GB2120449A - Power cables - Google Patents

Abstract

A high voltage power cable has the dielectric (3,6) made of polyethylene glycol terephthalate (PEGT). Conductive sheaths (2,4) of polyetherester with carbon black are provided and a braided earth 5. PEGT withstands high voltage breakdown and high temperatures better than high density polythene, and has a greater tensile strength than the rest of the cable. <IMAGE>

Description

SPECIFICATION Power cables This invention relates to power cables.

Present underground power cables conventionally use crosslinked polyethylene as the dielectric material to insulate adjacent conductors and to insulate the conductors from earth material in the underground environment.

In an article in EPRI Journal December 1979, page 52, the problems of longterm insulation are highlighted, in particular small flaws which can be detected by laser detecting equipment in cable of the order of fifteen years old and operating at 25 kV. In the same journal on page 55 is a description of a gas insulated high temperature cable system which would hopefully offer a significant cost reduction over systems presently in use. The improvement in performance was to be achieved by using lower-cost components, by operating closer to the intrinsic thermal limit of the insulation system, and by possible introduction of forced cooling.

Although there are obvious advantages to the increased temperature limits associated with gasinsulated systems, a realistic upper limit had to be established so that an optimal balance between hardware cost, installation cost, and cost of losses could be established. Because this upper limit is a sensitive function of the three basic cost components and relative costs may vary significantly in future years, the feasibility of operating at a conductor hotspot temperature of 1500C was examined so maximum design freedom could be retained. The 150doc upper limit was chosen because of general agreement that SF6 can be operated at least up to this level.

Tentative conclusions or results were reached in six major research areas: general design, insulation, enclosures, conductor joints, heat transfer, and prototype development.

For general design, increased temperature limits lead to reduced present-worth cost for underground seif-cooled systems above a certain power level, but the optimized conductor temperatures still remain substantially below 1500C. It appears economically desirable to operate continuously with conductor temperatures in the 100--1100C range. For short-term overload capability, however, systems should be built with a hot spot capability of 1 500 C, even if that temperature is rarely attained.

The insulation researched consisted of 63 insulator base materials and 26 coatings; all were.

tested to find the optimal material for operation at 1500C and 5.905 kV/mm (150 kV1in); Two materials were identified as the best, based on a combination of such properties as resistance to arc tracking, glass transition temperature, dielectric constant dissipation factor, and dielectric strength. Samples of both these insulating materials were exposed to a hotdielectric life test at 1500C and 7.874 kV/mm (200 kV/in) for more than 15,000 hours without failure.

A further article appeared in EPRI Journal December 1979, on pages 15 to 17 discussing the economics between underground power transmission systems and overhead systems. It made the point that cost is becoming an increasingly strong reason to think twice about overhead construction when planning transmission expansion. As demand for electricity grows, greater amounts of power flow from generator to consumer and from an economic standpoint, the best way to push more power over long distances is to use high voltage. Because there ard aesthetic as well as other objections to overhead transmission, some utilities and researchers are planning to increase the small percentage of underground lines instead of constructing additional overhead lines.

However costs of overhead construction are rising. Power is beind transmitted at ever-greater voltages, and voltage determines such costrelated factors as the horizontal spacing between aerial lines and the length of the string of insulators from which the lines are suspended.

In underground power transmission systems High Pressure Oil Filled cables are used as well as the extruded crosslinked polyethylene insulated cables mentioned at the beginning. HPOF cables have limitations, among them a dangerous rise in temperature with severe overload, high losses in the insulation, and excessive charging amounts with increased voltage and distances. Extruded polyethylene cables are easier to handle. They cost less than HPOF cables to produce, install and operate. However the reliability depends on purity of the polyethylene and strict controls are necessary to test the extruded insulation.

According to the present invention there is provided a high voltage power cable comprising at least two conductors of electrically conductive material, such as copper wires, separated from each other by a layer of polyethylene glycol terephthalate acting as the main dielectric material between the two conductors.

In order that the invention can be clearly understood reference will now be made to the accompanying drawings in which Figure 1 is a cross-section of a high voltage power cable embodying the present invention; Figure 2 is a graph of kV/AC versus time for PEGT and for H.D. polythene.

Referring to Figure 1 of the drawings there is shown in cross section of a single-cored high voltage power cable. By "high voltage" we mean 6 kV and upwards. The cable comprises a central conductor 1 of copper or aluminium wires which have been stranded and laid up by conventional techniques. In this embodiment there are forty wires each of 1 8 S.W.G. copper wire.

Extruded over the central conductor is a high strength conductive sheath 2 comprising electrically conducting polyetherester to form a conductor shield. The material used in this embodiment is sold under the trade name Arnitel with carbon black added and is extruded over the conductor.

Over the conductor shield is extruded a layer 3 of polyethylene glycol terephthalate to act both as a dielectric and longitudinal strength member. It has a high voltage breakdown strength and high temperature operational capability better than for example high density polythene. Furthermore it is extremely strong -- stronger than high density polythene and stronger than the conductor it surrounds.

Around the PEGT dielectric is extruded a second conductor shield 4 of electrically conducting polyetherester. The conducting layers improve the high voltage breakdown capability of the cable by providing smooth exactly circular conductive dielectric interfaces with no voids because there is a bond at the interface caused by the heat of extrusion and by the compatibility of the two plastic materials.

Over the conducting layer 4 is laid up an earth layer 5 such as a braid of tinned copper wire.

Over the earth braid 5 is extruded a second layer 6 of PEGT, as sold under the trade name ARNITE to act as a protective sheath and provide additional strength to the cable.

The exact dimensions of the conductor, the dielectric, the protective sheath and conductive shield layers would be chosen according to the particular electrical strength and operational temperature requirements of the cable but in general a cable according to the present invention will be stronger and cope with higher voltages at higher temperature than cables according to the prior art.

Furthermore the extrusion of the second layer 4 of conducting Arnitel would be carried out as a separate operation from the extrusion of the PEGT dielectric 3, whereas with a cable having a high density polythene dielectric and a conducting polythene shield according to the prior art, the two layers need to be extruded concurrently through a special double head extruder which is expensive, difficult to control and relatively slow. With the present invention it is possible to separately extrude the conducting Arnitel using a single head extruder and much higher speed yet nevertheless achieving a void-free bonded interface.

It is also less prone to deformation when the cable is wound on the take up drum -whereas polyethylene tends to stick to itself requiring interface layers of paper on the drum, PEGT does not and so is easier to handie.

The cable described could form part of a multi core cable such as a two core or a three core cable in which case the PEGT outer sheath 6 would not be individual to the core 1; instead the insulated cable core 1, 2, 3, would be laid up with one or more other similar insulated cores to form a composite cable, such as a three core power cable, preferably suitably shaped to fit together as parts of a circle, the PEGT acting as electrical insulation and strength member for the three cores. An outer earth conductor could be applied if desired, followed by the protective Arnite sheath.

Such a cable could operate continuously at over 1 000C, e.g. 100-11 00C with a hot spot capability of 1 500C simply by the replacement of conventional dielectric material e.g. polyethylene, by PEGT as presently proposed, in a cable which is otherwise conventional. The prdvision of PEGT conductor shields provides an added manufacturing advantage.

Referring now to Fig. 2 of the drawings there is shown a graph of kV/AC versus time for a single core incorporating high density polythene insulation and a physically identical core with the high density polythene replaced by polyethylene glycol terephthalate. The test was carried out on small insulated wires immersed in a water tank and with the following dimensions.

Core diameter 0.025 inches Insulation outer diameter 0.035 inches The insulated cores were tested in a conventional high voltage breakdown test environment in a water tank and the test voltage was applied until breakdown occurred, and the time taken to breakdown and the breakdown voltage recorded.

Claims (8)

1. A high voltage power cable comprising at least two conductors of electrically conductive material, such as copper wires, separated from each other by a layer of polyethylene glycol terephthalate acting as the main dielectric material between the two conductors.

2. A cable as claimed in claim 1, wherein at least one of the conductors is in intimate contact with a conductor shield which comprises an electrically conducting polyetherester.

3. A cable as claimed in claim 2, wherein the conductor shield is made of Arnitel (RTM).

4. A cable as claimed in any preceding claim comprising a protective layer around the outer conductor made of polyethylene glycol terephthalate (PEGT).

5. A cable as claimed in claim 4, wherein the PEGT is one sold under the trade name Arnite (RTM).

6. A cable as claimed in any preceding claim, wherein the dielectric layer has a greater tensile strength than any other layer in the cable.

7. A cable as claimed in claim 6, wherein the dielectric layer has a greater tensile strength than either of the conductors.

8. A cable substantially as hereinbefore described with reference to the accompanying drawings.