HK1201995B - Space dampers for four-conductors bundles - Google Patents

Abstract

A space damper for 4-cable bundles of overhead power transmission lines is disclosed, comprising a framework (10) where- from 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.

Description

Damping spacer for four-conductor wire harness

Technical Field

The invention relates to a spacer for an electric power harness conductor of an overhead transmission line, in particular to a spacer for a four-cable harness.

Background

Overhead power lines are known to represent one of the most widely used remote power lines to date. Which includes a plurality of conductors extending between support poles. Typically, especially for high voltage remote power lines, each line conductor consists of a cable harness or a basic conductor harness, as the harness is able to increase the power that can be transmitted, limiting leakage and electromagnetic interference fields. The most commonly used wiring harnesses consist of two, three or four elementary cables.

Obviously, the individual harness conductors must be kept at the correct distance from the other harness conductors, preferably over the entire span from one support pole to the other. For this purpose, it is known to use spacers which are constructed in different ways depending on the number of conductors constituting the wire harness. In the present specification, a spacer for a four-cable harness, i.e., a spacer adapted to maintain positional consistency between four cables belonging to the same conductor harness, will be discussed.

In a static condition, a few spacer rods located on the length of the span (varying between 200 and 1000 metres in length) are theoretically sufficient to keep the cables at the correct mutual distance to define a series of wide subspans. However, the overhead transmission line is exposed to variable atmospheric conditions that can change the state of the overhead transmission line and cause external disturbing forces: typically, wind has a significant effect on overhead transmission lines, affecting their dynamic performance.

The action of wind causes three types of vibrations/shocks on the overhead transmission line. The wind-induced vibrations due to cyclonic separation typically exhibit high frequencies and low amplitudes. Another mode of vibration is the mode known as "galloping", which mode, with low frequency and high amplitude, originates from the same span (i.e. between two support poles) and causes the cable harness to vibrate in the vertical plane. Galloping occurs under very specific environmental conditions (typically when there is ice deposition around the cable). There is finally subspan vibration which manifests itself in separate subspans separated from each other by spacers and caused by aerodynamic effects (wake effect) between the windward and leeward cables.

The subspan vibration will be discussed essentially below. This phenomenon proves to be particularly evident in a four-cable bundle, where there is a typical case with a pair of upwind cables and a pair of leeward cables that produce a severe wake effect. These vibrations, also referred to as "subspan" vibrations, can cause collisions between the bundle sub-conductors, significant stresses on the conductors and at the damped spacer clamps and potentially resulting cable breakdowns.

All four-cable damped spacers proposed so far in the prior art provide a central quadrangular frame at the vertices of which fastening means are arranged, supported by respective small arms, at the ends of which the cables of the conductive wire bundles are tensioned. Typically the structure is symmetrical to maintain the four cables sufficiently separated from each other at the vertices of the quadrilateral and with evenly distributed static loads.

In order to cope with the subspan vibrations, different arrangements have been proposed over time.

Conventionally, the cable fastening arm is mounted on the quadrilateral frame by means of a damping hinge, which has brought about a damping effect for vibrations/shocks. Spacer dampers of this type are described, for example, in patent documents EP0244624, US4554403 and US 4188502.

Figure 1, herein appended, shows a prior art spacer-damper in elevation view. It can be noted that the spacer damper consists of a quadrangular frame, at the vertices of which four identical arms 2a, 2b, 2c, 2d are articulated, at the distal ends of which arms 2a, 2b, 2c, 2d a conductive cable (not shown) is fastened by means of a clamp of known type. Each hinge 3a, 3b, 3c, 3d constraining the frame 1 to the arms 2a, 2b, 2c, 2d has a configuration suitable to support the clamp in the desired position and to dampen any vibrations. In particular, it can be noted in fig. 1 that the clamp is supported so that the cable is almost at the same height as the centre of rotation of the hinges 3a, 3b, 3c, 3d (i.e. the fastening point of the cable and the axis of rotation of the hinges are on one substantially horizontal line). This design is considered to be optimum in this field, since it allows to best cope with wind-induced vibrations in addition to creating a symmetrical pressure load. In fact, said wind-induced vibrations are typically triggered in a vertical plane (coinciding with the y-axis in fig. 1), and it is therefore useful to find a good lever arm defined by the arms 2a, 2b, 2c, 2d with respect to the rotation axis of the hinges 3a, 3b, 3c, 3d, in order for the damping system to function properly. Figure 2 shows a geometry from which the spacer damper of figure 1 is schematized for numerical dynamic simulation. It has been found that the near horizontal attitude of the arms and the need to space the cables as far apart from each other as possible makes the central frame generally rectangular and of significant dimensions (the height of the central frame is almost the same as the vertical distance between the cables).

Another effective measure in order to reduce the extension of the subspans and thus improve the aerodynamic performance of the beam is of course to increase the number of damped spacers on the transmission line. When the electrically conductive wiring harness is particularly light and weather conditions are particularly severe (high temperature, reduced tension on the cable and frequent and violent winds), subspan vibration is a major problem and it may be necessary to shorten the subspan to about 40-45 meters. It can be guessed that this is particularly undesirable as it affects material costs as well as installation and maintenance costs.

Alternatively or additionally, it is proposed that it is suitable for a damping device provided with counterweights adapted to the subspan line, which damping device acts in particular on torsional regular modes. However, this solution also affects costs.

Disclosure of Invention

It is therefore an object of the present invention to provide an innovative spacer-damper that solves the above mentioned problem, i.e. that of reducing subspan vibrations as much as possible without forcing a reduction of the subspan extension.

This object is achieved by a device substantially as defined in claim 1 appended hereto. The dependent claims describe preferred features of the invention.

Specifically, according to a first aspect of the present invention, there is provided a spacer damper for a four-cable harness of an overhead power transmission line, comprising a frame from which four small support arms project, at the distal ends of which are provided clamps for fastening power cables, said small arms being constrained to the frame by respective hinges, wherein the spacer damper is configured such that its vertical modal natural frequency is higher than its corresponding horizontal modal natural frequency.

According to yet another aspect, there is provided a spacer damper wherein the small arms have reference axes connecting the fastening points of the respective cables with the rotation centers of the respective hinges so as to form angles with the horizontal axis, two of said angles being the same and different from the other two.

According to another aspect, the damped hinge is located at the apex of a quadrilateral frame having at least one side inclined with respect to the horizontal. Also, in accordance with a final aspect of the invention, the quadrilateral frame has non-orthogonal adjacent sides.

Drawings

Further characteristics and advantages of the invention will become more apparent from the following detailed description of a preferred embodiment, given purely by way of non-limiting example and illustrated in the accompanying drawings, in which:

figures 1 and 2 show a front view and a schematic view, respectively, of a prior art four-cable harness spacer-damper as already described;

FIG. 3 is a front view of one embodiment of a spacer-damper according to the present invention;

figure 4 shows a geometry by which the spacer damper shown in figure 3 is schematised for digital dynamic simulation purposes.

Detailed Description

The inventors have in this regard assumed that, although it is currently the most common for wind-excited vibration to be stopped on four-cable harnesses (where the phenomenon of subspan vibration is equally important), it is advantageous to go out of the integrated approach and try to determine a compromise solution, giving up part of the utility for wind-excited vibration.

Starting from this approach, the inventors have realized that subspan vibrations are related to the phenomenon of instability of the mechanical structure subjected to aerodynamic forces (flutter) in two degrees of freedom. In particular, instability is caused by coupling between the horizontal and vertical subspan vibrational natural modes (torsional modes that may originate from the entire span).

The frequencies of the two natural modes are structurally different. In particular, with the known configuration of the symmetrical four-cable harness spacer damper and with the use of arms of the same direction (as in the solutions of fig. 1 and 2), the frequency of the vertical mode is lower than that of the horizontal mode.

It has been recognised that the non-conservative force field (typically in the horizontal plane) of the windward to leeward conductors caused by the aerodynamic wake effect gradually changes the frequency of the vertical mode, raising it and making it the same as the frequency of the corresponding horizontal mode. When the frequencies of a pair of modes match, a typical unstable pattern begins to develop (flutter) with the elliptical motion of the two harness cables, which expands to a limit cycle where the cables collide with each other or even break down.

According to the present invention, by abandoning the integration method of the symmetrical structure, a spacer damper is provided having such a configuration that the vertical mode frequency is higher than the frequency of the corresponding horizontal mode, although by a small amount. In this case, the effect of aerodynamic forces tends to move further away from the frequency, thus avoiding two modes from causing subspan vibration through coupling.

Figure 3 illustrates one possible embodiment of a four-cable harness spacer-damper following the inventive principles set forth above.

It comprises, in a manner known per se, a frame 10 provided with four peripheral supporting arms 20a, 20b, 20c, 20d, which peripheral supporting arms 20a, 20b, 20c, 20d support at their distal ends tensioning clamps for four respective electrically conductive cables (not shown). The support arms 20a, 20b, 20c, 20d are constrained to the frame 10 by damper hinges 30a, 30b, 30c, 30 d.

According to the invention, the spacer damper is no longer constructed in a symmetrical manner, although the cable remains at the four vertices of an ideal rectangular quadrilateral with horizontal and vertical sides. In particular, the four hinges 30a, 30b, 30c, 30d are located at the four vertices of a quadrilateral having at least one side inclined with respect to the horizontal (fig. 3), and possibly also having non-orthogonal adjacent sides, for example a parallelogram (fig. 4).

Furthermore, the support arms 20a, 20b, 20c, 20d have reference axes (i.e. axes connecting the cable fastening point with the rotation axis of the respective hinge 30a, 30b, 30c, 30d) which have different angles with respect to the horizontal axis fig. 4 shows a situation in which the reference axes of the support arms have an inclination angle with respect to the horizontal which is the same in pairs, i.e. the two opposite arms define an angle α with respect to the horizontal₁And the other two opposing arms define a plane other than α₁Reference angle α₂These angles vary between 35 and 65 degrees, preferably α₁Between 30 and 40 degrees, and α₂Varying between 60 and 70 degrees.

By this arrangement, superior average performance of the spacer damper is obtained with respect to typical vibrations. In fact, it produces horizontal natural mode frequencies that are lower compared to the frequencies of the corresponding vertical natural modes, thus strongly suppressing the development of sub-span vibrations at the expense of a slight deterioration of the wind-induced vibrations, but the expense remains within acceptable limits.

The superior performance of spacer damper according to the present invention has been demonstrated on a theoretical level by numerical simulations. It has been found that spacer-damper according to the invention allows a significant extension of the sub-span length up to 60 meters under severe environmental conditions without significant deteriorating impact on the performance with respect to wind vibrations.

The configuration according to the invention allows, among other things, to define a central frame of smaller dimensions compared to the prior art, so as to further reduce the relative costs.

It is understood, however, that the invention is not limited to the configurations described and illustrated above, which represent only non-limiting examples of the scope of the invention, and that numerous modifications, which may be realized by those skilled in the art, are possible without departing from the scope of the invention.

Claims (4)

1. Spacer damper for a four-cable harness of an overhead power transmission line, comprising a one-piece quadrangular frame (10), from which four supporting arms (20a, 20b, 20c, 20d) protrude, at the distal ends of which supporting arms (20a, 20b, 20c, 20d) clamps are provided for fastening power cables, said arms (20a, 20b, 20c, 20d) being constrained to said frame (10) by respective damped hinges (30a, 30b, 30c, 30d), wherein said spacer damper has a vertical natural modal frequency higher than their respective horizontal natural modal frequency.

2. Spacer damper as claimed in claim 1, wherein the arms (20a, 20b, 20c, 20d) have reference axes connecting the fastening points of the respective cables with the centre of rotation of the respective hinges (30a, 30b, 30c, 30d) to form an angle (α) with the horizontal axis₁，α₂) Said angle (α)₁，α₂) The pairs are identical and the pairs are different from each other.

3. Spacer damper as claimed in claim 1 or 2, wherein said shock-absorbing hinge is located at the apex of said unitary quadrilateral frame (10), said unitary quadrilateral frame (10) having at least one side inclined with respect to the horizontal.

4. A spacer-damper as claimed in claim 3 wherein said quadrilateral frame has non-orthogonal adjacent sides.