WO1998048482A1

WO1998048482A1 - Electrical connection clamp

Info

Publication number: WO1998048482A1
Application number: PCT/IB1998/000526
Authority: WO
Inventors: Laurent Christian Royer; Christian Lahaye
Original assignee: The Whitaker Corporation
Priority date: 1997-04-21
Filing date: 1998-04-09
Publication date: 1998-10-29
Also published as: AU6513898A

Abstract

A clamp (2) for connecting large power supply cables (26) to power generation or transmission support members (3) is stamped and formed from austenitic stainless steel. The clamp comprises an upstanding peripheral wall (8) with a certain height H to provide the clamp with sufficient rigidity. The side walls (8) are provided with inwardly bent portions (32) that endow the clamp with a certain bending elasticity such that the conductor is clamped with a certain spring force to compensate for stress relaxation of the aluminium conductor.

Description

ELECTRICAL CONNECTION CLAMP

This invention is related to a clamp for connecting an electrical conductor such as a cable or solid cylindrical conductor to a conducting support of a power generation or transmission device.

Electrical cables for the transmission of electrical power are commonly interconnected to power generation or transmission devices by clamps positioned over the cable and tightened by bolts to a conducting base member of the structure to which the cable is electrically interconnected. The clamps and the support are typically cast or machined from an aluminium alloy in view of the good electrical conductivity, and the non-magnetic property of aluminium (which is important for high power electrical transmission) . The cables for electrical connection thereto are also of aluminium alloy. A high clamping force is required to ensure a sufficiently large airtight electrical contact between the cable and the connection support, particularly in order to overcome the problem of oxide layers forming at the contact surfaces. Oxide layers significantly increase contact resistance.

One of the problems with aluminium clamps and cables is the stress relaxation of aluminium over time (i.e. aluminium "flows" under stress) thereby reducing the clamping force of the cable to the connection support. Clamped connections therefore often degrade over time, leading either to failure thereof or the need for increased maintenance. This effect is increased where high electrical currents are carried, in view of the heat generated by power losses in the conductors and at the contact region, particularly where there is high contact resistance, for example due to oxide layers. Thermal oxides accelerate the relaxation of stress. It would be desirable to provide a clamped connection for high power electrical supply systems that ensures a reliable lasting electrical connection. It is also a continuous requirement to provide a clamp which is cost-effective to manufacture.

In the electrical connection industry in general, it is known to provide clamps for screwing or bolting electrical conductors to a complementary conductor where the clamping part is made of a material other than aluminium and for example stamped and formed from sheet metal. Existing designs, for example stamped and formed from sheet metal, are however not appropriate for connecting high power electrical cables which may have relatively large diameters (e.g. 30-50mm) . In particular, such designs are not adapted to support the particularly high clamping forces required and supply electrical currents at high voltages that may attain 500 000 volts. For this reason, amongst others such as the need for a nonmagnetic material, clamping parts have typically been cast or machined into relatively robust solid components.

A further reason that encourages the use of cast or machined components from aluminium alloys, is the desire to use similar metals in connection elements because of anodic

(galvanic) potential differences. Different metals have different potentials, which in the presence of an electrolyte encourages formation of oxide layers at the contact interfaces. Although clamps are often not contact members through which the electrical current transits, but merely mechanical clamping devices, it has not been usual in the electrical power industry to differentiate between clamping and connection members in terms of electrical properties. One of the reasons for such thinking comes from the fact that clamping parts are nevertheless subjected to the high voltages and require outer contours that are designed to prevent sparking due to the effect of electrical charges accumulating at sharp points, protrusions, or edges. Clamping members thus have to be designed like contact members when used at high voltages to prevent electrical charge accumulation and magnetic behaviour.

It is an object of this invention to provide a clamp for connecting high voltage electrical cables for power generation and transmission systems, that ensures reliable lasting electrical connection. It would be advantageous to provide a clamp that is cost-effective to manufacture. Reduced weight and material usage would be further advantageous .

Objects of this invention have been achieved by providing the electrical power cable connection clamp according to claim 1. Disclosed herein is an electrical connection clamp for clamping a high power electrical cable to a conducting support by means of bolts tightening the clamp towards the conducting support for compressing the cable thereagainst, the clamp comprising a pair of bolt holes and an arcuate pressure surface arranged therebetween for abutment against the cable, the bolt holes and pressure surface arranged generally along a base wall of the clamp, wherein the clamp is stamped and formed from sheet metal, the clamp further comprising a wall extending roughly orthogonally from the base wall to form a substantially continuous peripheral enclosure within which nuts or boltheads are receivable in alignment with the bolt holes, the peripheral wall defining side walls extending between the position of the bolt holes and extending substantially from the base wall clamping portion. The side walls may have a non-planar spring portion for enhancing the resilient flexibility of bending of the clamping portion. Advantageously therefore, the peripheral wall provides a robust clamp for a high clamping force, whilst the side walls provide large elastic deformability to ensure that a substantially constant elastic clamping force on the cable is provided. Stress relaxation of the cable is compensated whilst maintaining the clamping force.

The sheet metal may advantageously be of an austenitic stainless steel that provides a high strength elastic material without magnetic properties.

The side walls may be joined together at either end by substantially cylindrically shaped end walls that surround the bolt receiving portions of the clamp. The end walls and side walls thus contribute to providing a rigid clamp with relatively large elastic deformability.

The clamping portion may be provided with protrusions extending along the corner joining the side wall and the arcuate clamping portion, the protrusions thus providing rigid high pressure clamping ribs.

Tabs may be struck from the base wall at either longitudinal side of the arcuate clamping portion in order to assist in locating the clamp across large cables.

The non planar side walls may comprise an arcuate indent on each side wall such that an upper free edge of each side wall is inclined towards each other. The smooth contours reduce the risk of breakage of the clamp (thereby enhancing its strength and reliability) .

Further advantageous aspects of this invention will be described in the claims or will be apparent from the following description and drawings.

An embodiment of this invention will now be described by way of example with reference to the drawings in which:

Figure 1 is an isometric view looking towards a base wall of a clamp according to this invention;

Figure 2 is an isometric view of the clamp looking towards a top end; Figure 3 is a plan top view of the clamps;

Figure 4 is a cross-sectional view through lines 4-4 of Figure 3 with nuts positioned over the bolt holes;

Figure 5 is a cross-sectional view through lines 5-5 of Figure 3;

Figure 6 is a top plan view of a pair of clamps clamping a cable to a portion of a conductor support;

Figure 7 is a view with partial cross section looking in the direction of arrow 7 of Figure 6; Figure 8 is a view similar to Figure 7 but with smaller diameter cable being clamped;

Figure 9 is a side view of the clamp with a protective cover;

Figure 10 is a cross-sectional view through lines 10- 10 of Figure 9.

Referring to the Figures a clamp 2 comprises a base wall 4 and a peripheral wall 6 upstanding therefrom, the peripheral wall 6 extending generally orthogonally from the base wall 4 and having opposed side walls 8 extending in a longitudinal direction L in a spaced apart roughly parallel manner joined together at ends 10,12 by end walls 14. The end walls 14 are arcuate and have a substantially cylindrical shape.

The base wall 4 comprises a clamping portion 16 and bolt support portions 18 at either longitudinal end 20 of the clamping portion 16, a circular bolt hole 22 being cut out of each of the bolt support portions 18 for passage of a bolt therethrough. The peripheral wall and base wall forms an enclosure having an upper edge 24. Nuts 23 for coupling to complementary bolts 21 are received in the end wall enclosure portions 25 as best seen in Figures 4 and 7.

The clamping portion 16 is arcuately shaped in order to clamp around a substantially cylindrical portion of conductor or cable 26 or 26' as shown in Figures 7 and 8. The cables 26,26' are for transporting large electrical currents, and may have diameters in the range of 20-50mm for example. The clamping portion is provided with ribs 28 as shown in Figure 5, the ribs arranged at each corner of the side walls 8 with the clamping portion 16 of the base wall and directed towards the conductor 26,26' to be clamped. The ribs 28 provide a rigid pressure member for effective clamping of the conductor against a conductor support 3 at defined positions and with a high localized clamping force. In particular, a more effective retention of the cable to the clamp and conductor support 3 is ensured, whilst also rigidifying the clamping portion of the clamp with respect to bending forces about the cable, especially where the cable 26' has a small diameter as shown in Figure 8 (i.e. the curvature of the clamping portion has a larger radius than the cable) . Locating tabs 30 are stamped from the base 4 and extend roughly in the direction of side walls 8 towards the cable to be clamped. The locating tabs 30 are positioned at ends 20 of the clamping portion 16 and serve to locate the clamping member with respect to a cable 26 as shown in Figure 7. The tabs 30 are particularly useful for keeping strands of a straded cable together during clamping (i.e. limiting splaying of the cable) . The locating tabs 30 are particularly useful where large diameter cables, such as shown in Figure 7, which have a radius substantially equivalent or larger than the radius of curvature of the clamping portion 16 are to be clamped. A person clamping large cables can easily seat the clamp across the cable the tabs 30 providing a good seating of the clamp on the cable prior to bolting. The holes 31 from which the tabs 30 are stamped, usefully provide drains to allow liquids (water) from being evacuated, thereby reducing corrosion and dirt accumulation in the clamp. The peripheral wall 8 is provided with a height H (see Figures 1 or 4 or 7) between the base wall 4 and the upper edge 24 that is relatively deep such that a nut or bolthead is fully received within the height. The large height of the clamp peripheral wall endows the clamp with sufficient rigidity to withstand high clamping forces. The upper edge 24 is curved such that the height of the side walls is substantially maintained across the arcuate clamping portion 16. The latter enables sheet metal of relatively low thickness to be used for the manufacture of the clamp, which improves ease of manufacture thereby reducing costs, whilst also reducing material usage and weight. It is also important to fully receive the nut or bolt head within the peripheral wall in order to provide the nut or bolt with voltage discharge protection in view of the sharp angles and pointed corners of the bolt or nut head. At very high voltages the charges accumulating at pointed portions of a conductor may lead to corona electrical discharge by ionization of the air surrounding the conductor, which leads to power losses and erosion of the bolt. For very high voltage utilization, for example above 300 000 volts, the clamp may be provided with a protective cover or cap 40 stamped and formed from a conductive material which may be the same as that of the clamp and positionable over the upper edges 24 of the clamp. The particularly smooth outer profile of the clamp would thus provide corona protection at yet higher voltages.

In order to provide the clamp with a large range of elastic deformability to ensure that the clamping force between the conductor 26,26' and the support conductor 3 is maintained over a long period of time, the side walls 8 are provided with a non-planar spring portion 32. The spring portion 32 comprises a smooth arcuate indent of the side walls 8 such that a central portion 33 of the side walls (substantially centrally positioned between the bolt holes 22) is inwardly inclined i.e. opposing spring portions 32 of the opposing side walls 8 are obliquely inclined towards each other as best seen in Figure 5. Pressure of bolts on each of the bolt support platforms 18 thus causes the clamping member to flex elastically around the cable 26,26' whereby the spring portions 32 are slightly straightened during the bending process. The straightening of the curvature of the spring portions 32 provides additional resiliency to the clamping member that enables a more constant contact pressure between the conductor 26 and the support connector 30 taking into account slight deformation of the elements over time, particularly if they are of aluminium alloy. Furthermore, the metal of the clamp can be provided from an stainless steel that does not undergo any significant stress relaxation when subject to mechanical and cyclic thermal stresses to which the connector is subject (contrary to aluminium) . The steel clamp maintains its resilient properties even under stress and large temperature changes. It would also be possible to provide the side walls as planar, wherein the long and relatively high side walls stamped from a stainless steel provides a relatively large elastic deformation of the clamping portion. The provision of indents or no indents thus depends on the degree of elastic deformation that is desired. The side walls therefore provide the clamping member with a bending spring portion whether they are planar or non-planar. A further important advantage of the enhanced elasticity of the clamp is to prevent disconnection of the bolt. In certain applications the connector may be subjected to mechanical vibration that could loosen the bolt. It is therefore typical to provide a locking feature in prior art clamping systems. With an elastic compression force provided by the clamp, the bolt can be used without locking system.

Claims

1. An electrical connection clamp (2) for clamping a high power electrical cable (26,26') to a conducting support (3) by means of bolts (21) tightening the clamp towards the conducting support for compressing the cable thereagainst, the clamp comprising a pair of bolt holes (22) and a concave clamping portion (16) arranged therebetween for abutment against the cable, the bolt holes and clamping portion arranged generally along a base wall (4) of the clamp, wherein the clamp is stamped and formed from sheet metal, the clamp further comprising a peripheral (6) wall extending roughly orthogonally from the base wall to form a substantially continuous peripheral enclosure within which nuts (23) or boltheads (19) are receivable in alignment with the bolt holes (22), the peripheral wall defining side walls (8) extending between the bolt holes and extending substantially from the base wall (4) clamping portion (16), the side walls having or defining a spring portion (32) for enhancing the elastic range of bending of the clamping portion.

2. The clamp of claim 1 wherein the spring portion (32) comprises a non-planar portion (33) with respect to the side wall (8) .

3. The clamp of claim 2 wherein the spring portion (32) comprises a substantially smooth arcuate indent.

4. The clamp of claim 3 wherein the indents (33) extend from an upper edge (24) of the peripheral wall towards the clamping portion (16) , the indent extending substantially fully across the clamping portion (16) between the bolt holes (22) .

5. The clamp of any one of claims 2-4 wherein the non- planar spring portions of the side walls are inclined towards each other.

6. The clamp of any one of the preceding claims wherein the peripheral wall (6) has a height (h) greater than the height of a nut (23) or bolthead (19) for positioning above the boltholes (22) .

7. The clamp of any one of the preceding claims wherein the peripheral wall (6) has an upper edge (24) which has a convex shape such that the side walls above the clamping portion of the base wall have substantially said height (h) .

8. The clamp of any one of the preceding claims wherein positioning tabs (30) are struck from the base at either end of the clamping portion proximate the boltholes, for locating the clamp about a cable (26) to be clamped.

9. The clamp of any one of the preceding claims wherein the clamping portion (16) is provided with pressure ribs (28) formed substantially at corners joining the base wall with the side walls, the ribs projecting towards a cable to be clamped for enhancing local clamping pressure thereagainst.