AU2002224765A1 - An insulation system, in particular for electric power cables - Google Patents

Description

An insulation system, in particular for electric power cables

The invention relates to an insulation system, in particular for electric power cables.

A typical insulated electric power cable generally comprises one or more high potential conductors in a cable core that is surrounded by several layers of polymeric materials including a first semiconducting shield layer, an insulating layer, a second semiconducting layer, a metallic wire or tape shield used as ground phase, and- a protective jacket. Additional layers within this construction such as moisture impervious materials are often incorporated.

Polymeric semiconducting shields have been utilized in multilayered power cable construction for many decades. Generally they are used to fabricate solid dielectric power cables rated for voltages higher than 6 kiloVolt

(kV) . These shields are used to provide layers of intermediate resistivity between the high potential conductor and a primary insulation, and between the high potential conductor and the primary ^'insulation and the 1 ground or neutral potentials.

The primary purpose of the semiconduc ing stress control shield between the conductor and insulation within an electrical power cable construction is to ensure the long term viability of the primary solid insulation. The use of extruded semiconducting shields essentially eliminates partial discharge within the cable construction at the interface of the conductive and dielectric layers. Longer cable life is also realized through improvement of the conductor shield interfacial smoothness, which minimizes any localized electrical stress concentration. Polymeric conductor shields with improved smoothness have been demonstrated to extend the cable life through accelerated testing.

Semiconductors for insulation systems' used for electric power cables are normally produced by mixing a polymer with carbon black. When using 35-40 % of the conductive carbon black, the polymer composition obtains a much higher conductivity.

Today polymers such as ethylene vinyl acetate (EVA) , ethylene ethyl acrylate (EEA) and ethylene butyl acrylate (EBA) are preferred in the manufacture of semiconductors. Common for these amorphous polymers are good properties with regard to the high filling of the conductive substance as well as the ability to be extruded. Unfortunately residual polar monomers and other residues are present or develop^' during the compounding, and/or crosslinking can deteriorate the electric properties of the semiconductor.

Furthermore there may be a diffusion of polar volatile substances into the polymeric insulation, thus changing the conductivity and other electrical properties, which is not desirable.

WO 0079543 discloses an electric cable comprising at least one lead-free polymer insulation, wherein the lead- free polymer insulation is made from a specific elastomer terpolymer EPDM composition. The insulation layer of lead-free polymer material is a replacement of lead- containing insulating polymer compositions used in the art and provides the cable with similar properties with respect to the ability to prevent decay of dielectric strength with time and the formation of water trees. It is mentioned that the electric cable may comprise two semiconductive layers with the insulation layer in between the semiconductive layers, wherein all three layers may be based on the same polymer material . As for the other polymers mentioned above, the polymer composition used in WO 0079543 as insulating layer and semiconductive layers may result in diffusion residual polar monomers during the compounding and/or crosslinking or diffusion of polar volatile substances into the polymeric insulation, which may deteriorate the electric properties of the semiconductor.

Consequently there is a need for an insulation system with equal or better electric properties than the known types, but without the drawbacks of the known insulation systems .

One object of the present invention is to provide an alternative insulation system which provides excellent insulating properties.

Another object of the present invention is to provide an insulating system where the semiconducting layers and the insulating layer can be extruded to obtain a smooth surface .

A further object of the present invention is to provide an insulating system suitable for high power cables in which the semiconducting layers and the insulating layer are fully compatible with each other. Yet a further object is to provide an insulating system suitable for high power cables which reduces or even prevents diffusion of polar volatile substances, thereby adding to the stability of the cable conductivity.

These and other objects are achieved by the present invention as defined in the claims.

By the term high power cables is meant cables capable of transmission of above 6 kV or preferably above 32 kV, such as above 72 kV, 150 kV, 400 kV or even more e.g 600 kV.

The invention provides an insulating system by which it is possible to optimise the insulating properties due to the compatibility between the semiconducting layers and the insulating layers of the system. Consequently it is possible to produce electric power cables with insulation systems that are long lasting and have excellent insulation properties.

One aspect the invention relates to an insulation system in particular for electric power cables, which system comprises at least three adjacent layers constituted by - a first layer of a first semiconducting composition,

- a second layer of an insulating composition, and

- a third layer of a second semiconducting composition wherein the first semiconducting composition, the insulating composition and the second semiconducting composition are based on the same polymer material.

The semiconducting and the insulation compositions are based on the same polymer material. This means that the major part of the polymeric substance in the polymer matrix of the system is the same polymer. The amount of this polymer constitutes at least 50, preferably at least 60 e.g from 75 to 100 % by weight of the total amount of polymer exclusive fillers in the polymer matrix of the first and the second semiconducting composition and the insulation composition, respectively.

In the insulation system according to the invention, the polymer material on which the compositions are based is low density metallocene catalysed polyethylene having a density from 0.825 to 0.920 g/cm³. Low density metallocene catalysed polyethylene has excellent properties with regard to electric insulation and has a potential for receiving large amounts of conductive filling material or other filling material due to the low crystallinity of the low density metallocene catalysed polyethylene .

The layers of the polymer composition thus comprise at least 50 %, preferably at least 70 %, more preferably at least 80 %, such as more than 90 % by weight of the total amount of polymer of low density metallocene catalysed polyethylene having a density from 0.825 to 0.920 g/cm³.

Preferably the amount of low density metallocene catalysed polyethylene having a density from 0.825 to 0.920 g/cm³ constitutes at least 50 %, such as at least 60 % or even 80 % by weight of the insulating layer or layers, and the low density metallocene catalysed polyethylene having a density from 0.825 to 0.920 g/cm³ constitutes at least 30 %, such as at least 40 % or even 60 % by weight of the semiconducting layer or layers. In order to make further improvements in the electric qualities in the insulated electric direct current cable according to the invention, the polyethylene in a preferred embodiment therefore comprises one or more oils in an amount up to 5% by weight, and preferably from 0,2 to 2% by weight .

In a preferred embodiment the oil is dielectric oil selected from the group consisting of mineral and synthetic oils, where the synthetic oils are chosen among polyisobutylene, silicon oils and lower molecular PE waxes .

Moreover, it is preferred that the semiconducting compositions comprise carbon black in an amount of 15 to 55 % by weight, preferably in an amount of 30 to 50 % by weight, and even more preferably in an amount of 35 to 45 % by weight. Any types of carbon black normally used for polymerbased semiconductors can be used according to the invention. The skilled person is able to select the optimal type of carbon black for a specific purpose and hereby the best semiconductive properties. Thus the possibility of designing an insulation system with the desired semiconductive properties is obtained.

Furthermore, it is preferred that the polymer material comprises low density metallocene polyethylene having a density from 0.855 to 0.915 g/cm³, more preferably from 0.865 to 0.910 g/cm³. By use of such low density metallocene polyethylene both the semiconducting compositions and the insulating composition achieve good and stable electric properties. It is preferred that the polyethylene is cross-linked, preferably in a degree of 40-95%, more preferably in a degree of 60-85%.

Preferably the polyethylene is cross-linked by use of a radical former.

The radical former is preferably peroxide, more preferably it is dicumylperoxide .

I

In a preferred embodiment of the insulation system according to the invention, the radical former is added in an amount of preferably 0.2 to 5 % by weight, more preferably 1 to 2 % by weight .

Preferably in the insulation system according to the invention the semiconducting compositions have a volume resistance of less than 1000 ohm cm at ambient and working temperature. Hereby the best properties of the insulation system are obtained.

Furthermore it is preferred that the ambient and working temperature of the semiconducting compositions is between 0 and 100°C.

The invention also relates to the use of the insulation system as described above in medium or high voltage power cables for alternating or direct current (AC or DC) .

Moreover, the invention relates to a method of producing an electric power cable with a polymer-based insulation system comprising the steps of extruding a first layer of a semiconducting composition around a conductor, extruding a layer of an insulating composition around the first layer of semiconducting composition, and extruding a second layer of a semiconducting composition around said layer of an insulating composition wherein the first semiconducting composition, the insulating composition and the second semiconducting composition are based on the same polymer material, which is to be understood in such a way that the major part of the polymeric substance in the polymer matrix of the system is the same polymer. The amount of this polymer constitutes from 50 to 100 %, preferably from 40 to 100

%, more preferably from 75 to 100 % by weight of the total amount of polymer in the polymer matrix of the first and the second semiconducting composition and the insulation composition, respectively. The polymer composition of the semiconducting composition and the insulation composition, respectively, should be as described above for the insulation system.

In a preferred embodiment of the method according to the invention, the polymer material substantially consists of low density metallocene polyethylene, i.e. at least 95 % by weight of the polymer is low density metallocene polyethylene with a density below 0,920 g/cm³. This composition appears to have particularly good electric properties and is very suitable for serving as the polymer basis in an insulation system, in particular for electric power cables.

The following example serves to illustrate the invention. Example

A metallocene PE with a density of about 0.885 and a MFI of 3 g/10 min are melted during mixing in a Haake Reocord at 170 °C for 5 min. 37 % by weight of the PE of Carbon Black (Acetylene Black) is added as well as an antioxidant (Irganox 1010) . The composition is blended at 170 °C for 10 min. at a speed of 50 RPM. The compound is granulated. Subsequently a second portion of metallocene PE with a density of about 0.885 and a MFI of 3 g/10 min are used for the production of an insulating layer and a power cable with an insulation system composed of a first semiconductive layer made from the PE/carbon black blending, a second insulating layer of the PE and a third semiconductive layer made from the PE/carbon black blending are produced by triple extrusion and cured continuously in a pressure chamber. A cable with a insulation system according to the invention is then obtained.

Claims (11)

C l a i m s

1. An insulation system, in particular for electric power cables, comprising at least three adjacent layers constituted by

- a first layer of a first semiconducting composition,

- a second layer of an insulating composition, and

- a third layer of a second semiconducting composition wherein the first semiconducting composition, the insulating composition and the second semiconducting composition are of materials comprising at least 50 % by weight of the total amount of polymer of low density metallocene catalysed polyethylene having a density below 0.920 g/cm³

2. An insulation system according to claim 1 wherein the semiconducting compositions comprise carbon black in an amount of 15 to 55 % by weight, preferably in an amount of 30 to 50 % by weight, and even more preferably in an amount of 35 to 45 % by weight.

3. An insulation system according to any one of the claims 1 and 2 wherein the polymer material comprises low density metallocene polyethylene having a density from 0.825 to 0.920 g/cm³, preferably from 0.855 to 0.915 g/cm³ and even more preferably from 0.865 to 0.910 g/cm³.

4. An insulation system according to any one of the preceding claims wherein the polyethylene is cross- linked, preferably in a degree of 40-95%, more preferably in a degree of 60-85%.

5. An insulation system according to claim 4 wherein the polyethylene is cross-linked by the use of a radical former.

6. An insulation system according to claim 5 wherein said radical former preferably is a peroxide, more preferably dicumylperoxide .

7. An insulation system according to any one of the claims 6 and 7 wherein said radical former is added in an amount of preferably 0.2 to 5 % by weight , more preferably 1 to 2 % by weight .

8. An insulation system according to any one of the preceding claims wherein the volume resistance of the semiconducting compositions is less than 1000 ohm cm at ambient and working temperature .

9. An insulation system according to claim 9 wherein the ambient and working temperature of the semiconducting compositions is between 0 and 100°C.

10. Use of an insulation system according to any one of claims 1-10 in medium or high voltage power cables for alternating or direct current (AC or DC) .

11. A method of producing an electric power cable comprising the steps of extruding a first layer of a semiconducting composition around a conductor, extruding a layer of an insulating composition around the first layer of semiconducting composition, and extruding a second layer of a semiconducting composition around said layer of an insulating composition wherein the first semiconducting composition, the insulating composition and the second semiconducting composition are based on low density metallocene polyethylene having a density from 0.825 to 0.920 g/cm³polymer material.