Space dampers for four-conductors bundles

Abstract

A space damper for 4-cable bundles of overhead power transmission lines is disclosed, comprising a framework (10) wherefrom four support arms (20a-20b) depart, at the distal ends of which there are provided clamps for fastening electric cables, said arms (20a-20c) being constrained to the framework (10) through respective dampening hinges (30a-30c), wherein the spacer damper is configured so that the vertical, natural-mode frequencies thereof are higher than the corresponding horizontal, natural-mode frequencies thereof.

Claims

1. A spacer damper for 4-cable bundles of overhead power transmission lines, comprising an integral quadrilateral framework (10) wherefrom four support arms (20a-20b) depart, wherein one distal end of each of said four support arms is provided with a clamp for fastening an electric cable, each of said four support arms (20a-20c) being constrained to said integral framework (10) at another distal end of each of said four support arms through respective dampening hinges (30a-30c); wherein said spacer damper is configured not symmetrical, said each electric cable being nevertheless kept at each of the four vertexes of an ideal rectangular quadrilateral with horizontal and vertical sides; and said dampening hinges (30a-30b) are at vertexes of the integral quadrilateral framework having at least one side of the horizontal and vertical sides inclined with respect to a horizontal line; so that the vertical, natural-mode frequencies of the spacer damper are higher than the corresponding horizontal, natural-mode frequencies thereof.

2. The spacer damper as claimed in claim 1, wherein each of said arms (20a-20c) have reference axis, which join the fastening point of the respective electric cable with the centre of rotation of the respective dampening hinges (30a-30c), said reference axis which have different angles (1, 2) with respect to a horizontal axis.

3. The spacer damper as claimed in claim 2 wherein said reference axis form first and second different angles (1, 2) with the horizontal axis which are identical in pairs and mutually different.

4. The spacer damper as claimed in claim 3, wherein said first and second different angles range between 35 and 65 degrees, preferably with the first angle 1 between 30 and 40 degrees and the second angle 2 between 60 and 70 degrees.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features and advantages of the invention become in any case more evident from the following detailed description of a preferred embodiment, given purely by way of a non-limiting example and illustrated in the attached drawings, wherein:

(2) FIGS. 1 and 2, as already mentioned, represent an elevation view and a schematic view, respectively, of a prior-art, 4-cable bundle spacer damper;

(3) FIG. 3 is an elevation front view of an embodiment of a spacer damper according to the invention; and

(4) FIG. 4 represents a geometric diagram by which the spacer damper of FIG. 3 is schematized, for the purpose of a numeric dynamic simulation.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(5) The inventor started off from the consideration that, although aeolian vibrations are by far the most frequent ones, on the 4-cable bundleswhere the phenomenon of subspan oscillations is equally importantit could be advantageous to move away from the consolidated approach and try to identify a compromise solution, renouncing part of the effectiveness against aeolian vibrations.

(6) Starting from this approach, the inventor could realise that subspan oscillations are referable to an instability phenomenon at two degrees of freedom of the mechanical structure subject to aerodynamic forces (flutter). In particular, the instability is due to the coupling of a subspan horizontal oscillation natural mode with a vertical subspan oscillation natural mode (which may possibly derive from a torsional mode of the entire span).

(7) The frequencies of the two natural modes are structurally different. In particular, by the known configuration of the symmetrical 4-cable bundle spacer dampers and with identically oriented arms (as in the solution of FIGS. 1 and 2), the frequency of the vertical mode is lower than that of the horizontal mode.

(8) It has been realised that the non-conservative force field due to the aerodynamic wake effectof the windward conductor over the leeward one, typically in the horizontal plane progressively changes the frequencies of the vertical modes, increasing them and making them the same as the ones of the corresponding horizontal modes. When the frequencies of a pair of modes match, the typical instability form sets in (flutter), with elliptical motion of the two bundle cables which expands up to a limit cycle which can cause the cables to collide against each other or even to breakage.

(9) According to the invention, by abandoning the consolidated approach of a symmetrical structure, a spacer damper is provided with such a configuration that the vertical-mode frequencies thereof are higher, even by a small measure, than the frequencies of the corresponding horizontal modes. In such case, the effect of the aerodynamic forces tends to move further away such frequencies, hence avoiding the two modes from coupling originating subspan oscillations.

(10) FIG. 3 shows a possible embodiment of a 4-cable bundle spacer damper which obeys the above set forth inventive principle. It consists, in a way known per se, of a framework 10 provided with four peripheral support arms 20a-20c which bear, at the distal ends thereof, tightening clamps for four corresponding conductive cables (not shown). Support arms 20a-20c are constrained to framework 10 through dampening hinges 30a-30b.

(11) According to the invention, the spacer damper is no longer configured in a symmetrical way, although the cables are nevertheless kept at the four vertexes of an ideal rectangular quadrilateral with horizontal and vertical sides. In particular, the four hinges 30a-30b are at the four vertexes of a quadrilateral having at least one side inclined with respect to a horizontal line (FIG. 3) an can also have non-orthogonal adjacent sides, such as for example a parallelogram (FIG. 4).

(12) Moreover, support arms 20a-20b have reference axes (i.e. the axis which connects the cable-fastening point with the rotation axis of the respective hinge 30a-30b) which have different angles with respect to a horizontal axis. FIG. 4 shows a case wherein the reference axes of the support arms have inclination angles with respect to the horizontal which are identical in pairs, i.e. two opposite arms define an angle a1 with respect to the horizontal, while the other two opposite arms define a reference angle a.sub.2 different from a1. These angles range between 35 and 65 degrees, preferably with a1 between 30 and 40 degrees and a2 between 60 and 70 degrees.

(13) By this configuration an excellent average performance of the spacer damper is obtained with respect to typical oscillations. As a matter of fact, it produces lower horizontal natural mode frequencies than the frequencies of the corresponding vertical natural modes, hence strongly inhibiting the development of subspan oscillations, at the price of a slight worsening over aeolian vibrations, which, however, remain within acceptable limits.

(14) The excellent performance of the spacer damper according to the invention has been proved at a theoretical level through numerical simulation. It has been detected that a space damper according to the invention allows to dramatically extend the length of subspans also in critical environmental conditions, up to by 60 meters, without significant worsening effects in the performance being noticeable in relation to wind vibrations.

(15) The construction according to the invention allows, among other things, to define a central framework of a smaller size compared to the prior art, which may further reduce the relative costs.

(16) However, it is understood that the invention is not limited to the specific configurations described above and illustrated, which represent only non-limiting examples of the scope of the invention, but that a number of variants are possible, all within the reach of a person skilled in the field, without departing from the scope of the invention.