WO1999029023A1

WO1999029023A1 - Insulated electrical conductor for high voltage use

Info

Publication number: WO1999029023A1
Application number: PCT/EP1998/007733
Authority: WO
Inventors: Peter Carstensen; Torbjörn IMRELL; Åke ÖBERG; Anders Nordström; Gunnar Kylander; Udo Fromm
Original assignee: Abb Ab
Priority date: 1997-11-28
Filing date: 1998-11-30
Publication date: 1999-06-10
Also published as: ZA9810943B; GB9725326D0; DE19882838T1; GB2331871A9; GB2331871A; AU2051199A

Abstract

An electrical conductor for high voltage (10kV-800kV) windings comprises central conductive means and an outer semiconductive layer (18) to which an electrically conductive foil is bonded, for example by means of an actuator (22) reciprocating at an ultrasonic frequency. A grounding wire may then be soldered to the foil or connected thereto by means of a resilient device.

Description

INSULATED ELECTRICAL CONDUCTOR FOR HIGH VOLTAGE USE

The present invention relates to an insulated electrical conductor. More specifically, the invention relates to an insulated conductor, for use in high-voltage windings, having an outer layer of (at least semi-) conductive material which is contacted for grounding purposes . The conductor is intended to be used in large motors, generators and transformers at voltages in excess of 10 kV, in particular in excess of 36 kV, and preferably more than 72.5 kV up to very high transmission voltages, such as 400 kV to 800 kV or higher. In addition, the invention relates to a method of establishing electrical contact with (semiconductive) polymeric material .

A particular conductor which can be used in the invention is shown in cross section in Figure 1. The conductor 10 comprises strands 12, for example of copper, the majority of which are insulated, surrounded by a first conductive layer 14. An insulating layer 16, for example of cross-linked polyethylene (XLPE) surrounds the first conductive layer 14 and is in turn surrounded by a second conductive layer 18.

Whilst the layers 14, 18 are described as "conductive" they are in practice formed from a base polymer mixed with carbon black or metallic particles and have a volume resistivity of between 1 and 10⁵ Ω-cm, preferably between 10 and 500 Ω-cm. Suitable base polymers for the layers 14, 18 (and for the insulating layer 16) include ethylene vinyl acetate copolymer/nitrile rubber, butyl grafted polythene, ethylene butyl acrylate copolymer, ethylene ethyl acrylate copolymer, ethylene propene rubber, polyethylenes of low density, poly butylene, poly methyl pentene, and ethylene acrylate copolymer.

The first conductive layer 14 is rigidly connected to the insulating layer 16 over the entire interface therebetween. Similarly, the second conductive layer 18 is rigidly connected to the insulating layer 16 over the entire interface therebetween. The layers 14 - 16 form a solid insulation system and are conveniently extruded together around the strands 12.

Whilst the conductivity of the first conductive layer 14 is lower than that of the electrically conductive strands 12, it is still sufficient to equalise the potential over its surface. Accordingly, the electric field is distributed uniformly around the circumference of the insulating layer 16 and the risk of localised field enhancement and partial discharge is minimized.

The potential at the second conductive layer 18, which should be zero or ground, is equalized at this value by the conductivity of the layer. At the same time, the conductive layer 18 has sufficient resistivity to enclose the electric field. In view of this resistivity, it is desirable to connect the semiconductive polymeric layer 18 to ground at intervals therealong.

A problem experienced in making electrical contact with polymeric layers is that they expand in use, due to their high thermal expansion coefficient, and also creep under mechanical loading .

It is an aim of the invention to maintain the second conductive layer substantially at ground by providing contact means therefor whilst avoiding damage to this layer.

Accordingly, the present invention provides an electrical conductor for high-voltage windings, comprising central conductive means and an outer semiconductive layer, characterised in that an electrically conductive foil is bonded to a portion of said outer semiconductive layer to facilitate electrical connection therewith. In a preferred embodiment, the central conductive means comprises one or more strands of wire and is surrounded in turn by an inner layer of lower conductivity than the wire, then by an electrically insulating layer and then by the outer layer, which preferably has a higher conductivity than the insulating layer.

The conductive foil is preferably metallic and may in particular comprise silver foil, which, when bonded, is electrically and thermally stable.

A plurality of pieces of foil may be bonded at intervals along the conductor. One or more grounding wires may be soldered, or connected by resilient spring-type devices, to the or each piece of foil.

The foil of the invention can carry high currents, even in the presence of elevated temperatures, strong electric fields and/or mechanical stresses.

The present invention also provides a method of establishing electrical contact with a semiconductive polymeric material, comprising bonding a conductive foil to the surface of the material.

Preferably, the conductive foil is metallic and more preferably, it comprises silver foil.

In a preferred embodiment, the bonding is performed by sandwiching the foil between the surface of the semiconductive polymeric material and an actuator and moving the actuator over the surface of the foil so as to cause plastic deformation at the foil/polymeric material interface. Preferably, the actuator is moved in a reciprocating manner at an ultrasonic frequency. This method is easy to perform at different locations along the conductor and involves only localized and limited heating of the semiconductive polymer. An embodiment of the invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which: -

Figure 1 is a transverse section through a conductor according to the invention, but not showing the conductive foil; and

Figure 2 is a fragmentary longitudinal section showing the foil being bonded to the outer layer of the conductor.

As shown in Figure 2, a piece of silver foil 20 is applied to the second conductive polymeric layer 18. The piece of foil 20 may be any convenient size or shape but advantageously comprises a strip circumferentially surrounding the conductor 10.

In order to bond the foil to the conductor, an actuator 22 is used to press the foil 20 against the polymeric layer 18 and is then set in reciprocating motion, parallel to the surface of the layer, at ultrasonic speed. This causes a localized plastic deformation in the interface between the foil and the conductive polymeric layer 18. Eventually, the foil 20 is bonded to the layer 18, establishing good electrical contact .

The method of the invention only exerts small mechanical stresses on the outer polymeric layer. A further advantage lies in the absence of any chemical reaction. The method is compatible with the extrusion process by which the conductor

(which may be a superconductor) is produced.

The electrical insulation of an electrical conductor according to the invention is intended to be able to handle very high voltages, e.g. up tp 800 kV or higher, and the consequent electric and thermal loads which may arise at these voltages. By way of example, electrical conductors according to the invention may comprise windings of power transformers having rated powers from a few hundred kVA up to more than 1000 MVA and with rated voltages from 3 - 4 kV up to very high transmission voltages of from 400 - 800 kV or more. At high operating voltages, partial discharges, or PD, constitute a serious problem for known insulation systems. If cavities or pores are present in the insulation, internal corona discharge may arise whereby the insulating material is gradually degraded eventually leading to breakdown of the insulation. The electric load on the electrical insulation in use of an electrical conductor according to the present invention is reduced by ensuring that the inner layer of (semi) conductive material of the insulation system is at substantially the same electric potential as conductors of the central electrically conductive means which it surrounds and the (semi) conductive outer layer is at a controlled, e.g. earth, potential. Thus the electric field in the electrically insulating layer between these inner and outer layers is distributed substantially uniformly over the thickness of the intermediate layer. By having materials with similar thermal properties and with few defects in these layers of the insulation system, the possibility of PD is reduced at given operating voltages. The electrical conductor can thus be designed to withstand very high operating voltages, typically up to 800 kV or higher.

Claims

1. An electrical conductor for high-voltage windings, comprising central conductive means and an outer semiconductive layer, characterised in that an electrically conductive foil is bonded to a portion of said outer semiconductive layer to facilitate electrical connection therewith.

2. A conductor according to claim 1, wherein the central conductive means comprises one or more strands of wire and is surrounded, in turn, by an inner layer of lower conductivity than the wire, then by an electrically insulating layer and then by the outer semiconductive layer.

3. A conductor according to claim 1 or 2 , wherein the outer semiconductive layer comprises at least one polymer and carbon black, and has a volume resistivity between 1 and 10⁵ ╬⌐-cm.

4. A conductor according to claim 3 , wherein the resistivity of the outer polymeric layer is between 10 and 500 ╬⌐-cm.

5. A conductor according to claim 1, 2, 3 or 4 , wherein the conductive foil is metallic.

6. A conductor according to claim 5, wherein the conductive foil comprises silver foil.

7. A conductor according to any one of the preceding claims, wherein a plurality of pieces of foil are bonded at intervals along the conductor.

8. A conductor according to any one of the preceding claims, wherein at least one grounding wire is connected to the foil or to each piece of foil.

9. A conductor according to claim 8, wherein the or each grounding wire is soldered to the foil.

10. A conductor according to claim 8, wherein the or each grounding wire engages the foil by means of a resilient device.

11. A conductor according to any one of the preceding claims, characterised in that the central conductive means and outer semiconductive layer are designed for high voltage, suitably in excess of 10 kV, in particular in excess of 36 kV, and preferably more than 72.5 kV up to very high transmission voltages, such as 400 kV to 800 kV or higher.

12. A conductor according to any one of the preceding claims, characterised in that the central conductive means and outer semiconductive layer are designed for a power range in excess of 0.5 MVA, preferably in excess of 30 MVA and up to 1000 MVA.

13. A method of establishing electrical contact with a semiconductive polymeric material, comprising bonding an electrically conductive foil to the surface of the material.

14. A method according to claim 13, wherein the conductive foil comprises silver foil.

15. A method according to claim 13 or 14, comprising a preliminary step of sandwiching the foil between the surface of the polymeric material and an actuator.

16. A method according to claim 15, wherein the actuator is moved over the surface of the foil so as to cause plastic deformation at the foil/polymeric material interface.

17. A method according to claim 16, wherein the actuator is moved in a reciprocating manner.

18. A method according to claim 17, wherein the actuator is moved at an ultrasonic frequency.

19. A method of establishing electrical contact with an outer semiconductive polymeric layer of an electrical conductor, according to any one of claims 13 to 18.

20. A method according to claim 19, including grounding said outer conductive polymeric layer by connecting at least one grounding wire to the foil.