EP0012560A1

EP0012560A1 - Electrical equipment employing insulants

Info

Publication number: EP0012560A1
Application number: EP79302793A
Authority: EP
Inventors: Edward Arthur Burton
Original assignee: DIELECTRIC INTERNATIONAL Ltd
Current assignee: DIELECTRIC INTERNATIONAL Ltd
Priority date: 1978-12-11
Filing date: 1979-12-05
Publication date: 1980-06-25
Also published as: JPS55126911A

Abstract

A cable has a stranded conductor (1) having a central duct (2) filled with insulating oil under pressure. The conductor (1) is surrounded by an oil-pervious taped insulant (4) in turn surrounded by an insulant (6) in the form of particles of solid insulating material in the insulating fluid. The insulating fluid acts as a discharge supressant and is in the form of a thin film.

Description

This invention relates to electrical equipment employing insulants.
Our British Patent Specification No. 1 296 032 discloses electrical equipment having a conductor member surrounded by insulants. The insulant comprises finely divided particles of a solid insulating material in an insulating fluid, which fluid acts as a discharge suppressant which is effectively in the form of a thin film as the fluid e.g. mineral fills the space between adjacent particles. In the embodiments illustrated in that specification means are provided for maintaining the fluid above atmospheric pressure and further means are provided for maintaining the interparticle pressure of the insulating material, that is, for applying a pressure to the insulant which limits movement of the particles. Prevention of particle movement ensures that the fluid remains in the form of a thin fluid. Particle movement tends to occur during thermal cycling and limitation of particle movement permits increased current loading.
It is an object of this invention to improve the transient electrical strength of electrical equipment employing insulants; transient electrical strength is that, e.g., during switching and when lightning strikes the electrical equipment.
It is a further object of this invention to provide improved electrical equipment employing an insulant in which there is a conductive member adjacent to which there is a region of highest electrical stress. It will be appreciated that in a cable there may be only one conductive member for the load current, insulation being provided between that conductive member and earth.
According to this invention there is provided electrical equipment comprising a conductive member surrounded by a taped or extruded insulant in turn surrounded by finely divided particles of solid insulating material in an insulating fluid, which insulating fluid acts as a discharge suppressant and is effectively in the form of a thin film, and means for maintaining the insulating fluid above atmospheric pressure.
In a preferred embodiment of this invention there are provided means for maintaining the interparticle pressure; such means apply pressure to the insulant to inhibit particle movement of the insulant.
If the density of the particles is greater than that of the fluid, gravitational effects may be adequate to prevent particle movement which would otherwise occur in certain types of equipment due to the electrical field, then mechanical means for maintaining the interparticle pressure may not be required.
A suitable tape insulant is fused silica tape or polypropelene/paper tape; if an extruded insulant is used it may, for example, be an extrusion of polypropelene or polytetrafluorethylene.
The solid insulating material may, for example, be quartz, fused silica, mica, glass or porcelain.
The fluid may be a mineral oil, fluorocarbon, silicon oil or sulphur hexafluoride.
Maintaining the interparticle pressure has advantages, for example, to inhibit conductor migration through the particle insulant in the case of a cable. The insulation of such a cable may have good thermal conductivity, low dielectric loss and high transient and quiescent electrical strength.
A supertension cable in accordance with this invention will now be described, by way of example, with reference to the accompanying drawing which is a sectional elevation of the supertension cable.
The illustrated supertension cable is generally similar to that shown in Figure 2 of British Patent Specification No. 1 296 032 and comprises a stranded cable 1 which is hollow, the duct 2 formed within the conductor 1 being filled with mineral oil under pressure and it will be appreciated that oil from the duct 2 can pass through the stranded conductor 1. The oil pressure in the duct 2 is maintained by a pump connected to the duct. The conductor 2 is surrounded by an oil permeable membrane 3 which may consist of paper tape wound onto the conductor 1; the membrane 4 has wound onto it a layer of fused silica tape 4, the tape 4 having a thickness of 100 to 130 microns.
A concentric membrane 5 surrounds the tape 4, the space therebetween being filled with an insulant 6 comprising finely divided particles of a solid insulating material in mineral oil. The oil from the duct 2 cannot only pass through the stranded conductor 1 and the membrane 3 but can also pass through the layer of tape 4 to reach the insulant 6. It should however be noted that the membrane 3 is impermeable to particles and will also act as an electrostatic screen. Membrane 5 is impermeable to oil and particles and is surrounded by an electrostatic screen 7 in turn surrounded by a helically wound layer of metal tape 8; the metal tape which is under tension, exerts an inward radial force on the impermeable membrane 5. The layer of metal tape 8 is surrounded by an impermeable metal sheath 9.
A pressure is applied to the mineral oil in the s duct 2 and this extends to the oil in the insulant 6. The inward force of the metal tape 8 provides the interparticle pressure. The oil within the space between the tape 4 and the membrane 5 is maintained in the form of a thin film by the particles and acts as a discharge suppressant.
The particles may be of any shape but in many applications are desirably spherical for ease of composition.
Various modifications may be made to the illustrated embodiment, for example the layer of metal tape may be replaced by a space containing oil under pressure as in Figure 1 of British Patent Specification No. 1 296 032.
The provision of the layer of fused silica tape in the region of high electrical stress increases the electrical strength under transient voltages and the use of silica tape as opposed to tapes of organic material has the advantage of relatively great thermal conductivity.
The layer of fused silica tape may be replaced by an extruded insulant. An extruded insulant is normally impervious to oil and consequently it will be necessary to connect an oil pump to the space between the membrane 5 and the extruded insulant.
The invention is applicable to electrical equipment other than a cable, for example, a transformer bushing and any two electrical member construction which is asymmetrical.
A bushing in accordance with this invention may comprise a conductor surrounded by a casing and a corona shield of sheath material which surrounds the central part of the conductor and is secured to the casing so as to define a space which is filled with the fluid which acts as a discharge suppressant, above atmospheric pressure, the remainder of the space within the casing being filled with the said insulant, there being formed a passage permeable to the said fluid but not to the particles of insulating material between the first mentioned space and the remainder of the space within the casing.
The particle and fluid insulant is electrically isotropic which has certain advantages in electrical equipment where the electrical stress departs from the radial direction. By way of example only this has the advantage in the jointing and terminating of transmission cables providing economy in length of the accessory since such a composite being isotropic in nature can tolerate axial stresses within the accessory greater than those which are tolerable using a taped construction.

Claims

1. Electrical equipment comprising a conductive member (1) surrounded by an insulant (6) of finely divided particles of a solid insulant in an insulating fluid, which fluid acts as a discharge suppressant and is in the form of a thin film, and means for maintaining the insulating fluid above atmospheric pressure, characterised by a taped or extruded insulant (4) which surrounds the conductive member (1) and is surrounded by the insulant (6).

2. Electrical equipment according to claim 1 characterised by means (8) for maintaining the interparticle pressure of the insulant (6).

3. Electrical equipment according to claim 1 or claim 2 characterised in that the solid insulating material is quartz, fused silica, mica, glass or porcelain.

4. Electrical equipment according to any of claims 1 to 3 characterised in that a taped insulant (4) of fused silica or polypropelene/paper tape is employed.

5. Electrical equipment according to claims 1 to 3 characterised in that an extruded insulant of polypropelene or polytetrafluorethylene is employed.

6. Electrical equipment in the form of a cable and according to any of claims 1 to 4 characterised in that the cable has a stranded conductor (1) having a central duct,(2) filled with the insulant fluid, in that a taped insulant (4) pervious to the insulating fluid is employed, and in that the means for maintaining the insulating fluid above atmospheric pressure comprise a pump connected to the duct (2).

7. A cable according to claims 2 and 6 characterised in that the means for maintaining the interparticle pressure of the insulant (6) comprises a layer of metal tape (8) surrounding an impervious membrane enclosing the insulant (6).