The structure of electrical power cables may vary according to the voltages used in their intended applications. In general, electrical power cables may be categorized as low voltage, medium voltage, or high voltage. Typically, “low voltage” means a voltage up to 5 kV, “medium voltage” means a voltage of from 5 kV to 46 kV, and “high voltage” means a voltage greater than 46 kV.
Medium and high voltage power cables include four major elements. From interior to exterior, these power cables include at least an electrical conductive element, an electrical insulation layer, a metallic screen or sheath layer, and a jacket. Additional layers may also be present. One example is a semi-conductive conductor shield between the conductive element and the electrical insulation layer. Another example is a semi-conductive insulation shield between the electrical insulation layer and the metallic screen or sheath layer.
In the present description and claims, an “insulated cable core” means the interior of an electrical power cable under the jacket and comprising at least one conductive element, at least one insulation layer, and a metallic screen or sheath layer.
The thickness of each of the layers in an insulated cable core is determined by voltage rating and conductor size and is specified by industry standards such as those published by the Insulated Conductors Engineering Association (ICEA), the Association of Edison Illuminating Companies (AEIC), and Underwriters Laboratories (UL). Electrical cable performance criteria are specified and tested according to AEIC and ICEA standards.
The term “conductive element” may mean a conductor of the electrical type or of the mixed electrical/optical type. An electrical type conductor may be made of copper, aluminum, or aluminum alloy. Also, an electrical type conductor may be either solid or stranded metal, with stranding adding flexibility to the cable. If stranded, the electrical type conductor for medium voltage cables and often also for high voltage cables often includes strand seal to fill its interstices, which helps prevent water migration along the conductor. A mixed electrical/optical type conductor may comprise mixed power/telecommunications cables, which include an optical fiber element in addition to the electrical conductive element for telecommunication purposes.
An inner semi-conductive layer typically surrounds the electrical conductor. The inner semi-conductive layer is most often a semiconducting crosslinked polymer layer applied by extrusion around the conductive element.
Arranged in a position radially external to the inner semi-conductive layer, an electrical insulation layer is usually made of a thermoplastic or thermoset material. Examples include crosslinked polyethylene (XLPE), ethylene-propylene rubber (EPR), or polyvinyl chloride (PVC). The insulation layer may include additives to enhance the life of the insulation. For example, tree retardant additives are often added to XLPE to inhibit the growth of water trees in the insulation layer.
An intermediate semi-conductive layer made, for example, of a semiconducting polymer, can be extruded over the insulation layer. The intermediate semi-conductive layer is usually adhered to the insulation layer by extrusion, or, particularly for certain high voltage cables, may be bonded to the insulation layer by other means.
A metallic shield overlaying the insulation shield may comprise a metallic screen or sheath layer. Usually, metallic screen or sheath layer is made of aluminum, steel, lead, or copper. In general, the metallic screen or sheath layer is a continuous tubular component or a metallic sheet folded on itself and welded or sealed to form the tubular component. More particularly, the metallic shield may be formed, for example, as a longitudinally applied corrugated copper tape with an overlapped seam or welded seam, helically applied wires (i.e. drain wires or concentric neutral wires), or flat copper straps. The intermediate semi-conductive layer is advantageously in electrical contact with the metallic shield.
For the purposes of the present description, the expression “unipolar cable” means a cable provided with an insulated cable core having a single conductive element as defined above, while the expression “multipolar cable” means a cable provided with at least one pair of conductive elements. In greater detail, when the multipolar cable has a number of conductive elements equal to two, the cable is technically defined as being a “bipolar cable,” if there are three conductive elements, the cable is known as a “tripolar cable,” and so on.
In the case of a multipolar cable for medium voltage power transmission or distribution, the conductive elements of the cable, each surrounded by semi-conductive and insulating layers and a metal sheath discussed above, are generally combined together, for example by means of a helical winding of predetermined pitch. The winding results in the formation of a plurality of interstitial zones, which are filled with a filling material. The filling material serves to give the multipolar cable a circular cross section. The filling material may be of conventional type, for example a polymeric material applied by extrusion, or may be an expanded polymeric material.
U.S. Pat. No. 5,281,757, herein incorporated by reference, discloses an example of an insulated cable core for an electrical power cable. In the '757 patent, an electrical power cable has a stranded conductor, a semi-conductive stress control layer around the conductor, a layer of insulation around the stress control layer, a semi-conductive insulation shield layer around the layer of insulation, and an imperforate metal strip with overlapping edge portions around the shield layer. The strip is free to move with respect to the jacket and the shield layer with expansion and contraction of the cable elements with temperature changes. The overlapping edge portions of the strip are bonded together by an adhesive which permits the edge portions to move relative to each other with such temperature changes without creating fluid passageways between the edge portions.
Electrical power cables may include a protective jacket arranged radially external to the insulated cable core. The jacket is typically a polymeric material applied by extrusion.
Any defect in and/or damage to the protective jacket of the cable constitutes a discontinuity in the polymeric layer, which may give rise to problems that reduce, even drastically, the cable's capacity for power transmission and distribution, and also the cable's life. For example, the presence of an incision in the jacket of the cable represents a preferential route for the entry of water or moisture to the interior (that is to say towards the core) of the cable.
The entry of water into a cable is particularly undesirable since, in the absence of suitable solutions provided to stop the leak, once the water has entered, it is able to run freely inside the cable. This particularly causes damages in terms of the integrity of the cable, since corrosion problems (affecting, for example, the armoring, if present, or the metal screen) may arise inside the cable, as well as problems of premature ageing with degradation of the electrical properties of the insulating layer. This phenomenon of premature ageing is better known with the term “water treeing” and is manifested by the formation of micro-fractures of branched shape (“trees”) due to the combined action of the electrical field generated by the passage of current in the conductor, and of the moisture that has penetrated into the insulating layer.
Testing methods used to evaluate the structural integrity of the protective jacket of an electrical cable are called jacket integrity tests. These tests involve installing an electrically conductive or semi-conductive layer placed in a position radially external to the jacket.
One jacket integrity test is known as the DC withstand test and may be conducted according to methods known in the art, such as the ICEA (Insulated Cable Engineers Association, Inc.) Standard S-108-720-2004 for Extruded Insulation Power Cables Rated Above 46 Through 345 kV (Section E5.2). In the test, a semi-conductive coating, such as a layer of graphite in liquid or solid form, is applied to the jacket and serves as a first electrode. The second electrode is represented by the metal component arranged in a radially internal position relative to the sheath to be tested, such as the metal screen or sheath. A DC voltage of about 150 V/mil (6 kV/mm) and up to a maximum of 24 kV is applied between the metallic screen and the semi-conductive layer to verify the integrity of the outer jacket dielectric.
In the absence of defects and/or damages, the jacket is capable of withstanding the voltage applied between the electrodes. That is, in the absence of defects in and/or damages to the jacket, the voltage measured according to a relevant standard at the end of the cable that is opposite to the end at which the DC voltage is applied between the first and second electrodes will be substantially unchanged relative to the applied voltage. This result will occur because the electrical current will be able to pass undisturbed in the semi-conductive coating and in the metal component immediately below the jacket from one end of the cable to the other, apart from a small reduction in voltage due to the resistance of the jacket.
If, however, the jacket has a defect and/or damage such as to create an electrically conductive path in the thickness of the jacket between the electrodes in the test, a short-circuit condition will exist and an overcurrent will be produced. The establishment of the overcurrent condition thus enables a person skilled in the art to confirm the presence of damage to and/or a defect in the protective jacket of the cable.
In general, the DC withstand test of the jacket is performed directly at the production plant after the process for producing the cable. Sometimes, the DC withstand test is also repeated once the cable has been installed, so as to check for any evidence of damage produced in the outer jacket due to the laying operations of the cable. Repeating the testing once the cable has been installed is desirable, especially in the case of underground installations in which the electrical cable is placed directly in the ground without the aid of conduits to contain it.
Graphite has traditionally been used for the outer semi-conductive layer because it can be easily removed at one end of the cable, as is required for conducting the DC withstand test. However, after the cable has been buried, graphite may offer problems during maintenance testing because the graphite is messy and it may have rubbed off during installation.
Instead of applying graphite around the jacket, a thin layer of semi-conductive polymeric material may alternatively be extruded over the jacket. A discussion of various semi-conductive materials can be found for example in the Background section of U.S. Pat. No. 7,208,682, which is incorporated herein by reference for that subject. Typically, the jacket and the outer semi-conductive layer are co-extruded, which bonds them together. As a result, the semi-conductive layer does not buckle due to friction or sidewall bearing forces during installation.
Another benefit to co-extruding the two layers is that the semi-conductive layer can help contribute to sunlight resistance of the cable. Although the semi-conductive layer over the outer cable jacket is not generally relied on for sunlight resistance, depending on its thickness, the semi-conductive layer could impart more sunlight resistance to the cable. Industry standards, for example ICEA S-108-720-2004 (Section 7.3), provide for an extruded semi-conductive layer over the jacket in a thickness up to 20% of the combined wall thickness of the semi-conductive layer and the jacket. Thus, a sufficiently thick semi-conductive layer would be able to impart sunlight resistance to the cable.
While the outer jacket of an electrical power cable is typically black, it is known to make the jacket non-black for particular applications. In these situations, which are more expensive to manufacture, customers request different colored jackets in order to identify one cable from another. When colored jackets are used, a semi-conductive layer is not applied over the jacket, as it defeats the purpose of the colored jacket.
WO 03/046592, which is incorporated by reference, relates to a modified electrical cable in which a semi-conductive polymeric layer is arranged in a position radially external to the outer protective polymeric sheath that coats the cable. In particular, the cable comprises a semi-conductive polymeric layer in a position radially external to the protective polymeric layer. The thickness of the semi-conductive polymeric layer is preferably between 0.05 mm and 3 mm and more preferably between 0.2 mm and 0.8 mm. In the examples, the outer protective sheath is made of MDPE with a thickness of 1.8 mm and is deposited on the cable thus obtained by extrusion; a semi-conductive polymeric layer is deposited on the outer protective sheath, by extrusion, with a thickness of 1 mm. The semi-conductive polymeric layer is disclosed as possibly being a foamed material.
Other cables are known with semi-conductive jackets. For example, U.S. Pat. No. 5,144,098 discloses a conductively-jacketed electrical cable, which provides continuous electrical contact from a drain wire through a metal-coated tape wrapped shield, a semi-conductive adhesive layer applied to the tape on the reverse side from the metal coating, and a semi-conductive outer jacket. The semi-conductive outer jacket is a conductive carbon-filled polymer material such as a thermoplastic fluoropolymer.
U.S. Pat. No. 4,986,372 discloses an electric cable that may include an optional outer jacket, which is substantially cylindrical, and may be composed of either an insulating non-conductive material or a semi-conductive material, for example low density polyethylene, linear low density polyethylene, semi-conducting polyethylene, or polyvinyl chloride.
As mentioned above, the semi-conductive material layer, whether made of graphite or of an extruded polymer material, must be removed at either end of the cable at the beginning of the DC withstand test. Additionally, the semi-conductive layer must be removed from joints and splices.
Applicant has found that the conventional approaches to co-extruding a semi-conductive polymeric layer with the polymeric jacket can lead to problems when removing the semi-conductive material layer to perform the DC withstand test. In particular, Applicant has observed that the jacket and the outer semi-conductive layer lack attributes to make them sufficiently distinguishable from each other to a worker in the field.
The co-extruded jacket and outer semi-conductive layer are both generally black. As discussed above, the jacket may be a color other than black in special circumstances to help distinguish one cable from another, but not when the cable includes an outer semi-conductive layer. The jacket is also black to aid with sunlight resistance. The semi-conductive layer may be black in color from the conductive filler, which is often carbon black. Therefore, due to the color similarity between the jacket and the outer semi-conductive layer, Applicant has found that it is difficult for a worker to distinguish the two layers from each other by sight.
U.S. Pat. No. 6,717,058 discloses a multi-conductor cable with a twisted pair section and a parallel section, wrapped in a transparent plastic jacket to form a generally uniform round-shaped cable. The transparent jacket allows the flat section to be identified so that the jacket may be removed at this location and the conductors in the flat section prepared for attachment to a connector. The cable of the '058 patent is concerned with communication cables having twisted pairs and not with electrical power cables traditionally having a black jacket, required sunlight resistance, or jacket integrity tests.
Accordingly, Applicant has observed that, in the absence of sufficient distinguishing visual characteristics between the jacket and the outer semi-conductive layer of an electrical power cable, a worker may damage the underlying jacket when attempting to remove a portion of the semi-conductive layer to perform a test like the DC withstand one. Damaging the jacket needs to be avoided because, as discussed above, a defect in and/or damage to the jacket can constitute a discontinuity, which may reduce the cable's capacity for power transmission and distribution and the cable's life.
For the purpose of the present description and of the appended claims, 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.
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