GB2204747A - Diaphragm for reducing electrical stress in a cable coupling - Google Patents

Abstract

A diaphragm (11) for an electrical cable coupling device comprises an insulating diaphragm body comprising an annular flange (14), a central boss (13), a central electrically conducted column (17) and three screening fins (19) extending from the column (17). The fins (19) are located between three holes (16) in the body through which connecting pins (22) associated with the coupling device extend. The diaphragm (11) is mounted at the junction between two cable halves. The fins (19) are connected to earth through the column (17) which is provided at both ends with a connecting slot surrounded by a spring clip which engages a suitable connector rod inside the casing for this purpose. The earthed fins (19) of the diaphragm alleviate the concentrated electrical stress caused by a reduction in the insulation gap between the high voltage cables, thus reducing the risk of electrical breakdown. <IMAGE>

Description

DIAPHRAGM FOR ELECTRICAL CABLE COUPLING DEVICES The present invention relates to electrical cable coupling devices and in particular to diaphragms for use in these devices.

A problem associated with coupling devices for high voltage (e.g. 3.3 Kv) conductors is voltage stress between conductors or between a conductor and some other metallic component, when a high potential difference exists.

Figure 1 illustrates the basic concept, showing a conductor 1 carrying a voltage 3.3 Kv or greater positioned within a metallic tube 2 at zero potential.

With voltage applied the air insulation between the conductor 1 and the tube 2 is subject to electrical stress at a linear voltage gradient from conductor volts to zero, the dielectric strength of air in volts/mil is known therefore the air insulation thickness between the conductor 1 and the metallic tube 2 can be calculated to prevent electrical breakdown of the air insulation.

Figure 2 shows two metallic tubes 2a, 2b (which are insulated by a moisture proof seal) in the same configuration as Figure 1, but the zero potential control is interrupted by a gap at the joint position of the two tubes 2a, 2b.

Where either tube 2a, 2b is parallel with the conductor 1, the condition shown in Figure 1 still applies.

However at the flange position, the voltage which is controlled by the zero voltage of each side of the joint now tends to centralise the voltage gradient to a central position 3, and there is a concentrated voltage gradient at the point where it rises vertically.

The voltage stress gradient therefore exceeds the dielectric strength of the smaller air gap, and air insulation breakdown will occur. If an insulated seal is used in place of the air gap to prevent insulation breakdown, it would have to be thick enough for the insulation purpose. This may be unacceptable in practice. It would also have to protect against voltage stress concentration at the corners of each joint.

In the case of a cable coupling device or connector, there is a danger of insulation breakdown in this way between the connector pins at the diaphragm which may form a moisture-proof seal between two parts of a coupling device.

It is therefore an object of the present invention to provide a diaphragm for a cable coupling which minimises the risk of insulation breakdown between connector pins.

According to the invention, there is provided a diaphragm for an electrical cable coupling device, the diaphragm being made from a resilient insulating material and comprising an outer annular flange and a central enlarged boss, the diaphragm being formed with a plurality of holes arranged to accommodate connector pins, and the boss including a plurality of radially extending screening fins located between the positions of the holes.

The diaphragm may be located at the interface between two halves of a cable coupling device.

An equivalent theoretical treatment of the invention is shown in Figure 3. Here, the apparatus shown in Figure 2 incorporates a thin section insulated moisture proof seal 4 with an integral electrical screen 5 moulded internally. In-his configuration the screen 5 is at earth potential.

With this arrangement theidesign substantially restores the linear zero voltage potential and the voltage stress gradient to that described in Figure 1 and the insulation of the material for sealing requirements now assumes normal and practical proportions.

By producing an interface arrangement between the two zero volt metallic tubes there can be slight distortion of the voltage stress gradient but this is within practical limits.

Where three conductors are placed within two metallic tubes, as in the case of of the present invention, it has been found by testing that flat screen plates spaced at 1200C interfaced between the conductors, whilst producing slight distortion of the voltage gradient have nevertheless been found effective for voltage gradient stress control within practical limits. This concept is believed to be at the heart of the present invention.

Preferably, the flange has a thickened circumferential region, with an annular groove at the outer periphery. This helps to ensure that effective displacement occurs to prevent the ingress of moisture and dust when it is displaced by pressure between the two mating coupling device surfaces which in turn incorporate grooves for material displacement.

The holes are preferably symmetrically arranged and pass through both the flange-and the boss. The number of holes preferably, corresponds to the number of cable cores to be connected.

At the centre of the boss there may be a low resistance conductive barrel for connection to earth for example by means of an insulated connector rod in each half of the coupling device. The screening fins preferably extend from the barrel and are preferably perforated.

The material of the diaphragm may be any suitable resilient or elastomeric material such as rubber or indeed a synthetic rubber, for example, neoprene.

This arrangement may provide a unique interface electrical screening arrangement around the interconnecting pins at the minimum insulation spacing between cable cores within the two coupling device in halves at the position most sensitive to voltage corona.

The invention may be carried into practice in various ways and one embodiment will now be described by way of example with reference to the accompanying drawings, in which: FIGURE 1 is a vertical section through a diaphragm in accordance with the invention; FIGURE 2 is a plan view of the diaphragm shown in Figure 1, and; FIGURE 3 is a section through a coupling device with the diaphragm of FIGURES 1 and 2 in position.

The diaphragm 11, which is made from a resilient rubber material, is generally circular and comprises an annular flange 12 and an enlarged central boss 13. At the outer periphery of the flange 12 there is a thickened rim 14 which has a peripheral groove 15.

This groove 15 helps to ensure that effective material displacement occurs when the diaphragm 11 is gripped between the two halves 21 of a cable connector.

Three symmetrically arranged holes 16 are formed in the diaphragm 11 to accommodate three connector pins 22 which provide current transmission between the two halves 21 of the connector. The holes extend through the flange 12 and the boss 13.

At the centre of the boss 13 there is a low resistance metallic barrel 17 for connection to earth.

At each end, the barrel 17 incorporates a slot 18 surrounded by a spring clip to provide an effective female socket with a push on connection feature.

Three radially extending perforated metal screening fins 19 are connected to the barrel 17. The fins 19 are wholly contained within the boss 13 and are located between the holes 16.

FIGURE 3 illustrates the assembly of two cable connector halves 21 with the diaphragm 11 in place at the interface connection. In the arrangement shown each half cable connector 21 incorporates various parts essentially to facilitate location of the cable sockets, the cable gland entry, and internal earthing facilities.

The cable gland comprises a frusto cone arrangement 23 and a compression member to form an effective clamp for the cable armour 25 which provides a collective screen for a cable 26 which in turn is connected to earth. The body 27 of each half 21 provides a continuation of the collective screen by being connected to the cable gland. Stiffening rods 28 produce mechanical restraint for a female socket assembly 29 within each half 21. The resulting internal area is filled with a setting type insulation material via a filler plug 31 until totally filled and all air exhausted, enabling a riser plug 32 and the filler plug 31 to be screwed and sealed.

The female socket assembly 29 includes three sockets 33 into which the cable cores 34 are inserted and fixed by means of retaining screws or by crimping.

The three male connector pins 22 enter corresponding female sockets 33 in each of the two halves 21.

A central screening pin in the form of an insulated connector rod 35 is located in the slot 18 in the barrel 17 of each half 21. The central screening pin 35 is connected to the designed internal earth position (by screws or other means) which forms part of the cable coupler body 27.

The two halves 21 of the coupling device are then connected together face to face and by means of bolt tightening pressure the resilient rubber diaphragm 11 is secured into place.

Claims (12)

CLAIMS:

1. A diaphragm for an electrical cable coupling device, the diaphragm being maderom a resilient insulating material and comprising an outer annular flange and a central enlarged boss, the diaphragm being formed with a plurality of holes arranged to accommodate connector pins, and the boss including a plurality of radially extending screening pins located between the positions of the holes.

2. A diaphragm as claimed in Claim 1, in which the flange has a thickened circumferential region with an annular groove at the periphery.

3. A diaphragm as claimed in Claim 1 or Claim 2 in which the holes are symmetrically arranged and passed through both the flange and the boss.

4. A diaphragm as claimed in any preceding claim in which, at the centre of the boss there is a low resistance conductive barrel for connection to earth.

5. A diaphragm as claimed in any preceding claim in which the screening pins extend from the barrel.

6. A diaphragm as claimed in any preceding claim in which the fins are perforated.

7. A diaphragm as claimed in any preceding claim made from a resilient or elastomeric material.

8. A diaphragm for an electrical coupling device constructed and arranged substantially as herein specifically described with reference to and as shown in Figures 4 and 5 of the accompanying drawings.

9. An electrical cable coupling device comprising two coupling halves with a diaphragm as claimed in any preceding claim located at their interface.

10. A coupling device as claimed in Claim 9 in which the two coupling device halves include two mating surfaces which incorporate grooves to receive the thickened circumferential region of the flange when it is displaced by pressure between the two mating surfaces.

11. A coupling device as claimed in Claim 9 or Claim 10 including an insulated connector rod in each half of the coupling device for connection to earth and to the conductive barrel of the diaphragm.

12. An electrical cable coupling device constructed and arranged substantially as herein specifically described with reference to and as shown in Figure 6 of the accompanying drawings.