GB2115988A - Insulating bushing - Google Patents

Abstract

An insulating bushing 5, 6, 8 adapted to convey an electrical conductor 4 through the casing (1) of electrical apparatus exposed to the weather comprises a generally tubular insulating member 2 incorporating an exterior portion 5, the said member being enclosed, over at least the major part of its exterior portion, by a plurality of insulating axially-overlapping weather- resistant dished collars 6, 8. The bushing may be made of a relatively cheap non weather-resistant insulating material such as epoxy resin or polyurethane. The collars may be made of EPDM rubber. <IMAGE>

Description

SPECIFICATION Insulating bushing The present invention relates to electrically insulating bushings and the like suitable for conveying electrical conductors through the casing of electrical equipment to the outside atmosphere. The invention is especially applicable to high power, high voltage electrical equipment such as power transformers and switchgear which is situated in the open air and incorporates one or more rigid or semi-rigid metallic conductors which extend outwards through a metallic casing.

A major problem associated with the design of bushings for such equipment arises from the high rated voltages involved, which may range from 12 to 36kV. As a result, very intense electric fields are produced between the conductors and the casing, so that the conductor-bushings must have an extremely high dielectric strength. Hitherto, the only suitable plastic material which has also possessed the necessary weather-resistance has been cyclo-aliphatic resin. Because this material is relatively expensive and cannot easily be moulded into the required shape, it has not proved economical to use this material for the large bushings which are required to withstand voltages of 15kV or more. Consequently hollow, oil-filled porcelain bushings have been generally used on high voltage equipment.Usually a cast metal bushing cap incorporating a sight glass for oil-level indication has been found necessary on bushings of this type. Porcelain bushings are therefore complicated and expensive, and, being relative lyfragile, are susceptible to mechanical damage.

An object of the present invention is to provide a simple weatherproof bushing which is able to withstand high voltages.

According to the present invention, an insulating bushing adapted to convey an electrical conductor through the casing of electrical apparatus exposed to the weather comprises a generally tubular insulating member incorporating an exterior portion adapted to protrude outside the said casing and an interior portion adapted to protrude inside the said casing, the said member being enclosed, over at least the major part of its said exterior portion, by a weather-resistant insulating sheath of resilientflexi- ble material. By a weather-resistant sheath we mean a sheath which is more resistant to the weather than the insulating bushing itself.

Preferably the said sheath is detachable from the exterior portion of the bushing and comprises a plurality of axially overlapping collars extending the length of the exterior portion of the bushing. Preferably the end face of the said exterior portion is covered by an insulating cap of resilient flexible material.

The said collars are preferably dished in order to increase the creepage and flashover lengths and to prevent tracking. The joints between the collars may be sealed with weather-resistant grease.

The use of a plurality of weather resistant collars as described above enables the said tubular member to be made from cast or moulded epoxy resin, polyurethane resin, or other relatively cheap insulating resins. Such materials have excellent insulating properties but generally deteriorate on exposure to the weather. Thus a bushing in accordance with the invention suitable for use at 36kV can be manufactured at a considerably lower cost than an equivalent cyclo-aliphatic-resin bushing.

Preferably the exterior portion of the said tubulr member is in the form of a plain tube, so that bushings suitable for various working voltages may be produced simply by varying the length of the exterior portion and covering it with the appropriate number of insulating collars.

We have found that EPDM rubber, which is a terpolymer of ethylene, propylene, and a nonconjugated diene, is a suitable material for the construction of the insulating collars and end caps.

The interior portion of the said tubular member is preferably of greater diameter than the exterior portion and supports a metal flange which in turn is attachable to the casing of the associated electrical apparatus. The flange is preferably incorporated into the tubular member during the casting or moulding of the latter. The flange may incorporate a collar which extends between the interior and exterior portions of the bushing. A current transformer may conveniently be supported by the combined flange and collar.

One example of a bushing in accordacne with the invention will now be described with reference to the accompanying drawing.

The drawing is a sectional view of a bushing fixed to the casing 1 of a high voltage electrical apparatus (not shown), which may be a transformer or a circuit-breaker, for example. All the parts are generally circular in radial cross section. The bushing comprises a generally tubular insulating member 2 made of epoxy or polyurethane resin incorporating a support member 3 in the form of a combined flange and collar made of cast aluminium. The support member 3 is shrunk onto and bonded to the tubular member 2, and therefore forms a gas-tight seal. A conducting stud 4 extends from the interior of the apparatus through the tubular member 2 and forms a gas-tight seal with the latter. The exterior portion 5 of the tubular member 2 is enclosed within a weather resistant sheath comprising a plurality of axially overlapping dished collars 6 made of EPDM rubber.

It will be appreciated that the length of the exterior portion 5 and the corresponding number of dished collars may be appropriately chosen to suit the potential of the conducting stud 4. For example, the exterior portion may be approximately 500 mm long and 90 mm in diameter, when the rated voltage is 36kV, and is suitably covered by approximately ten dished insulating collars.

The outer end of the bushing shown is covered by an insulating cap 7 of EPDM rubber. A stress cone 8, in the form of an elongated dished collar of poorly conducting plastic material surrounds the portion of the tubular member 2 adjacent the support member 3 and distributes the electric filed in this region.

Stress cones and dished collars are widely used for the termination of cables onto overhead lines and busbars, and their design and construction is therefore well known to those skilled in high voltage electrical engineering. Earth collars 9 and 10 conduct leakage currents to earth, and the collar 10 also compresses the end of the stress cone 8 onto the collar portion of the support member 3. The overlapping portions 11 of the dished collars 6, cap 7, and stress cone 8 are sealed with weatherproof grease (not shown). The stress cone 8 may be dispensed with for voltages of 12kV and below.

A cover plate 12 is optionally supported by the collar portion of the support 3, and in such a case a current transformer (not shown) may be accommodated in the space 13 between the flange of the support 3 and the plate 12. The gap between the support 3 and the plate 12 is preferably sealed with a suitable weather-seal (not shown).

The interior part 14 of the tubular member 2 is provided with a ridged surface and may be surrounded by an atmosphere of sulphur hexafluoride within the apparatus casing. Alternatively the casing may be filled with oil or other suitable insulating fluid.

Claims (12)

1. An insulating bushing adapted to convey an electrical conductor through the casing of electrical apparatus exposed to the weather, comprising a generally tubular insulating member incorporating an exterior portion adapted to protrude outside the said casing and an interior portion adapted to protrude inside the said casing, said member being enclosed, over at least the major part of its said exterior portion by a weather-resistant insulating sheath of weather-resistant insulating material.

2. A bushing according to Claim 1 wherein said sheath is detachable from the exterior portion of the bushing and is composed of resilient flexible material.

3. A bushing according to Claim 2 wherein said sheath comprises a plurality of axially overlapping collars extending the length of said exterior portion.

4. A busbing according to Claim 3 wherein at least one of said collars is dished.

5. A bushing according to any of Claims 2,3 or 4 in which said sheath is composed of EPDM rubber.

6. A bushing according to any of Claims 3,4 or 5 wherein said tubular member has a plain cylindrical outer surface.

7. A bushing according to Claim 2 wherein the end face of said exterior portion is covered by an insulating cap of resilient flexible material.

8. A bushing according to any preceding Claim wherein said interior portion of said tubular member is of generally greater diameter than said exterior portion and is sealed to a metal supporting flange which in turn is adapted to be attached to the casing of associated electrical apparatus.

9. A bushing according to Claim 8 wherein said flange is provided with an integral collar portion which is adapted to support a current transformer on the outside of associated high voltage electrical apparatus.

10. A bushing according to any preceding Claim wherein said tubular member is composed of cast or moulded epoxy resin or polyurethane resin.

11. A bushing substantially as described hereinabove with reference to the accompanying drawing.

12. High voltage electrical apparatus incorporating a bushing as claimed in any preceding Claim.