Corona-resistant Solid Dielectric Cable

Abstract

A conductor is surrounded by an inner shielding layer, an insulating layer of solid dielectric material such as polyethylene, and an outer shielding layer of polymeric material which adheres but is unbonded to the insulating layer and which has a resistivity of at least 1.0 million ohms-cm., whereby the outer shielding layer substantially suppresses the effects of corona discharge and can be readily stripped from the insulating layer for splicing or terminating the cable.

Description

This invention relates to electrical cables for high voltage use and more particularly to high-voltage cables of the type in which the insulating layer surrounding the conductor comprises a solid dielectric material such as polyethylene (which may be cross-linked), ethylene propylene, isobutylene isoprene, etc.

High-voltage cables of the above-noted type have been largely successful due to the effectiveness of extruded insulation shields at the inner and outer annular surfaces of the insulating layer. These shielding layers are commonly referred to as "semiconducting" layers; and a primary requisite of the shielding material according to the prior art is that it be of low resistivity, preferably in the order of 100 ohms-cm. but in any case less than about 50,000 ohms-cm. With such insulation shielding employing so-called semiconducting material, the most efficient cable design provides a bond between the shielding layer and the adjacent surface of the solid dielectric layer.

It has been found, however, that under service conditions creating severe corona action at the insulation surface, these prior cables are adversely affected by corona discharge, particularly in that they suffer deterioration or acquire instability as exhibited, for example, by changes in corona measurements or power factor measurements. Moreover, the bond between the shielding and insulating layers makes it difficult to strip the shielding layer from the adjacent layer when splicing or terminating the cable.

An object of the present invention is to provide a cable of the type described which overcomes the above-noted disadvantages.

I have discovered that by providing the outer shielding layer with a resistivity far in excess of that previously considered desirable or even acceptable, this layer can adequately perform its insulation-shielding function while eliminating or at least substantially suppressing the adverse effects of corona discharge, and that the latter advantage will be obtained with the outer shielding layer adhering but unbonded to the insulating layer so that such stripping of this shielding layer can be readily effected.

In a cable made according to the invention, the outer shielding layer comprises a polymeric material having a resistivity of at least 1.0 million ohms-cm. Such resistivity can be considerably higher than this minimum value. In fact, the desired results can be obtained when the resistivity is as high as 100 million ohms-cm. Also, this shielding layer of the new cable, although adhering this shielding layer of the new cable, although adhering to the insulating layer, is free of bonding to the insulating layer. When the shielding layer is formed by an extrudable material, it can be provided with a small degree of adhesion to the insulating layer, while avoiding bonding, by properly controlling the extruding conditions, as is well known in the art,-- such conditions including the extruding temperature and pressure, running speed, curing tube temperature and pressure, rate of cooling, etc.

The composition used for the insulation shield may be compounded from compositions conventionally used for so-called semiconductive shields, such as homopolymer (polyethylene) or a copolymer of ethylene. However, the proportion of carbon-black with which this composition is physically blended, according to the invention, is considerably less than the proportion previously used to provide the desired degree of conductivity. For example, a standard commercial semi-conductive product such as Bakelite's 0580 compound is an ethylene copolymer designed to provide an electrical resistance of about 100 ohm-cm. or less, and which comprises 30-35 parts of carbon-black in a total of 100 parts of blended copolymer (for instance, 35 parts of carbon-black and 65 parts of the copolymer). According to the present invention, however, this compound is modified to provide a resistivity of 1 million ohm-cms. or higher, by substantially reducing the proportion of carbon-black to a value, for example, of 10-20 parts in the total of 100 parts of the blended copolymer.

A further advantage in providing the outer shielding layer with this high resistivity is that its relatively low content of carbon-black eliminates any concern about brittleness. In conventional shielding layers, the amount of carbon-black approaches or is at the practical upper limit in regard to control of flexibility and brittleness.

The shielding function with this high-resistance shielding layer may be considered as involving the movement of electrons under the influence of an energy field to fill or neutralize adjacent ionized atoms. In another sense, the high-resistance shield serves to dissipate the corona (ionization) energy. Another important advantage of this high-resistance shield is that it remains stable (resistance remains essentially unchanged) under heat aging and physical flexure, whereas the lack of this type of stability has been a major problem with prior cables of the type described and has limited their use.

The functioning of the present invention can be further explained by the following, after noting that conventional shielding layers are actual conductive rather than semiconductive, whereas the outer shielding layer of the invention is truly semiconductive. In comparing conductive shielding and semiconductive shielding, the true semiconductor can function more effectively to absorb and neutralize corona discharge energy, because of the relative disposition of electron orbits and energy levels about the atomic nucleus. The electrons of any atom exist in different energy levels, or orbits, about the nucleus. Each orbit can accommodate only a certain number of electrons, so that additional electrons must move to a new outer orbit, and are known as valence electrons. As the orbits of valence electrons interreact to form bonds, their energy levels may split into energy bands called valence bands or conduction bands. In conductive materials, the valence band overlaps the conduction band, so that with the addition of only minute external energy (such as discharge) valence electrons are transferred to the conduction band. With insulators, a very considerable energy gap exists between bands, so that relatively little charge transfer occurs. In semiconductors, the energy gap is moderate enough so that electrons can jump the gap under an external energy force. In terms of corona discharge effects, with true semiconductors corona may be completely absorbed or dissipated in providing the charge-transfer energy required for electrons to jump the gap from valence to conduction band. With conductive shielding, however, charge-transfer of electrons may be easily initiated with only minute additional increments of energy, so that total corona energy is not readily absorbed. Hence the corona discharge is rendered relatively innocuous and harmless with true semiconductors, but not with conductive shielding.

The invention will be described further in connection with the accompanying drawing, in which the single illustration is a cross-sectional view of one embodiment of the new cable.

As illustrated, the high-voltage cable comprises a central conductor 1, which may be solid or stranded copper or aluminum. Immediately surrounding this conductor and extruded thereon is an inner shielding layer 2. Extruded over the shielding layer 2 is surrounding insulating layer 3 of solid dielectric material, such as ethylene propylene or isobutylene isoprene but preferably polyethylene (which may be cross-linked). Immediately surrounding the insulating layer 3 and extruded thereon is an outer shielding layer 4 having a resistivity of at least one million ohm-cm., preferably somewhat greater but not in excess of 100 million ohm-cm. The shielding layer 4 is closely surrounded by the usual metallic sheath 5.

The shielding layer 4 may be compounded in accordance with the example previously described. As previously mentioned, this layer is not bonded to the insulating layer 3 but adheres thereto, so that it can be easily removed incident to splicing or terminating the cable; and it will be understood from the foregoing that the unbonded adhesion can be effected in a conventional manner by controlling the conditions under which the shielding layer 4 is extruded on the insulating layer 3.

The inner shielding layer 2 is preferably well bonded to the surrounding insulation 3, since this has the effect of avoiding corona. Such a bond can be obtained by using a conventional technique incident to extruding the insulation 3 over the layer 2. The composition used for the shielding layer 2 may be conventional; that is, it may have a resistivity in the order of 100 ohms-cm., for example. However, I prefer to use for this layer a composition of high resistivity similar to that used for the outer shielding layer 4. In this way, if the bonding of the layers 2 and 3 is not perfect and voids form at their interface, the conductor shield 2 will perform better than in the case of a conventional shielding layer.

As an example of the effectiveness of the present invention, a length of 69 KV power cable having a conventional insulating layer 3 of cross-linked polyethylene, but with high-resistance shielding layers 2 and 4 as previously described, was load cycled under twice the rated voltage to conductor temperatures up to 85.degree. C. and 105.degree. C. for several months. Although limited corona discharge occurred during the test period, the cable did not show any evidence of significant instability either on the basis of change in corona measurements or power factor measurements. Physical (dissection) examination after testing likewise failed to reveal any evidence of deterioration.

Although I have referred to the shielding layers 2 and 4 as being formed by extrusion, it will be understood that they may be formed by a tape or tapes of the high-resistance polymeric material and which can be applied in lapped or butted fashion.

Claims

I claim:

1. In a high-voltage electrical cable, the combination of a conductor, an inner shielding layer surrounding the conductor, an insulating layer of solid dielectric material surrounding said shielding layer and having an outer surface, and, adhering to said outer surface but free of bonding thereto whereby it is readily stripable therefrom, an outer shielding layer of polymeric material surrounding the conductor and having a resistivity of at least 1.0 million ohms-cm., said insulating and outer shielding layers extending along substantially the entire length of the conductor whereby said outer shielding layer is operable to substantially suppress the effects of corona discharge.

2. The combination according to claim 1, in which said inner shielding layer is bonded to the insulating layer.

3. The combination according to claim 2, in which said inner shielding layer has a resistivity of at least 1.0 million ohms-cm.

4. The combination according to claim 1, in which said outer shielding layer is the homogeneous product of extruding said shielding material around the insulated conductor.

5. The combination according to claim 1, in which said outer shielding layer comprises said shielding material in tape form.

6. The combination according to claim 1, in which said insulating layer is a polymeric material.

7. The combination according to claim 1, in which said insulating layer is polyethylene.

8. The combination according to claim 1, in which said insulating material is an elastomer.