FIELD OF THE INVENTION This invention relates to overhead conductor suspension systems and particularly to an apparatus and method for suspension of overhead power transmission line conductor with increased electrical clearances to ground, underlying objects and grounded structures for uprating, upgrading and other needs.

BACKGROUND OF THE INVENTION Electricity supply companies commonly wish to exceed the capacity of transmission lines without incurring the expense of building new pylons. The capacity of an electrical transmission line is a function of the line voltage and current that can be carried and these, in turn, are a function of the clearance between adjacent conductors as well as between a conductor and ground. Known ways to increase the system's capacity are uprating. when higher current is permitted in the conductors, and upgrading, when a higher line voltage is used.

When the lines are uprated, the conductor temperature rises as a result of larger ohmic losses in the conductors, and the legal requirements of clearance to underlying objects can be violated owing to heat expansion of the conductors. This requires that power transmission lines built for conductor temperatures, as a rule, between 50 to 100°C, be modified to fit the clearance requirements at higher conductor temperatures. When the lines are upgraded. the clearance requirements both to underlying objects and to supporting constructions can be changed.

Enlarged clearances can also be helpful for other needs. e. a. wildlife protection.

Sometimes, the additional clearances are obtained by using shorter insulator strings. V-shaped insulators installation etc. which permit the conductor suspension point to be raised, but in many cases it is not enough.

U. S. Patent No. 5,777,262 (Nourai et al.) discloses an apparatus and method for increasing electrical clearances on power lines so as to increase the conductor to pylon clearance and insulation length while avoiding diminution of the conductor clearance to ground and other underlying objects. The apparatus disclosed by Nourai et al. is intended for connection to the lower extremity of an existing vertical insulator string mounted on the pylon and to which the conductors are nonnally coupled. The insulator string is of sufficient length to provide adequate conductor to ground electrical isolation before upgrading. As explained by Nourai et al., when it is required to increase the line voltage. the length of the insulator must be increased so as to accommodate the higher voltage. This is achieved by fixing to the lower extremity of the existing vertical insulator string a yoke plate, to which a pair of additional substantially horizontal auxiliary insulator strings is mounted on opposite sides thereof. The conductors are attached to the free ends of the auxiliary insulator strings and are shorted by a bypass loop connected to each of the conductors as well as being separated from the yoke by a post insulator.

Such an arrangement allows for the electrical clearance to be increased without risking dielectric breakdown between the conductors and the point of attachment to the support structure. However, it does not address the complemen- tary requirement to increase the conductor to ground clearance.

According to another embodiment disclosed by Nourai et al., the original insulator is removed and a yoke plate is connected directly to the suspension structure. In this case, the ground clearance rises, but there is lost one of the two functions of the suspension device, namely the equalization of the conductor tension on both sides of the structure and the avoidance of forces being applied to the structure in the direction of the conductor.

Therefore, there exists a need for a device that permits more significant elevation of the conductor suspension point, not limited bx the insulator length or

electrical conductor spacing requirements. Such an apparatus should be adapted for mounting on existing suspension structures and conform to the power transmission line design specifications. Preferably, it should be possible to attach the apparatus live-line without modifying the existing structure.

SUMMARY OF THE INVENTION It is an object of the invention to provide a device which allows the conductor suspension point on an electricity pylon to be raised in manner not provided for by hitherto proposed systems.

A primary objective of the invention is to provide enlarged clearances between energized overhead power transmission line conductors to underlying objects. while employing existing support structures. A fiirther objective is to allow increased conductor clearance to the underlying cross ann. either to exploit the enlarged clearances for other purposes, or to allow lower structures having reduced space between the cross arms to be used.

The invention achieves this by separating between the functions of conductor mechanical suspension and electrical insulation, which are integrated in existing conductor support devices.

In accordance with a first aspect of the invention, there is provided an apparatus for supporting an overhead electrical conductor on a grounded support structure, the apparatus comprising: a hanging member attached to the support structure. a pair of insulators each attached at respective first ends thereof to the hanging member, each of the insulators being clamped at respective second ends thereof to the electrical conductor such that the insulators extend in a direction substantially collinear with the electrical conductor at respective points of attachment. the insulators being sufficient for ground insulation of the conductor, and

a loop formed by the electrical conductor between said points of attachment. producing an air gap between the conductor and the hanging member : characterized in that: the hanging member is displaceable along said direction substantially collinear with the electrical conductor. and the insulators and the air gap are sufficient for ground insulation of the conductor.

According to a second aspect of the invention there is provided a method for increasing electrical clearances of an overhead electrical conductor to underlying objects and a support structure, the overhead conductor being supported by at least one first insulator attached to the support structure. the method comprising the steps of : (a) providing a hanging member adapted for mounting to the support structure so as to allow movement of at least a section of the hanging member relative to the support structure substantially in the conductor direction, and for attaching a pair of second insulators to said section, (b) attaching to the hanging member a first end of respective second insulators, each insulator sufficient for ground insulation of the conductor, (c) clamping respective second ends of the second insulators to the electrical conductor at respective points of attachment on opposite sides of the first insulator such that the second insulators extend in a direction substantially collinear with the electrical conductor.

(d) removing the at least one first insulator from the electrical conductor, (e) removing the at least one first insulator from the support structure, (f) mounting the hanging member to the support structure so as to have limited movement along said direction, and (t) cutting the electrical conductor between the respective points of attachment and bridging the cuts by an additional conductor section

forming a loop of sufficient distance from the hanging member to provide ground insulation of the conductor.

In a variation of such a method, the loop is formed bv pulling the conductor into the section between the clamps and so there is no need to cut the conductor and bridge its ends.

BRIEF DESCRIPTION OF THE DRAWINGS In order to understand the invention and to see how it may be carried out in practice, some preferred embodiments will now be described, by way of non-limiting examples only, with reference to the accompanying drawings. in which: Fig. 1 is a pictorial perspective view showing a first embodiment of the invention for use with an overhead conductor suspension: Fig. 2 is a pictorial perspective view showing in detail a section of a slider mount for use with the first embodiment; Fig. 3 shows pictorially a detail of a support structure having two cross arms each for supporting an overhead conductor using the mount illustrated in Fig. 2; Figs. 4 to 7 are pictorial perspective views showing details of further embodiments of the invention; Fig. 8 is a pictorial perspective view showing a support structure having two cross arms supporting overhead cables according to prior art suspension schemes and having a third cross arm supporting an overhead cable according to the invention: and Figs. 9 and 10 are pictorial perspective views showing details of two further embodiments of the invention.

In all figures, similar components are identified bv identical reference numerals.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Figs. 1 and 2 show a suspension apparatus depicted generallv as 1 for mounting on a cross arm 2 of a support structure (not shown) for supporting a conventional electrical conductor 3. The suspension apparatus 1 comprises a hanging member in the form of a stainless steel rope 4 slidably mounted within a recess in the slider mount 5 attached to the cross arm 2 of the support structure. An insulator 6 is pivotally anchored at a first end thereof to each end of the stainless steel rope 4. A second end of each insulator 6 is clamped at a respective second end thereof to the conductor 3 by means of respective dead-end clamps 7. Near the ends of the hanging member 4, stops 8 are fixed to limit its movement in the conductor direction in the event that the conductor is broken at one side thereof. The conductor forms a loop (jumper) between the two dead-end clamps 7 by-passing the stainless steel rope 4 and providing an air insulation gap of size required by the power line voltage.

In such an arrangement, any slight imbalance between the forces acting on the two ends of the conductor 3, is accommodated by movement of the hanging member resulting from the stainless steel rope sliding in the recess of the slider mount. thus allowing the conductor 3 to move slightly from side to side.

Fig. 3 shows a suspension pole 9 of a power transmission line having a pair of cross arms 2 each bearing an overhead conductor 3 using the apparatus 1 described above with particular reference to Figs. 1 and 2. Each conductor 3 is suspended on the respective cross arm 2 such that the vertical distance between the conductors is kept equal to the vertical distance between the cross arms. This is usually the case in power transmission lines. However. unlike previously known approaches where each conductor suspension point is lower than the corresponding cross arm. in the invention each conductor 3 is substantially level with the fixing point of the cross arm 2. As a result, clearances of the lower conductor to underlying objects. and of the upper conductor to the lower cross anm. are increased thereby allowing higher voltages or currents to be carried by the overhead cables 3 without violating safety requirements.

Fig. 4 shows a modified hanging member 4 in the form a flexible loop fixed to the cross arm 2 by a mount 10. Pulleys 11 are disposed at opposite sides of the loop and serve as anchoring points for anchoring thereto the first ends of the respective insulators 6. The conjunct movement of the anchoring points in the conductor direction is provided by rolling or sliding the pullevs along the loop.

Fig. 5 shows an alternative arrangement wherein the hanging member includes a pendulous member in the form of a substantially upright bar 13 attached at an upper end thereof by means of a pivot 14 to the cross arm 2. The insulators 6 are themselves pivotally anchored towards a lower end of the upright bar 13. so as to depend from opposite sides thereof. In such an arrangement, any slight imbalance between the forces acting on the two ends of the conductor 3, is accommodated by movement of the hanging member resulting from the upright bar swinging about the pivot 14, thus allowing the conductor 3 to move slightly from side to side.

Fig. 6 shows a pendulous hanging member in the form of a substantially upright bar 13 supporting a yoke plate 16 at a lower end thereof. The insulators 6 are pivotally anchored to opposing sides of the yoke plate 16. In the event of one side of the conductor 3 breaking, the unbalanced tension in the other side of the conductor is applied via the insulator 6 and bar 13 to the cross arm 2. In hitherto proposed support schemes, if the conductor breaks on one side of the insulator, the latter is pulled by the remaining conductor section and its length is effectively added to that of the conductor, thereby reducing somewhat the excess tension. In the present case, the short hanging member cannot reduce the surviving conductor tension to the same extent as the long insulator employed in hitherto proposed support schemes. In order to prevent mechanical overload to the structure, the pivot 14 by means of which the upper end of the upright bar is attached to the cross arm 2 may be constituted by a shear pin. A stainless steel wire 15 connects the cross arm 2 with the upright bar 13 so as to prevent the conductor from falling when the pin is sheared. A safen device can equally well be employed with any of the other

embodiments of the hanging member, so that in the event of an unbalanced load being applied thereto, overload to the structure will be prevented.

Fig. 7 shows an arrangement similar to that shown in Fig. 5 wherein the hanging member includes a substantially upright bar 13 pivotally attached at an upper end thereof to the cross arm 2 by means of a pivot 14. The insulators 6 are pivotally attached toward a lower end of the upright bar 13 so as to depend from opposite sides thereof at different heights. By such means. different spans of a power transmission line differing in length and terrain conditions can accommodate different conductor tension. No equivalent provision is made is hitherto proposed suspension devices enables for changing the conductor tension at a suspension structure.

Fig. 8 shows a suspension pole 9 of a power transmission line having three cross arms 2 extending in the same direction, and each supporting an overhead conductor 3. At an uppermost cross arm, a known suspension device 17 is used for the conductor suspension, consisting of an upright insulator string 19 supported by a pivot 14 at an upper end thereof to the cross arm and supporting at a lower end thereof a suspension clamp 20. The conductor 3 is fixedlv mounted within the clamp 20 so as to be supported thereby. At the middle cross arm, another known suspension device 18 is used to support the conductor 3 at a slightly decreased drop C from the cross arm with respect to the drop D achieved by the regular suspension 17. Thus, there are provided two insulator strings 19 commonly mounted at respective upper ends to the cross arm 2 by a pivot 14. The lower ends of the two insulators are respectively connected to the conductor at two spaced-apart points using suspension clamps 20. At the lowest cross arm. a suspension device 12 according to the invention supports the conductor 3, thereby achieving an even smaller drop B from the cross arm 2. The invention thus allows the overhead cable 3 to be elevated relative to the cross arm to an extent not provided for in hitherto proposed suspension systems.

Overhead power transmission lines commonly consist of tension and suspension poles which are grounded structures beanns a plurality of energized

conductors. which must be electrically insulated from the structures. As opposed to tension poles. the limited strength of suspension poles allows a very limited load to be applied thereto in the direction of the conductor. It thus emerges that there are two principal functionalities of the suspension device: electrical insulation and prevention of forces in the conductor direction from the structure. The electrical insulation is provided by insulators and air gaps. Excessive forces in the conductor direction are prevented by means of movement of the hanging member. In hitherto proposed suspension devices, both functionalities are integrated, which means that the insulators are used as the suspension member. As shown in Fig. 8, the vertical distance between the cross arm 2 and the conductor 3 is not less than the insulator length D required by the line voltage for the suspension 17 or than the required air gap C for the suspension 18.

In the invention, the two functionalities are separated. This enables significant elevation of the conductor suspension point, which is vertically displaced from the cross arm by only the short length A of the hanging member, thereby raising both spans of the conductor towards the cross arm Fig. 9 shows another embodiment wherein a hanging member in the form of an upright bar 13 is pivotally attached to the cross arm by a pivot 14 intermediate the upper and lower ends of the upright bar 13. Insulators 6 are attached to the upright bar 13 at its upper end above the pivot 14 so as to provide even greater elevation of the conductor 3 suspension point. The hanging member 13 is balanced at its lower end by a counterweight 21. The conductor jumper is spaced from the grounded parts by a post-insulator 22 fixed at one end to the cross arm 2 or to upright bar 13 and being clamped at an opposite end to the conductor by a conventional suspension clamp 20, as shown in Fig. 8.

The counterweight 21 serves to counteract the effect of wind forces acting in a direction normal to the axis of the conductor when the pivot 14 is bi-directional.

The same effect could be realized using springs, thus obviating the need for the counterweight 21 and allowing the upright bar 13 to be pivotally mounted to the cross arm 2 substantially at the lower end of the upright bar l 3.

Fig. 10 shows yet a further embodiment of a hanging member 23 formed of flexible stainless steel rope having a substantially inverted T-shaped configuration.

An upper end of the upright of the T-shaped loop is pivotally mounted to the cross arm 2. The base of the T-shaped loop forms an open loop 24 within opposite ends of which the insulators 6 are anchored by sliders or rollers 11. This provides another way to avoid damage to the support structure in the event of unbalanced loading caused by a broken conductor.

The invention also contemplates methods for increasing electrical clearances of an overhead electrical conductor to underlying objects and a support structure, when the overhead conductor is originally supported by at least one first insulator attached to the support structure. Such methods provide a way to produce the apparatus of the invention at an existing power transmission line. At its simplest, two insulators are dead-end clamped to the conductor at pre-determined points thereof, while the distance between the clamps is defined by final conductor loop dimension requirements. The conductor is pulled into the section between the clamps until the distance between the clamps enables attachment the free ends of the insulators to the hanging member. When the insulators are attached to the hanging member, the hanging member or the conductor is supported by a suitable platform so that to loosen the conductor at the original suspension area. The original suspension accessories are then removed from the conductor and the support structure, the hanging member is attached to the structure, and finally the platfonn is removed.

An alternative way to produce the apparatus of the invention at an existing power transmission line can be used when there is not enough spare conductor length available for to produce the loop. The hanging member is pre-assembled with the two insulators and the assembly is aligned with the conductor. The insulators are dead-ended to the conductor at their free ends so as to provide maximum distance between the clamps. Then the hanging member is supported by a suitable platform so that to loosen the conductor at the original suspension area.

The original suspension accessories are removed from the conductor and the

support structure, the hanging member is attached to the structure, and the platform is removed. The conductor is cut between the two dead-end clamps and its ends are bridged by an additional piece of conductor. so that the conductor section between the dead-end clamps will form a loop of required size.

In the method claims which follow, alphabetic characters used to designate claim steps are provided for convenience only and do not imply any particular order of performing the steps.