GB2209091A - Extra-high-tension power cable - Google Patents

Abstract

An extra-high tension power cable is provided with extruded insulation over a conductor 11 thereof. The insulation comprises an EPR based on a copolymer formed from at least ethylene and propylene monomers in which copolymer the ethylene is present from about 50 to about 80% by weight. The insulation is extruded in two layers 13 with the extrusion weld lines of the layers being mutually offset. In the inner layer the ethylene is present in the copolymer from about 75 to 80% by weight to optimise electrical strength properties of that layer whereas in the outer layer the ethylene is present in the copolymer from about 50 to 75% by weight to enable dielectric losses to be minimised in that layer. <IMAGE>

Description

EXTRA-HIGH-TENSION POWER CABLE This invention relates to extra-high-tension (EHT) power cables, that is power cables for voltages of 132 kV and above, and the invention is particularly applicable to cables for voltages of 275 kV and above.

A viable insulation for EHT cables should have high electric strength to withstand alternating, continuous and impulse stresses and additionally have a low dielectric loss. Heretofore the insulation for EHT cables has been formed by lapped dielectric tape or extruded dielectric material based on polyethylene which may be crosslinked or un-crosslinked.

Lapped dielectric tape insulation normally comprises windings of cellulose paper tape or alternate windings of cellulose paper tape and polypropylene film tape, the latter being utilised to reduce dielectric losses. The lapped tape is impregnated with a compound or oil and surrounded by a metal sheath which contains the impregnant. Extruded dielectric material based on polyethylene provides an insulation which usually has extremely low dielectric losses and a very high initial electric strength and which requires neither an impregnant nor a containing metal sheath therefor. However, this type of extruded insulation has the disadvantage of having non homogeneous electric strength and exhibiting a rapid decrease in electric strength and increase in dielectric loss when subjected to moisture.

An object of the invention is to provide an extra-high-tension cable with extruded insulation which overcomes at least one of the above-mentioned disadvantages.

To this end the invention includes an extra-hightension power cable provided with extruded insulation over a conductor thereof, said insulation comprising an elastomeric dielectric material comprising copolymer formed from at least ethylene and propylene monomers in which copolymer ethylene is present from about 50 to about 80% by weight.

Said copolymer may be an EPM copolymer (that is a copolymer formed from only ethylene and polypropylene monomers) or for example may be an EPDM terpolymer (that is a terpolymer formed from ethylene, propylene and diene monomers).

In addition to said copolymer said elastomeric dielectric material may typically include a polyethylene, at least one filler (for example kaolin, zinc oxide and/or lead oxide), an antioxidant and an extrusion lubricant, the resulting material forming an ethylene propylene rubber (EPR). The proportions of these ingredients which are mechanically mixed with the copolymer may advantageously be chosen to minimise dielectric losses.

Preferably the insulation is extruded in two layers such that the extrusion weld lines of the layers are mutually offset, for example by about 900.

Since electrical discharge paths through extruded insulation normally occur through the extrusion weld lines, the offsetting of these lines in the two layers increases the critical path for electrical discharges.

When ethylene is present in the copolymer from about 75 to 80% by weight the elastomeric dielectric material is found to have its highest values of electric strength.

When the ethylene is present in the copolymer from about 50 to 75% by weight, the elastomeric dielectric material is found to have high values of electric strength and to exhibit low dielectric losses.

Advantageously, when the insulation is extruded in two layers as aforesaid, in the inner of the said two layers the ethylene is present in said copolymer from about 75 to 80% by weight. Thus the layer nearer the conductor is formed with a material chosen for its high electric strength since this is the region of the insulation which is subjected to the highest electrical stresses. Further, advantageously, in the outer of said two layers the ethylene is present in said copolymer from about 50 to 75% by weight. Thus, the layer of the insulation which is not subjected to the highest electrical stress can be formed with a material chosen primarily for its low dielectric losses.

The invention also includes an extra-high-tension power cable provided with extruded insulation over a conductor thereof, which insulation is extruded in two layers such that the extrusion weld lines of the layers are mutually offset.

In order that the invention may be well understood, an embodiment thereof, which is given by way of example only, will now be described with reference to the accompanying drawings, in which: Figure 1 shows in plan and section a test piece for testing the electric strength of an elastomeric dielectric material; Figure 2 is a graph showing how the electric strength of an EPM resin and an EPR containing an EPM resin vary with the quantity of ethylene in the copolymer chain of the EPM, the electric strength in kV/mm being represented along the ordinate and the percentage by weight of ethylene in the copolymer being represented along the abscissa; Figure 3 is a graph showing the dielectric losses of an EPR containing an EPDM at given temperatures for predetermined percentages of kaolin in the EPR; and Figure 4 is a schematic cross-section of a 345 kV cable.

The inventors of the present invention have developed a test for testing the electric strength of a dielectric material and Figure 1 shows a test piece for such a test. This test piece comprises dielectric material 1, a semiconducting layer 2, a voltageelectrode 3 and an earth electrode 4. Test pieces were made with both EPM resin and an EPR compound containing EPM as the dielectric material with different percentages of ethylene in the EPM. These test pieces were tested to determine the electric strength of the dielectric material and the results are shown in Figure 2 where line 5 is representative of the electric strength of the EPR and line 6 of the EPM material. The results show a direct dependence between electric strength and the quantity of ethylene present in the EPM copolymer, whether it is tested by itself or compounded into an EPR.From these results it is deduced that the best results for electric strength are achieved when there is 80% ethylene and 20% propylene in the EPM or the EPR containing the EPM. However, it is to be understood that according to the application of the dielectric material, adequate electric strength may be achieved when the proportion of ethylene is less than 80%. For example the proportion of ethylene may be only 50% by weight.

It is believed that the relationship between the proportion of ethylene in the EPM and the electric strength of the dielectric material shown in Figure 2 is due to the molecular structure (molecular weight and molecular weight distribution) and crystalline morphology of the material. The above relationship is also found in copolymers formed from ethylene and propylene monomers other than EPM. An example of such an other copolymer is an EPDM terpolymer.

In addition to the copolymer formed from at least ethylene and propylene monomers in the EPR, the other ingredients of the EPR affect the electric strength of the EPR mixture. Other tests carried out using the above described test procedures show that an increase in the electric strength of the EPR mixture is achieved by adding polyethylene, whereas the addition of inorganic fillers and extrusion lubricants had a negative effect on the electric properties of the EPR mixture. However, all of these materials are required in the EPR mixture if it is to be used for extruded insulation in an extra-high-tension power cable in order to make the insulation economically viable, and the actual proportions of these ingredients used are chosen in accordance with the physical and rheological properties required for the EPR mixture.

Other tests have shown that the presence of kaolin in relatively large proportions in the EPR increases dielectric losses, particularly at temperatures above 700C. The results of these tests which were performed on an EPR containing EPDM are shown graphically in Figure 3 where the dielectric loss is plotted against temperature. In Figure 3 the line 7 shows the results where there is 0% kaolin in the EPR and the lines 8, 9 and 10 show the results where there is 13%, 23% and 30% respectively of kaolin in the EPR. Additionally, other tests we have carried out show that the absence of kaolin and the presence of other fillers contribute to the stabilisation of the value of dielectric losses in the temperature range 30 to 10 OOC, which is of particular interest with respect to the operation of extra-high-tension power cables.These tests have shown that it is possible to produce an extra-high-tension power cable provided with extruded EPR insulation which has a low dielectric loss (for example less than or equal to 1.5 x 10-3) and also has high electric strength in view of the molecular structure and crystalline morphology of the copolymer in the EPR and the appropriate dosage of the other ingredients thereof.

We have also found that the extrusion process itself is important in determining the final electric strength of the extruded insulation of an extra-hightension power cable. Preliminary research has shown that random electrical discharges do not occur along the cylindrical outer surface of extruded dielectric material which insulates the conductor. On the other hand, there are preferential zones for electrical discharge paths. It is known that the majority of electrical discharge paths tend to be located in the zones known as extrusion weld lines. The presence of such lines is a complicated phenomenon which is linked to the extruded materials rheology.In the case of extruding a layer over a cylindrical conductor, the features of the extrusion are such that there are two such extrusion weld lines and our research has confirmed that electrical discharge paths preferentially occur in the regions in or adjacent these lines.

Heretofore, attempts have been made to overcome the problem caused by the presence of such extrusion weld lines in extruded insulation by devices which mix the material being extruded in the extrusion head so that the flow junctions of the extruded flows of material do not form extrusion weld lines which would give rise to preferred discharge paths. In the embodiment of the present invention shown in Figure 4, rather than utilise such mixing devices in an extrusion head, the insulation is extruded in two layers 13 over the conductor 11 of the cable such that the extrusion weld lines 12 of the layers are mutually offset.As illustrated, the extrusion weld lines of the two layers are mutually offset by 900 and this maximises the critical path for any electrical discharge since the preferred path for discharges through the first layer are in or adjacent its weld lines and the preferred paths for electrical discharges in the outer layer are in or adjacent its weld lines. Also illustrated in Figure 4 are screening layers 14 disposed radially inwardly and outwardly of the extruded insulation.

The extrusion of the insulation in two layers not only allows the extrusion weld lines to be mutually offset as discussed above, it also enables the extrusion to be formed from two different EPR's. As will be appreciated from the above discussion, the use of a copolymer formed from at least ethylene and propylene monomers in which copolymer the ethylene is present in a quantity to maximise electric strength in the EPR produces an EPR in which the dielectric losses are not minimised due to the interaction of the copolymer with other ingredients in the EPR.However, by extruding the insulation in two layers, the inner of the two layers, i.e. the layer nearer the conductor, can be chosen for its high electric strength properties, and for the outer of the two layers, in which layer the electric strength is not so critical, the insulation can be chosen for its low dielectric loss properties and have reduced electric strength properties. Thus, in the embodiment illustrated in Figure 4, in the inner of the two layers the ethylene is present in the copolymer from about 75 to 80% by weight, whereas in the outer of the two layers the ethylene is present in the copolymer from about 50 to 75% by weight. Advantageously the two layers are co-extruded over the conductor.

The extra-high-tension power cable illustrated in Figure 4 has the same advantages as prior lower tension cables which have EPR extruded insulation, that is to say, flexibility, resistance to partial discharges, resistance to ozone and high resistance to 'treeing' and additionally better electrical performance when subjected to moisture.

Claims (15)

CLAIMS:

1. An extra-high-tension power cable provided with extruded insulation over a conductor thereof, said insulation comprising an elastomeric dielectric material comprising a copolymer formed from at least ethylene and propylene monomers in which copolymer the ethylene is present from about 50 to about 80% by weight.

2. A cable as claimed in claim 1, wherein said copolymer is an EPM copolymer.

3. A cable as claimed in claim 1, wherein said copolymer is an EPDM terpolymer.

4. A cable as claimed in claim 1, 2 or 3, wherein said elastomeric dielectric material includes, in addition to said copolymer, a polyethylene, at least one filler, an antioxidant and an extrusion lubricant.

5. A cable as claimed in any one of claims 1 to 4, wherein said insulation is extruded in two layers such that the extrusion weld lines of the layers are mutually offset.

6. A cable as claimed in claim 5, wherein the extrusion weld lines of the two layers are mutually offset by about 900.

7. A cable as claimed in any one of claims 1 to 6, wherein the ethylene is present in said copolymer from about 75 to about 80% by weight.

8. A cable as claimed in any one of claims 1 to 6, wherein the ethylene is present in said copolymer from about 50 to about 75% by weight.

9. A cable as claimed in any one of claims 1 to 6, wherein in the inner of said two layers the ethylene is present in said copolymer from about 75 to 80% by weight.

10. A cable as claimed in claim 9, wherein in the outer of said two layers the ethylene is present in said copolymer from about 50 to 75% by weight.

11. A cable as claimed in any one of claims 5 to 10, wherein said two layers are co-extruded.

12. An extra-high-tension power cable provided with extruded insulation over a conductor thereof, which insulation is extruded in two layers such that the extrusion weld lines of the layers are mutually offset.

13. A cable as claimed in claim 12, wherein said extrusion weld lines of the two layers are mutually offset by about 900.

14. A cable as claimed in claim 12 or 13, wherein each of said layers comprises an ethylene propylene rubber.

15. A power cable provided with extruded insulation and substantially as hereinbefore described.