AU598760B2 - Capacitor tubes for high voltage power cables terminations - Google Patents

Description

To: The Commissioner of Patents To: The Commissioner of Patents.

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PF/App/6/84

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I 59

AUSTRALIA

Form PATENTS ACT 1952 COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE Short Title: Int. CI: Application Number: 6 e-4 /g- Lodged: Complete Specification-Lodged: Accepted: Lapsed: 4- 4 '?,ririty Published: This document contains the amendments mad under Section 49 and is correct foi printing.

Related Art: TO BE COMPLETED BY APPLICANT Name of Applicant: Address of Applicant: PACIFIC DUNLOP LIMITED 23rd Floor, 500 Bourke Street, Melbourne, Victoria 3000.

Actual Inventor: JOHN ALBERT ADCOCK Address for Service: Complete Specification CLEMENT HACK CO., 601 St. Kilda Road, Melbourne, Victoria 3004, Australia.

for the invention entitled: "CAPACITOR TUBES FOR HIGH VOLTAGE POWER CABLES TERMINATIONS" The following statement is a full description of this invention, including the best method of performing it known to me:- PF/CPlF/2/80 L7_ 2 o Q 0 0 0000 q 00 CAPACITOR TUBES F, R HIGH VOLTAGE POWER CABLES TERMINATIONS Introduction This invention relates to capacitor tubes for medium and high voltage power cables terminations.

The term "medium and high voltage power cables" hereinafter referred to as "high voltace cables" refers to screened cables rated at 6.6 kv and above of the type defined I. in Australian Standards 1429 of 1985 and 2802 of 1985, also defined in International specifications such as IEC 502.

These cables are used for carrying power for generation and distribution systems. The primary insulation of these cables is conventional and includes an inner and outer semi-conductive screen known as the conductor screen and the e 3 o a a 0o 0 0000o 0 00 0 000 0 000 00: 0 u insulation screen respectively. Contact is made to the insulation screen by earthed screen wires and contact is made to the conductor screen by the main conductor. The term semi-conductive material is used to describe a carbon loaded or some other conductive material loaded polymeric material which has a relatively high conductivity.

Prior Art In Figure 1 there is illustrated a conventional high voltage transformer bushing. The transformer bushing comprises a outer porcelain shell a into which is placed a capacitor bushing b which may be in the form of oil impregnated paper or bakelite. The capacitor bushing grades the voltage stress both radially and axially of the bushing.

This is achieved by building co-axial foils c of decreasing length from the centre into the bakelite or oil impregnated paper bushing b. An outer foil d is earthed to a mounting ring e. The capacitance between each foil is approximately equal and therefore the voltage V between each foil is therefore approximately equal.

20 In Figure 2 there is illustrated a half sectional view of a standard cable termination. The cable comprises, from the centre or core outwardly, a conductor g terminating in a lug h, semi-conductive conductor screen n, a major polyiaeric insulation layer j, a semi-conductive insulator screen m, a wire screen p, an outer sheath q and a weather shield i. Since the cable g is insulated by the polymeric layer j, it is not possible to build in multiple coaxial foils or screens of the kind illustrated in Figure i. In the standard cable termination shown in Figure 2, the cable sheath q is removed and the wire screen p pulled back, semi-conductive insulator screen m is cut back to a position between the wire screen p and the lug h. The area around the termination of the outer semi-conductive insulator screen m is overlayed with a material k known as stress relief. The stress relief is constructed of a material having a high dielectric constant K or high relative permittivity. The s 4 stress relief k is polymeric material of high dielectric constant applied in the form of either a tape or a shrink tube. The outside of the cable is covered with a weather shield 1 of either polymeric material or a liquid filled shell.

The purpose of the stress relief k is to reduce corona at the fringe of the semi-conductive screen and to distribute the voltage field more evenly axially along the insulator. It achieves this by virtue of its higher dielectric constant resulting in a high capacitance in the area of the stress relief. This has the effect of extending the voltage flux field axially of the termination.

In practice however this flux control is inadequate under certain operating conditions especially under abnormal weather conditions.

Summar' of the Invention According to the present invention there is provided a capacitor tube for a high voltage cable termination comprising a shrinkable tube of polymeric material having high relative permittivity arranged to be shrunk onto the end of a high voltage power cable, the tube containing at least one capacitor electrode comprising a pair of radially and axially spaced co-axial rings of conductive or semi-conductive material positioned one on either side of an annular joining wall portion, the electrode being embedded within the polymeric material to form at least two capacitive steps between the tube and the cable to ensure a stepped gradation of the voltage from the axially inner end of the tube to the end of the cable.

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5 In a preferred embodiment the electrodes are constructed of semi-conductive or conductive material having the inherent elasticity to allow the tube to shrink onto the cable end. In one embodiment each electrode is positioned within the tube of polymeric material so that the material acts both as an underlay and an overlay to the electrode.

The invention also includes a method of applying a capacitor tube of the kind described above to a high voltage cable, the cable comprising, radially from the inside, a conductor, a semi-conductive screen, a polymeric insulating layer, a semi-conductive insulator screen, a wire screen and an outer polymeric sheath, the method comprising the steps of: removing a length of the outer sheath from the end of the cable, pulling back the wire screen to the sheath, removing a predetermined length of the *f insulator screen to ensure that a portion of the screen S extends axially past the sheath towards the cable end, applying the capacitor tube to the exposed length of the cable, shrinking the tube onto the end of the cable, and applying an outer weatherproof sheath to the end of the cable.

Description of the Drawings Various embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which:- Figure 1 is a cross-sectional view of a typical capacitor bushing according to the prior art; Figure 2 is a half-sectional view through a conventional cable termination; Figure 3 is a sectional view of a cable termination according to one embodiment of the present invention; Figure 4 is a half-sectional view of another form of cable termination in accordance with the invention; -6- Figure 5 is a half-sectional view of a still further cable termination in accordance with the invention; and Figure 6 is a half-sectional view of a simple form of cable termination in accordance with the present invention.

Description of Preferred Embodiments The embodiments described hereinafter with reference to Figures 3 to 6 of the accompanying drawings concern cable terminations for high voltage power cables. The expression "high voltage power cables" as used herein refers to screened cables rated at 6.6 kv and above of the type defined in Australian Standards 1429/1985 and 2802/1985, also covered in International specifications such as I.E.C. 502. These cables are used for carrying power for generation and distribution systems.

As shown in Figure 3, the cable consists of a number o of layers referred to hereunder in order from o the centre or core of the cable to the exterior. The cable oio comprises a multi-stranded conductor 25 terminating in a lug 21, the conductor 25 is covered in a semi-conductive conductor screen 26. A substantially thick polymeric insulating layer 27 covers the conductor screen 26. The insulation 27 is covered by a semi-conductive insulating screen 28 which is in turn, covered by a wire screen 29. Suitable polymeric outer sheathing 30 completes the cable The capacitor tube 10 for the cable 20 as shown in its simplest form in Figure 6 comprises a tube 11 of polymeric material having a high relative permittivity (high dielectric constant Embedded within the tube 11 is a capacitor electrode 12 of conductive or semi-conductive material. The electrode is formed from two co-axial rings 13 and 14 of differing diameters positioned one on either side of an inclined joining wall portion 15. The inner ring 14 is adjacent the interior surface of the 11 tube whilst the outer t're so, ring 15 is within the tube underneath a layer 17 of~high

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7 SdielectricAme The tube 11 is positioned on the end of i the cable 20 so that at least part of the tube extends over i the end of the insulator semi-conductive screen 28.

i The electrode 12 forms two capacitive steps, one i 5 between the ring 13 and the insulator screen 28 and one f between Lhe ring 14 and the conductor In Figure 3 a plurality of electrodes 12 are i axially spaced within the tube 11 of high dielectric constant so that the adjacent outer ring 13 of one electrode overlaps with the adjacent inner ring 14 of the adjacent electrode defining a gap 19 therebetween which is filled by the dielectric material. The electrodes 12 are also spaced axially along the length of the tube to ensure that there is an even stepped gradation of voltage along the length of the termination from its inner end to the end lug 21 of the cable In this way the electrodes form a plurality of adjacent on capacitors that operate to ensure a graded potential drop along the length of the termination 10. The tube 11 may be 0m covered by an outer sheath which may in turn incorporate 20 weather protective shields of the kind illustrated in Figure cao 2.

In a further embodiment illustrated in Figure 4 the electrodes formed within the tube 41 ofAdielectrc4iat are formed to provide multi-layered capacitors that have the effect of increasing the capacitance at the inner or screen end 43 of the cable 20. This embodiment is used when the capacitance towards the screen end become too high for a single layer capacitor. As can be seen from Figure 4 the axially inner or screen end 43 of the cable 20 is much thicker than the termination end 44 and the thicker end includes a first electrode 40 in the form of a closed U-shaped electrode, when viewed in half section comprising inner 46 and outer 47 spaced rings and a second electrodes 49 in the form of left hand inner and outer rings 50 and 51 and right hand inner and outer rings 52 and 53 joined together via an annular wall Ut

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I -8- K It is understood that a plurality of multi-layered capacitors may be used with the remaining electrodes are in the form of the embodiment illustrated in Figure 3.

In the embodiment illustrated in Figure 5, an overlay and underlay 60 and 61 of high dielectriclmei a is positioned between the inner 14 and outer 13 rings of each electrode 12. This provides an improved stress distribution on the outside of the tube 11.

An important aspect of the present invention is the ability for the tube of hi3h dielectric 4 f to shrink onto the cable end. In one embodiment, the tube is formed preshrunk and a plastics former is inserted into the interior of the tube to expand the tube to ensure that it fits over the cable end. Cnce the tube is placed on the cable end the plastics former can be removed allowing the tube to shrink onto the cable. In another alternative the tube is formed to heat shrink so that once it is placed on the cable end the application of heat will cause the tube to shrink onto the cable end.

The semi-conductive material that forms the electrodes of the capacitors is generally a polymeric material. Both the insulating and semi-conductive material is preferably based on silicon rubber or ethylene pro'ylene rubber or other appropriate polymers or polymeric mixtures containing fillers and other additives to achieve the necessary mechanical and electrical properties. The mineral fillers are added in the right proportions to achieve the i desired relative permittivity or dielectric constant, i.e.

above 2.2 and preferably approaching or exceeding 10. The fillers for semi-conductive materials are preferably carbon i black and/or other additives. If a conductive material is used instead of a semi-conductor, a metal which will shrink with the final assembly such as lead foil is preferable. The construction of the heat shrink or self shrink tube would be as follows. The dielectric 4 material would be in the form of an uncured polymeric cylinder. The semi-conductive or conductive electrodes of the capacitors are made of thin 'S.i 9 o0 0 0 0 0 0000 4., semi-conductive or conductive material which, when "fused" into an integral mass with the dielectric are capable shrinking with the body of the tube. Ideally the conductivity of the electrode material must be high enough so that the effective series resistance of each capacitor assembly is lower than the reactance of each capacitor when subject to the shortest time constant impulse wave. This means that any loss factor of the capacitor assembly due to the series resistance of the electrodes ideally should be less than 0.3 when measured at 500 kHz however, this requirement could be varied depending upon final performance. The electrode material may be in the form of a preformed film, a film or tape cut to size and placed in position or a semi-conductive or conductive paint. The components of the tube would be made to size, assembled together on a suitable mandrel and then pressed into a monolythic mass essentially free of voids. The assembly would then be cured and suitably processed extended and then cooled, to give the correct shrink properties.

The dielectric tube of high K effectively replaces 20 the capacitor bushing as used in transformer terminations and also replaces the stress relief used in standard terminations of the kind illustrated in Figure 2. The capacitance between each capacitor electrode and the capacitance of each electrode to the conductor must be such that the voltage drop between each capacitor elect :ode is approximately equal, thus the axial spacing between each electrode is approximately equal.

The voltage distribution over the length of the assembly will be such that flash over will not occur at the highest test voltage so long as the length of the termination is correct to normal design limits. The correct capacitance and voltage distribution can be achieved in design by grading the amount of overlap between the inner and outer rings of the electrodes. The last electrode would be either capacitively or directly connected to the conductor.

One advantage of the termination of the subject invention is the capacity for one site assembly. To illustrate the mode of assembly reference is made to Figure 3.

_Ili_~.l- 10 The outer sheath (not shown) is removed from the cable 20 and the wire screen 29 is pulled back to the inner end of the termination. The insulation screen 28 is then cut back to a position where it projects axially past the wire screen 29 towards the end lug 21 of the cable. A suitable siliconegel is applied to the exposed insulation 27 of the cable end. The shrink tube 11 is then applied to the cable end either preshrunk in a former or in expanded form prior to heating.

By removing the former (not shown) or heating the tube it is shrunk onto the end of the cable. A suitable outer sheath and weather protector of the kind shown in Figure 2 is then applied to the cable end thereby completing the termination.

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Claims (4)

1. A capacitor tube for a high voltage cable termination comprising a shrinkable tube of polymeric material having high relative permittivity arranged to be shrunk onto the end of a high voltage power cable, the tube containing at least one capacitor electrode comprising a pair of radially and axially spaced co-axial rings of conductive or semi-conductive material positioned one on either side of an annular joining wall portion, the electrode being embedded within the polymeric material to form at least two capacitive steps between the tube and the cable to ensure a stepped gradation of the voltage from the axially inner end of the tube to the end of the cable.

2. The capacitor tube according to Claim 1, wherein each electrode comprises pairs of parallel spaced rings axially spaced on either side of an annular wall portion.

3. The capacitor tube according to either Claim 1 or 2, wherein each electrode is constructed of semi-conductive or conductive material having the inherent elasticity to allow the tube to shrink onto the cable end.

4. The capacitor tube according to any one of the preceding claims, wherein each electrode is positioned within the tube of polymeric material so that the material acts both as an overlay and underlay to the electrode. The capacitor tube according to any one of the preceding claims, wherein the tube is preshrunk onto a plastics former which is removed once the tube is applied to the cable thereby allowing the tube to shrink onto the cable end. 64450/86 12 S The capacitor tube according to any one of Claims 1 5 toAT wherein the tube is heat shrunk onto the cable end. V7. A method of applying a capacitor tube according to any one of the preceding claims to a high voltage cable that comprises a conductor, a semi-conductive conductor screen, a polymeric insulating layer, a semi-conductive insulating screen, a wire screen and an outer polymeric sheath, the method comprising the steps of removing a length of the outer sheath from the end of the cable, pulling back the wire screen to the sheath, removing a predetermined length of the insulator screen to ensure that a portion of the screen extends axially past the sheath towards the cable end, applying the capacitor tube to the exposed length of the cable, 0oo15 shrinking the tube onto the end of the cable, and S applying an outer weatherproof sheath to the end of the cable. Sk-_ A high voltage cable termination substantially as described herein with reference to and as illustrated in Figures 3 to 6 of the accompanying drawings. J1;,9. A capacitor tube substantially as described herein with reference to and as illustrated in Figures 3 to 6 of the accompanying drawings. DATED this 27th day of October, 1986. PACIFIC DUNLOP LIMITED By Its Patent Attorneys CLEMENT HACK CO. Fellows Institute of Patent Attorneys of Australia.