum safe working load, the energy dissipating means is not permanently extensible in a rectilinear direction substantially parallel to the longitudinal axis of the insulator or string of insulators and, as the load on the or each sound insulator or sound string of insulators exceeds the maximum safe working load and increases towards the load at which the insulator or string of insulators will fracture, the energy dissipating means smoothly and continuously extends permanently in said rectilinear direction to reduce substantially risk of fracture of the insulator or string of insulators. Preferably, the energy dissipating means is of such a form that, as the load on the or each sound insulator or sound string of insulators increases to the maximum safe working load, the energy dissipating means is elastically extensible in said rectilinear direction Where the energy dissipating means is initially elastically extensible, the elastic extensibility of the energy dissipating means may be at least partially damped, the effectiveness of said damping ceasing to have effect as the value of the load on the or each sound insulator or sound string of insulators reaches or just exceeds the maximum safe working load. Where each insulator or each string of insulators of a group of at least two mutually spaced insulators or strings of insulators is separately suspended from a crossarm of a pylon, an energy dissipating means may be interconnected between said insulator or string of insulators and its point of suspension. Where the insulators or strings of insulators of a group of at least two mutually spaced insulators or strings of insulators are interconnected at their upper ends by at least one link and the link or links is or are suspended from a crossarm of a pylon at a single position, said energy dissipating means may be connected between the link or links and the single point of suspension and/or between the link or links and each insulator or string of insulators of the group. Energy dissipating means may additionally or alternatively be interconnected between the lower end of each insulator or string of insulators of a group of at least two mutually spaced insulators or strings of insulators and the link or links interconnecting the lower ends of the insulators or strings of insulators and/or between the lower end of the link or links interconnecting the lower ends of the insulators or strings of insulators and the or each conductor-supporting clamp. Preferably, the energy dissipating means comprises one or more than one substantially rigid elongate member of metal or metal alloy which is of such a transverse cross section and is so shaped between its ends that, as the load on the or each sound insulator or sound string of insulators increases to the maximum safe working load, the or each elongate member is substantially resilient and, as the load on the or each sound insulator or sound strings of insulators exceeds the maximum safe working load and increases towards the load at which the insulator or string of insulators will fracture, the or each elongate member will extend smoothly and continuously in a rectilinear direction substantially parallel to the longitudinal axis of the insulator or string of insulators. Where the energy dissipating means is a single substantially rigid elongate member, the member is preferably symmetrical about its longitudinal axis and is preferably formed between its ends into two loops lying in a common plane and on directly opposite sides of said longitudinal axis. Damping of the resilience of the substantially rigid member may be effected by forming the member into a shape approximating to that of a figure eight and clamping the adjacent central parts of the member together by a bolt or other means designed to fracture at a load equal to or slightly greater than the maximum safe working load of the insulator or string of insulators. Where the energy dissipating means comprises two or more substantially rigid elongate members, preferably it consists of two substantially rigid elongate members symmetrically arranged on opposite sides of the longitudinal axis of the insulator or string of insulators, or of the group of insulators or strings of insulators, each substantially rigid elongate member being formed between its ends into a curved portion of approximately semicircular shape, the semi-circular shaped portions of the two substantially rigid elongate members lying in a substantially common plane. In all cases, preferably the or each substantially rigid elongate member constituting the energy dissipating means is made of a mild steel or of an aluminium based alloy Energy dissipating means in accordance with the present invention may be used in combination with the energy dissipating means described and-claimed in the specification of our published UK Patent Application Mo. 2089142A (corresponding to our published European Patent Application No. 55047). The invention is further illustrated by a description, by way of example, of two preferred forms of energy dissipating means for use in en overhead electric transmission line with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a fragmental front view of an overhead electric transmission line; Figure 2 is Åa representation of the first form of energy dissipating means; and Figure 3 is a representation of the second form of energy dissipating means. Referring to the drawings, the overhead electric transmission line shown in Figure 1 comprises a crossarm 1 of a pylon (not shown) from whicb a single conductor 2 is suspended by two transversely spaced strings of insulators 3 and a link 4. Each string of insulators 3 is attached to the crossarm 1 by an energy dissipator 5. The energy dissipators 5 may each be of the form of energy dissipating means shown in Figure 2 or of the form of energy dissipating means shown in Figure 3. The energy dissipator shown in Figure 2 comprises two mild steel strips 7*each formed between its ends into a curved portion 8 of approximately semi-circular shape, the semi-circular shaped portions of the two strips lying in a substantially common plane, and the two strips are connected at their ends by bolts 9 to form, between their interconnécted ends, a substantially complete circle. The energy dissipating means shown in Figure 3 comprises a single mild steel strip 17 which, between its ends, is formed into two loops 18 lying in a common plane and on directly opposite sides of the longitudinal axis of the strip. With the energy dissipating means shown in Figure 2 or Figure 3 constituting the energy dissipators 5 employed in the transmission line shown in Figure 1, in the event of fracture of one of the insulator strings 5 as the load on the sound string of insulators increases to the maximum safe working load, each strip 7 or the strip 17 is substantially resilient and, as the load on the sound string of insulators exceeds the maximum safe working load and increases towards the load at which the sound string of insulators will fracture, each strip 7 or the strip 17 will extend smoothly and continuously in a rectilinear direction substantially parallel to the longitudinal axis of the sound string of insulators thereby to reduce substantially risk of fracture of the sound string of insulators

Claims: CLAIMS: 1. An overhead electric transmission line of the kind described wherein at at least one pylon, each group of at least two mutually spaced insulators or strings of insulators 3 supporting a single conductor 2 or the conductors of a group of transversely spaced conductors carries means 5 for dissipating the energy arising from fracture of an insulator or string of insulators of the group as the resultant additional load is applied to the or each sound insulator or sound string of insulators of the group, which energy dissipating means is of such a form that, as the load on the or each sound insulator or sound string of insulators increases to the maximum safe working load, the energy dissipating means is not permanently extensible in a rectilinear direction substantially parallel to the longitudinal axis of the insulator or string of insulators and, as the load on the or each sound insulator or sound string of insulators exceeds the maximum safe working load and increases towards the load at which the insulator or string of insulators will fracture, the energy dissipating means smoothly and continuously extends permanently in said rectilinear direction to reduce substantially risk of fracture of the insulator or string of insulators. 2. An overhead electric transmission line as claimed in Claim 1, wherein at each pylon, each group of at least two mutually spaced insulators or strings of insulators supporting a single conductor or the conductors of a group of transversely spaced conductors carries said energy dissipating means. 3. An overhead electric transmission line as claimed in Claim 1 or Claim 2, wherein the energy dissipating means is of such a form that, as the load on the or each sound insulator or sound string of insulators increases to the maximum safe working load, the energy dissipating means is elastically extensible in said rectilinear direction. 4. An overhead electric transmission line as claimed in Claim 3 in which the energy dissipating means is initially elastically extensible, wherein the elastic extensibility of the energy dissipating means is at least partially damped, the effectiveness of said damping ceasing to have effect as the value of the load on the or each sound insulator or sound string of insulators reaches or just exceeds the maximum safe working load 5. An overhead electric transmission line as claimed in any one of the preceding Claims in which each insulator or each string of insulators of a group of at least two mutually spaced insulators or strings of insulators is separately suspended from a crossarm 1 of a pylon, wherein an energy dissipating means 5 is interconnected between said insulator or string of insulators 3 and its point of suspension. 6. An overhead electric transmission line as claimed in any one of Claims 1 to 4 in which the insulators or strings of insulators of a group of at least two mutually spaced insulators or strings of insulators are interconnected at their upper ends by at least one link and the link or links is or are suspended from a crossarm of a pylon at a single position, wherein said energy dissipating means is connected between the link or links and the single point of suspension and/or between the link or links and each insulator or string of insulators of the group 7. An overhead electric transmission line as claimed in any one of the preceding Claims, wherein the energy dissipating means comprises one or more than one substantially rigid elongate member 7, 17 of metal or metal alloy which is of such a transverse cross-section and is so shaped between its ends that, as the load on the or each sound insulator or sound string of insulators increases to the maximum safe working load, the or each elongate member is substantially resilient and, as the load on the or each sound insulator or sound string of insulators exceeds the maximum safe working load and increases towards the load at which the insulator or string of insulators will fracture, the or each elongate member will extend smoothly and continuously in a rectilinear direction substantially parallel to the longitudinal axis of the insulator or string of insulators. 8. An overhead electric transmission line as claimed in Claim 7 in which the energy dissipating means is a single substantially rigid elongate member 17, wherein the member is symmetrical about its longitudinal axis and is formed between its ends into two loops 18 lying in a common plane and on directly opposite sides of said longitudinal axis. 9. An overhead electric transmission line as claimed in Claim 8, wherein the member is formed into a shape approximating to that of a figure eight and adjacent central parts of the member are clamped together by a bolt or other means designed to fracture at a load equal to or slightly greater than the maximum safe working load of the insulator or string of insulators. 10. An overhead electrical transmission line as claimed in Claim 7, wherein the energy dissipating means comprises two substantially rigid elongate members 7 symmetrically arranged on opposite sides of the longitudinal axis of the insulator or string of insulators, or of the group of insulators or strings of insulators, each substantially rigid elongate member being formed between its ends into a curved portion 8 of approximately semi-circular shape, the semi-circular shaped portions of the two substantially rigid elongate members lying in a substantially common plane.

Inventors: ADAM, JOHN FRANCIS
Applicants: BALFOUR BEATTY LIMITED
Application year: 1982
Main classification: H02G7
IPC codes: H02G7, H01B17