Patent Description:
In this specification, the expression "high voltage" (HV) indicates voltages equal or greater than <NUM> kV, and lower than <NUM> kV, the expression "medium voltage" (MV) indicates voltages of from <NUM> kV to <NUM> kV, and the expression "low voltage" (LV) indicates voltages of lower than <NUM> kV.

As it is known, the high voltage electricity distribution network infrastructures can comprise a plurality of pipes within which HV cables are installed and which can be buried into the ground in urban areas as for example in the city centers.

The need of modernization of the distribution networks for increasing the capacity of transporting electric energy and guaranteeing high safety standards demands the substitution of the HV cables and the relative pipes.

However, in this regard, the use of retrofit projects are currently growing in oder to avoid the difficult and complex replacement of the pipes, since these projects provide the installation of new HV cables inside existing pipes.

HV three-phase cables generally include three cable cores, each one usually formed by an electrically conductive metal conductor having a circular cross-section and covered by an extruded insulation system. Such HV cable cores include also a metal screen surrounding each insulation system. HV three-phase cables can comprise a water barrier and an armour.

The dimensions of the pipes can be smaller than the new HV cable cross section or, anyway, not sufficiently larger to provide a gap between cable outer diameter and internal diameter of the pipe suitable for a convenient and harmless cable deployment.

There is the need to create more compact HV cables to fit easily into the pipe.

Sector conductors are known to allow building cables with an outer diameter smaller than those comprising conductors with circular cross section, and thus easier to be pulled through existing pipes, as described, for example, in <CIT>.

Each sector conductor has a substantially triangular shape extending, for a three-phase cable, over <NUM>° thus forming a sector of a circle.

<CIT> discloses a method for producing a sector conductor for electric power cables, where "power cables" are low, medium and high-voltage cables with plastic-insulated conductors. The sector conductor can therefore be used for higher voltage power cables, with a corresponding insulation sheath.

<CIT> relates to a metallic conductor composed of a plurality of wires which adopts a predetermined polygonal cross-section. Each conductor was enveloped in a layer of an insulating thermoplastic or thermosetting polymeric material such as polyethylene, polyester, fluorinated polymer, polyolefin, polyamide, polyimide, polyurethane, polyvinyl chloride, thermoplastic elastomer, ethylene-propylene, polychloroprene or silicone rubber to maintain the predetermined cross-section that was imparted to it by a mechanical means of deformation.

<CIT> describes a three-phase cable comprising three cores wherein each core sequentially comprises an electric conductor, an inner semiconducting layer, an insulating layer, an outer semiconducting layer and a metallic layer, a tensile insulating layer and an inner sheath.

<CIT> describes an electric power cable that comprises one or more current conductors, a semi-conductive screen, a semi-conductive layer constituting the core screen, an insulating polymeric layer and a metallic screen.

<CIT> relates to a low voltage power cable comprising four conductors each of sector-shaped cross-section and each consisting of a plurality of compressed cores. The cores of each of the four conductors <NUM> to <NUM> are contained in individual sheaths of electrically insulating plastics material, and the conductors are all enclosed by a common plastics material sheath.

<CIT> states that sector cables, also known as segmental conductor cables, are relied upon in systems with voltage ratings from <NUM>,<NUM> to <NUM>,<NUM> volts. In particular, a sector cable comprises an outer protector covering, usually of a polymeric material, which overlies a lead sheath. Immediately under sheath is a semiconducting carbon black tape intercalated with a copper shielding tape. The copper shielding tape, in turn, is placed over cable insulation (usually paper) which is placed about strand shield. Conductor segments are covered with three layers of material in forming the insulation covering. It is an essential feature that all three layers of the insulation covering must be of uncreped paper.

The Applicant has faced the problem of exploiting the advantage of a sector conductor cable having an outer diameter smaller than that of a round conductor cable having substantially the same conductor cross section and substantially the same current capacity.

A sector conductor with a substantially triangular shape has three vertex portions with small curvature radii where charge accumulation occurs. While in the case of LV and MV cable, the charge accumulation results in an electric field gradient easily contained by the insulating layer, in the case of HV cable, the electric field generated at the conductor vertex portions (ranging, for example, from <NUM> kV/mm to <NUM> kV/mm) can be as high as to cause the insulating system perforation.

The Applicant has found that the above problem can be solved by making a three-core cable wherein each core comprises an electric conductor with a substantially triangular shaped cross-section surrounded by an insulating system made of an extruded polymeric material with a dielectric constant comprised from <NUM> to <NUM>.

The Applicant has found that such an insulating system is capable of bearing high electric field gradient without having its performance impaired even at the substantially angular portions.

Furthermore, the Applicant has observed that the metallic screen surrounding the insulating system of each core may be used to protect each core against any mechanical stresses and/or against the penetration of water and to discharge towards the electric ground possible short circuit currents.

Therefore, according to a first aspect, the present disclosure relates to a high voltage three-phase cable comprising:.

As apparent to a skilled person, the vertex portion of each electric conductor and, accordingly, of the insulating system surrounding it are rounded. This feature and the radius of rounded vertex portion depends upon various factors, for example upon the manufacturing apparatus.

The three electric conductors of the cable are positioned so that a first vertex portion thereof converges towards the longitudinal axis of the high voltage three-phase cable, while second vertex portions thereof are in a radially outer position with respect to the first vertex portion.

In an embodiment, in each electric conductor, the second vertex portions are connected each other by a major edge and each second vertex portion is connected to the first vertex portion by a respective minor edge. In a further embodiment, each electric conductor of the cable has a cross-section which is substantially an isosceles triangle where the first vertex portion defines an angle larger than those defined by the second vertex portions, and the major edge is longer than the minor two edges.

This particular shape of the electric conductors and their arrangement allows to reduce even more the dimensions of the cable.

In a non-claimed comparative embodiment, the insulating system of the present cable has a substantially constant thickness.

According to claim <NUM> of the invention, the insulating system has a thickness variable around the electric conductor cross-section. The thickness of the insulating system has variations substantially constant along the cable length.

In an embodiment, the thickness of the insulating system is greater at the vertex portions of the electric conductor cross-section than at the edges thereof.

In another embodiment, the thickness of the insulating system is greater at the second vertex portions than at the first vertex portion.

In a further embodiment, the thickness of the insulating system is greater at the major edge than at the minor edges.

In this way, the thickness of the insulating system is optimized and takes into account that the electric field gradient depends on the shape of the different portions of the electric conductors. In fact, Applicant found that the electric field gradient that may arise at radially outer parts of the insulating system is higher than that possibily arising in parts neighbouring the cable longitudinal axis.

In an embodiment, the metallic screen can be made of an electrically conductive metal such as copper or aluminium, in form, for example, of braids, wires, helically wound tape or longitudinally folded tape. When the metallic screen is in form made of a longitudinally folded tape, optionally welded, it can act as water-barrier, too.

In an embodiment, the HV cable of the disclosure is suitable for carrying current at <NUM>-<NUM> kV.

In an embodiment, the extruded polymeric material of the present cable is substantially devoid of contaminant particles with a size greater than <NUM> when measured in accordance to ICEA S-<NUM>-<NUM>-<NUM>, Appendix J, and the cable is suitable for carrying current at <NUM>-<NUM> kV. This allows minimizing the insulating system thickness.

It is apparent to the skilled person that the thickness of the insulating system can be calculated as a function of the voltage to be carried.

For the purpose of the present description and of the claims that follow, except where otherwise indicated, all numbers expressing amounts, quantities, percentages, and so forth, are to be understood as being modified in all instances by the term "about". Also, all ranges include any combination of the maximum and minimum points disclosed and include any intermediate ranges therein, which may or may not be specifically enumerated herein.

Also, the terms "a" and "an" are employed to describe elements and components of the disclosure. This is done merely for convenience and to give a general sense of the disclosure. This description should be read to include one or at least one, and the singular also includes the plural unless it is obvious that it is meant otherwise.

As "insulating layer" it is meant a layer made of a material having a conductivity comprised between <NUM>-<NUM> and <NUM>-<NUM> S/m.

As "semiconductive layer" it is meant a layer made of a material having a conductivity comprised between <NUM>-<NUM> and <NUM>/m.

As "radially outer" it is meant that an element or a portion of an element is radially farther from the cable longitudinally axis than another element or a portion of an element.

Further characteristics will be apparent from the detailed description given hereinafter with reference to the accompanying drawings, in which:.

A HV three-phase cable <NUM> according to a first embodiment of the present disclosure is shown in <FIG> and has a longitudinal axis L.

The HV three-phase cable <NUM> comprises three cores <NUM>. Each core <NUM> comprises an electric conductor <NUM>, an insulating system <NUM> comprising an inner semiconducting layer <NUM> surrounding the electric conductor <NUM>, an insulating layer <NUM> surrounding and in contact with the inner semiconducting layer <NUM>, and an outer semiconducting layer <NUM> surrounding and in contact with the insulating layer <NUM>.

A semiconductive water-swellable tape (not illustrated) may be present between the electric conductor <NUM> and the inner semiconducting layer <NUM>.

In an embodiment, the electric conductors <NUM> are made of copper or aluminium, in form of rod or bundled wires. For example, the electric conductors <NUM> are made of wires of class <NUM> or of class <NUM> according to IEC <NUM> (<NUM>).

When electric conductors <NUM> are made in form of rod or bundled wires, no metal tape surrounding the wires is necessary for keeping the the electric conductor <NUM> in shape.

A metallic screen <NUM> (visible in <FIG> which is an enlarged view of a portion of <FIG>) is provided to surround the insulating system <NUM>.

The cores <NUM> are positioned so as to assume the configuration with minimum radial dimension. This can entail the cores <NUM> to be in direct contact one another, though not necessarily. Moreover, in this configuration the metallic screens <NUM> result to be equipotential.

In an embodiment, the space between the three cores <NUM> can be filled with a bedding of polymeric material in form of extruded filler, shaped filler or threads.

In an embodiment, at least one optical fiber and/or at least one ground wire can be positioned in the space between the three cores <NUM>.

In the present embodiment, the power cable <NUM> further comprise a semiconductive tape <NUM> around the three cores <NUM> and a metal water barrier <NUM> surrounding it. The semiconductive tape <NUM> can be made in polyester or in nonwoven fabric, and charged with a semiconductive material such as carbon black, and, optionally, with water-swellable material such as superabsorbent powder. The semiconductive tape <NUM> can have a cushioning function while maintaining the electric contact between the screen of the cable core, and also a water blocking function in the case it contains water-swellable material.

A bedding <NUM> fills the portions between the semiconductive tape <NUM> and the cores <NUM>.

The metal water barrier <NUM> can be made of aluminium or copper. It can be in form of a longitudinally folded foil, welded around the cable cores <NUM> to form a tube. In radial external position to the metal water barrier <NUM>, a sheath <NUM> is provided and can be made of polymeric material like high density polyethylene.

In radial internal position with respect to the sheath <NUM>, an armour (not illustrated) can be present. This armour can be made of a layer of steel wires, for example flat steel wires.

As detailed in <FIG>, an electric conductor <NUM> has a substantially triangular shaped cross-section with vertex portions <NUM>, <NUM>, <NUM> and edges <NUM>, <NUM>, <NUM>.

In particular, the vertex portions <NUM>, <NUM>, <NUM> approximately define angular portions and the edges <NUM>, <NUM>, <NUM> can be substantially linear or curvilinear with a curvature radius substantially greater than those of the vertex portions <NUM>, <NUM>, <NUM>.

In the illustrated embodiments, an electric conductor <NUM> has a first vertex portion <NUM> pointing towards the longitudinal axis L of the high voltage three-phase cable, while second vertex portions <NUM>, <NUM> are in a radially outer position. In each electric conductor <NUM>, the second vertex portions <NUM>, <NUM> are connected by a major edge <NUM> and each second vertex <NUM>, <NUM> portion is connected to the first vertex portion <NUM> by a respective minor edge <NUM>, <NUM>.

In the embodiment of <FIG>, the three cores <NUM> are positioned so that the respective first vertex portions <NUM> face one to each other and converge towards the longitudinal axis L of the HV cable.

According to the present disclosure, the layers of the insulating system <NUM>, <NUM>, <NUM> are made of an extruded polymeric material having a dielectric constant εcomprised from <NUM> to <NUM>.

Extruded polymeric materials suitable for the insulating system of the present cable can be selected from crosslinkable polymeric materials. Such materials generally comprises a polyolefin, for example an ethylene homopolymer or copolymer of ethylene with at least one alpha-olefin C<NUM>-C<NUM>, having a density from <NUM>/cm<NUM> to <NUM>/cm<NUM>, for example from <NUM>/cm<NUM> to <NUM>/cm<NUM>.

In an embodiment, the polymeric material suitable for the insulating system of the present cable has a tanδ of from <NUM>-<NUM> to <NUM>-<NUM>.

In an embodiment, the ethylene homopolymer or copolymer is selected from: low density polyethylene (LDPE) and linear low density polyethylene (LLDPE) having a density from <NUM>/cm<NUM> to <NUM>/cm<NUM>.

The polyolefin can be crosslinked by reaction with an organic peroxide, such as: dicumyl peroxide, t-butyl cumyl peroxide, <NUM>,<NUM>-dimethyl-<NUM>,<NUM>-di(t-butylperoxy)-hexane, di-t-butyl peroxide, or mixtures thereof.

Alternatively, extruded polymeric materials suitable for the insulating system of the present cable can be selected from thermoplastic polymeric materials. In an embodiment, the thermoplastic polymeric material is selected from propylene homopolymers or copolymers of propylene with at least one α-olefin, possibily in admixture with at least one copolymer of ethylene with at least one α-olefin. In an embodiment, the thermoplastic material is in admixture with a dielectric fluid. The dielectric fluid may be selected from mineral oils, for example, naphthenic oils, aromatic oils, paraffinic oils, for example, alkyl benzenes, aliphatic esters; or mixtures thereof.

Suitable thermoplastic polymeric materials for the electrically insulating layer are described, e.g., in <CIT>, <CIT>, <CIT>, <CIT> and <CIT>.

Extruded polymeric materials suitable for the insulating system of the present cable, either crosslinked or thermoplastic, may further comprise an effective amount of one or more additives, selected e.g. from: antioxidants, heat stabilizers, voltage stabilizers, water-tree retardants, processing aids, antiscorching agents, inorganic fillers.

In an embodiment, the extruded polymeric material is substantially devoid of contaminant particles with a size greater than <NUM> according to the measurement protocol ICEA S-<NUM>-<NUM>-<NUM>, Appendix J.

The expression "substantially devoid of contaminant particles with a size greater than <NUM>" means that the number of such contaminants per kg of the extruded polymeric material is equal to <NUM> or less.

In the first embodiment of the present disclosure illustrated in <FIG> and <FIG>, the thickness of the insulating system <NUM>, <NUM>, <NUM> is substantially constant.

In this case the cores <NUM> have substantially the same shape of the electric conductors <NUM>.

<FIG> provides a comparison between the first embodiment and a second embodiment of the present disclosure. <FIG> show two cores <NUM> where the insulating system is sketched as a single structure <NUM>, but in view of the generally very small thickness of the semiconductive layers, it can also be assumed to be the insulating layer <NUM>.

While the thickness of the comparative insulating system <NUM> of <FIG> (first embodiment - not according to the invention) is substantially constant, the thickness of the insulating system <NUM> of <FIG> (second embodiment) is variable around the electric conductor cross-section.

In particular, the thickness of the insulating system <NUM> is greater at the vertex portions <NUM>, <NUM>, <NUM> of the electric conductor <NUM> than at the edges <NUM>, <NUM>, <NUM> thereof.

In a further embodiment, the thickness of the insulating system <NUM> is greater at the second vertex portions <NUM>, <NUM> than at the first vertex portion <NUM>. In a further embodiment, the thickness of the insulating system <NUM> is greater at the major edge <NUM> that at the minor edges <NUM>, <NUM>.

For example, while the thickness of the insulating layer <NUM> of the core <NUM> of <FIG> is of <NUM> all around the conductor cross-section, the thickness of the insulating layer <NUM> of the core <NUM> of <FIG> at the second vertex portions <NUM>, <NUM> is of <NUM>, at the first vertex portion <NUM> it is of <NUM>, at the major edge <NUM> is of <NUM>, and at the minor edges <NUM>, <NUM> is of <NUM>.

The above mentioned thickness values result in a maximum electric gradient of <NUM> kV/mm and a capacitance of <NUM> pF for the core <NUM> of <FIG>, and in a gradient of <NUM> kV/mm and a capacitance of <NUM> pF for the core <NUM> of <FIG> transporting <NUM> kV.

In an embodiment, the cable of the present disclosure can further comprise one or more optical fibers positioned, for example, along the cable longitudinal axis L and/or in the space between the three cores and the water barrier (or the semiconductive tape, if any). In an embodiment, each core of the cable of the present disclosure can further comprise a water-swellable tape between the insulating system <NUM> and the metallic screen <NUM>. For example, the water-swellable tape can be made of a material similar to those disclose for the semiconductive tape between the electric conductor <NUM> and the inner semiconducting layer <NUM> and/or the a semiconductive tape <NUM>.

In an embodiment, the metallic screen <NUM> is made as a welded metallic sheet wound around the insulating system <NUM>, <NUM>, <NUM> of each core <NUM>; in this case the metallic screen <NUM> results to be a barrier against the water.

Alternatively, the metallic screen <NUM> can be made as an overlapped metallic sheet or as a metallic tape.

For example the metallic screen <NUM> is made of aluminium or copper.

In order to understand the advantageous compactness that can be obtained by the HV three-phase cable according to the present disclosure, an examplary embodiment of cable for carrying current at <NUM> kV will be described in the following.

The cores had a substantially triangular shaped cross-section. The thickness of the inner semiconducting layer was <NUM>, the thickness of the insulating layer was <NUM>, the thickness of the outer semiconducting layer was <NUM>. The metallic screen was a copper helical tape having a thickness of <NUM>. An aluminium water barrier (also acting as armour against mechanical stress) had a thickness is <NUM>. The sheath was made of polyethylene and had a thickness of <NUM>.

The diameter of this HV three-phase cable was <NUM> with each conductor dimensions of <NUM>. <NUM> in cross-section, and its weigth was about <NUM>/m.

In comparison, a HV three-phase cable for carrying current at <NUM> KV and having electric conductors with circular cross-section with a diameter of <NUM> each had a diameter of about <NUM> and a weight of <NUM>,<NUM>/m. Such cable had an inner semiconducting layer <NUM> thick, an insulating layer is <NUM>,<NUM> thick, an outer semiconducting layer <NUM> thick. The metallic screen was a copper helical tape having a thickness of <NUM>. An armor made of <NUM> steel flat wires had a thickness of <NUM>. The sheath was made of polyethylene and has a thickness of <NUM>.

Claim 1:
High voltage three-phase cable (<NUM>) comprising:
- three cores (<NUM>) positioned so as to assume the configuration with minimum radial dimension; and
- a sheath (<NUM>) surrounding the three cores (<NUM>);
wherein each core (<NUM>) comprises:
- an electric conductor (<NUM>) having a substantially triangular shaped cross-section with vertex portions (<NUM>, <NUM>, <NUM>) and edges (<NUM>, <NUM>, <NUM>);
- an insulating system (<NUM>) surrounding the electric conductor (<NUM>), the insulating system (<NUM>) comprising an inner semiconducting layer (<NUM>) surrounding the electric conductor (<NUM>), an insulating layer (<NUM>) surrounding and in contact with the inner semiconducting layer (<NUM>) and an outer semiconducting layer (<NUM>) surrounding and in contact with the insulating layer (<NUM>), the layers (<NUM>, <NUM>, <NUM>) of the insulating system (<NUM>) being made of an extruded polymeric material having a dielectric constant ε comprised from <NUM> to <NUM>, the insulating system (<NUM>) having a thickness variable around the electric conductor cross-section, the thickness of the insulating system (<NUM>) being greater at the vertex portions (<NUM>, <NUM>, <NUM>) of the electric conductor cross-section than at the edges (<NUM>, <NUM>, <NUM>) thereof; and
- a metallic screen (<NUM>) surrounding the insulating system (<NUM>).