GB2110888A - Oil filled electric cable system - Google Patents

Abstract

In an oil-filled electric cable system, a joint between two cables includes an insulator 11 formed with a channel 18 extending to a region adjacent the jointed ends of the cable conductors and serving to facilitate initial impregnation of the joint insulation at that region. Oil feed to the cable in service of the cable system is not carried out through this channel, but is instead carried out directly into the space below the sheath 3 of each cable, for example through a lead 19 extending through the joint casing and into a closure 6 applied to the end of the cable sheath within the joint. Cable conductor 1 is mechanically and electrically connected to a joint ferrule 4 which also closes the end of the oil duct in the conductor. This ensures that little or no oil flows through the joint insulation in service of the system so avoiding weakness in the insulation caused e.g. by conducting particles. <IMAGE>

Description

SPECIFICATION Oil filled electric cable system This invention relates to an oil-filled cable system for carrying very high voltages, and including at least one joint interconnecting lengths of cable in such a system.

In a conventional very high voltage oil-filled system, successive lengths of cable are jointed end-to-end by straight or stop joints and the two ends of the system are terminated by sealing ends.

In the conventional single-core system, a central oil duct is provided in the conductor of each cable length and pressure tanks are connected to appropriate points such as the sealing ends and stop joints, thermal expansion and contraction of the oil during load cycling being catered for by oil flow directly along the conductor ducts to the pressure tanks at the sealing ends and stop joints. In each stop joint, a channel is formed through the joint insulation from each conductor duct to a space between the joint insulation and the inner surface of the joint casing, to provide for direct flow of oil between the external pressure tank and the conductor duct via that joint space.Experience has shown that very occasionally this oil channel may introduce a weakness in the electrical insulation not only when the joint is newly constructed but also especially after a period in service when conducting particles which may have become suspended in the flowing oil may then become deposited on the insulating surfaces of the oil channel.

In accordance with this invention, there is provided an oil-filled electric cable system, comprising at least two lengths of cable joined end-to end by a cable joint, each length of cable comprising a conductor formed with a longitudinal duct, insulation disposed around the conductor and a metal sheath enclosing the conductor and its insulation, the conductor duct being filled with insulating oil which also impregnates the cable insulation and fills a clearance space between the cable insulation and the cable sheath, and the cable joint comprising means electrically interconnecting the conductors of the two lengths of cabie, insulation serving to insulate the cable conductors over the jointing zone, and a casing enclosing the cables and joint insulation over the jointing zone, the joint insulation being formed with at least one channel extending to a region adjacent the jointed ends of the cable conductors and serving to facilitate initial impregnation of the joint insulation at that region, and the cable system further comprising at least one oil-feeding pressure tank arranged to communicate, in service of the system, with the clearance space of at least one cable, the arrangement being such that, in service of the system, oil flow between the oil-feeding pressure tank and the conductor duct of that cable takes place substantially by radial flow through its conductor and insulation and longitudinal flow through its said clearance space.

Thus, in service, there is no or no significant flow of oil through the channel with which the joint insulation is provided, because (as stated above) the oil flow between the oil-feeding pressure tank and the cable conductor duct takes place substantially by radial flow through the cable conductor and insulation and longitudinal flow through the cable undersheath clearance space. Accordingly there is minimal risk of deposition in that channel of any conducting particles suspended in the flowing oil. The cannel with which the joint insulation is provided ensures, during initial impregnation of the joint, that the joint insulation will firstly be thoroughly evacuated, even in the region adjacent the jointed ends of the conductors, and will then be thoroughly oil-impregnated.

In each embodiment to be described herein, the joint is a barrier joint, by which we mean the joint is arranged to form a barrier between the oil systems of the two cables and the in-service oil-feed to each oil system is by communication to the under-sheath clearance space of the respective cable. An oil-feed pipe leads through the joint casing and directly into the under-sheath clearance space of each cable, the end of each sheath being closed to separate the under-sheath clearance space from an adjacent oil-filled space within the joint. Respective said channels extend from respective such joint spaces (one for each cable) to the regions adjacent the jointed conductor ends. The ends of the conductor ducts are closed, in each embodiment to be described, and thus there is no communication through said channels for the direct flow of oil into the conductor ducts.Oil feed to the aforementioned joint spaces takes place, in service, radially outwards through the cable conductor and insulation from the conductor duct.

Embodiments of this invention will now be scribed, by way of examples only, with reference to the accompanying drawing, in which: Figure lisa diagrammatic longitudinal section of a barrier joint interconnecting two lengths of cable in an oil-filled electric cable system, and showing one half of the joint, the other half being substantially the same in this example; and Figure 2 is a similar section through a modified barrier joint.

Referring to Figure 1 of the drawings it will be understood that two lengths of very high voltage cable are jointed end-to-end by the barrier joint, although only one cable and one half of the joint are shown. Each cable (for carrying say 400 kV) compris es a conductor 1 formed with a central duct, insulation 2 (including insulating layers and inner and outer screening layers) disposed around the conductor, and a corrugated aluminium sheath 3 enclosing the conductor and its insulation, with a small clearance space remaining under the sheath between the insulation and the sheath itself. Insulat ing oil fills the conductor duct and impregnates the cable insulation and fills the under-sheath clearance space.

The sheath and various insulating layers are cut-back from the end of the cable conductor in conventional manner and the bared ends of the two cable conductors are electrically and mechanically interconnected by a joint ferrule 4. Each conductor duct is in fact closed, in the example shown, by a plug 5. An insulator 11 and ferrule 4 form a barrier between the oil systems of the two cables. The cut-back end of each sheath is closed by a tubular closure 6 which is sealed at one end around the sheath itself and at its other end around the cable insulation.The joint includes insulation 7 applied around the cable over a portion from which some or all of the cable insulation has been removed for jointing purposes, and also a preformed paper tu be insulator 8 (one for each cable being provided) which extends longitudinally outwards to surround the un-cut cable insulation over a certain length thereof. The joint further includes a cast epoxy resin insulator sleeve 9 surrounding the ferrule and having a tubular stress relief electrode 10 embedded therein which electrode is electrically connected to the ferrule. The joint moreover includes a cast tubular epoxy resin insulator 11 (one for each cable) which fits around the paper tube insulator 8 and is inserted into the respective end of the insulator 9.

Paper insulation 12 surrounds the insulators 9 and 11 and these integers are enclosed within a tubular metal joint sleeve 13, which is provided at each end with a closure plate 14 to which the outer end of the insulator 11 is secured. At each end of the joint a tubular metal end cap 15 is provided, being secured and sealed at its inner end to the closure plate 14 and at its outer end to the cable sheath 3. At the end of the joint shown, the end cap comprises two parts secured to opposite sides of an annular insulator 15a.

For initial impregnation of the joint, it is first necessary to evacuate the joint. At each end of the joint, the space 16 within the respective end cap and the respective paper insulation 7 and 8 are evacuated through a port 17 ofthe end cap. In order to ensure that the paper insulation 7 and 8 is thoroughly evacuated, even at the region remote from the joint space 16 and port 17 (i.e. the region adjacent the end of the cable conductor), the joint insulation is provided with a channel extending from each joint space 16 to the region in question. In the example shown, the channel is provided at 18 (for example by means of a longitudinal groove formed in the inner surface of insulator 11) between the insulator 11 and the papertube insulator 8, and extends from the joint space 16 to the inner end of the insulator 8.For example, four such channels may be provided.

Following the evacuation step, the joint is impregnated with oil and each space 16 and its associated insulation 7 and 8 are impregnated through port 17 and thorough impregnation is ensured with the channel 18 providing for flow of oil to the region of insulation adjacent the inner end of the cable conductor. The insulator 12 and space 21 within the central sleeve 13 are evacuated and then oil-filled by way of a port 22.

The joint furthermore comprises a pipe 19, leading from a port 20 on the end cap 15, into the sheath closure 6 and serving to provide direct communication of oil, in service of the system, from an external pressure tank and into the under-sheath clearance space of the respective cable. Thus, in service, oil feed from the pressure tank takes place directly into the under-sheath clearance space (which is separated from the respective joint space 16 by the closure 6) and from thence to the conductor duct by longitudinal flow in the under-sheath clearance space and radial flow through the cable insulation and conductor. In-service oil flow within the joint takes place from the conductor duct and radially outwards through the conductor and its insulation over the length of cable between the inner end of closure 6 and the ferrule 4.There is no or no significant oil flow through each channel 18, and the channel does not in any event provide direct communication into the conductor duct because the end of the latter is closed by the plug 5.

The joint space 21 in the example shown is in communication with the oil system of the other length of cable and this is fed, not at the joint shown, but at the joint at the opposite end of that other length of cable. Each length of cable in the system will be provided with a feed of oil from a pressure tank, but the feed is not necessarily effected at either of the joints for that length of cable and may instead be into the under-sheath clearance space at some other point along the cable length, for example directly through an aperture formed in the cable sheath itself specifically for this purpose.

Referring to Figure 2 of the drawings, a modified barrier joint is shown, and components in common with the components of Figure 1 are denoted by like reference numerals. In this modification, the channel or channels 18 communicate with the joint space 16 by passing radially outwards of an annular stress control electrode 11 a which is provided in the outer end of the insulator 11, instead of passing radially inwards of the stress control electrode 11 a as in the joint of Figure 1. Thus in Figure 2 the channel or channels 18 pass through relatively less stressed regions. The insulator 11 in Figure 2 comprises two parts, namely a tubular part or trumpet 11 b having a flared outer end, and a conical part 11 C provided with the electrode 1 1a and fitting into the flared end of the trumpet llbwith the channel or channels 18 extending between these two parts. For example, the channel or channels 18 may be formed as grooves in the inner surface of the trumpet 1 lbfrom its inner end to its junction with the conical part 11 C, and then continue as grooves formed in the outer surface of the conical part 1 c.

Claims (7)

1. An oil-filled electric cable system, comprising at least two lengths of cable joined end-to-end by a cable joint, each length of cable comprising a conductor formed with a longitudinal duct, insulation disposed around the conductor and a metal sheath enclosing the conductor and its insulation, the conductor duct being filled with insulating oil which also impregnates the cable insulation and fills a clearance space between the cable insulation and the cable sheath, and the cable joint comprising means electrically interconnecting the conductors of the two lengths of cable, insulation serving to insulate the cable conductors over the jointing zone, and a casing enclosing the cables and joint insulation over the jointing zone, the joint insulation being formed with at least one channel extending to a region adjacent the jointed ends of the cable conductors and serving to facilitate initial impregnation of the joint insulation at that region, and the cable system further comprising at least one oil-feeding pressure tank arranged to communicate, in service of the system, with the clearance space of at least one cable, the arrangement being such that, in service of the system, oil flow between the oil-feeding pressure tank and the conductor duct of that cable takes place substantially by radial flow through its conductor and insulation and longitudinal flow through its said clearance space.

2. An oil filled electric cable system as claimed in claim 1, in which a closure is applied to the end of tile conductor duct in each cable length within the joint.

3. An oil-filled electric cable system as claimed in claim 1 or 2, in which the joint insulation comprises a cast tubular insulator disposed around the end portion of each cable length, said channel comprising a longitudinal groove formed in the inner surface of said tubular insulator and communicating, at the outer end of said tubular insulator, with a joint space within the joint casing.

4. An oil-filled electric cable system as claimed in claim 1 or 2, in which the joint insulation comprises a two-part cast tubular insulator disposed around the end portion of each cable length, one part being a conical part fitting into the outer end of the other part, and said channel comprising a longitudinal groove formed in the inner surface of said other part from its inner end to its junction with the conical part, and then continuing between the two parts to communicate, at the outer end of the two part insulator, with a joint space within the joint casing.

5. An oil-filled electric cable system as claimed in any preceding claim, in which said oil-feeding pressure tank is connected through the joint casing and directly into the under-sheath clearance space of the respective cable through a closure which is applied to the end of the cable sheath.

6. An oil-filled electric cable system as claimed in any one of claims 1 to 4, in which said oil-feeding pressure tank is connected through the sheath of the respective cable and directly into its under-sheath clearance space, outside the joint.

7. An oil-filled electric cable system substantially as herein described with reference to Figure 1 or 2 of the accompanying drawings.