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High-voltage cable

A high-voltage cable (HV cable) is a cable used for electric power transmission at high voltage. A cable includes a conductor and insulation, and is suitable for being run underground or underwater. This is in contrast to a conductor, which does not have insulation. High-voltage cables of differing types have a variety of applications in instruments, ignition systems, and AC and DC power transmission. In all applications, the insulation of the cable must not deteriorate due to the high-voltage stress, ozone produced by electric discharges in air, or tracking. The cable system must prevent contact of the high-voltage conductor with other objects or persons, and must contain and control leakage current. Cable joints and terminals must be designed to control the high-voltage stress to prevent breakdown of the insulation. Often a high-voltage cable will have a metallic shield layer over the insulation, connected to the ground and designed to equalize the dielectric stress on the insulation layer.

A cross-section through a 400 kV cable, showing the stranded segmented copper conductor in the center, semiconducting and insulating layers, copper shield conductors, aluminum sheath and plastic outer jacket.
Like other power cables, high-voltage cables have the structural elements of one or more conductors, insulation, and a protective jacket. High-voltage cables differ from lower-voltage cables in that they have additional internal layers in the insulation jacket to control the electric field around the conductor.
For circuits operating at or above 2,000 volts between conductors, a conductive shield may surround each insulated conductor. This equalizes electrical stress on the cable insulation. This technique was patented by Martin Hochstadter in 1916; the shield is sometimes called a Hochstadter shield. The individual conductor shields of a cable are connected to the ground at the ends of the shield, and at splices. Stress relief cones are applied at the shield ends.
Cables for power distribution of 10 kV or higher may be insulated with oil and paper, and are run in a rigid steel pipe, semi-rigid aluminum or lead sheath. For higher voltages the oil may be kept under pressure to prevent formation of voids that would allow partial discharges within the cable insulation.
Sebastian Ziani de Ferranti was the first to demonstrate in 1887 that carefully dried and prepared paper could form satisfactory cable insulation at 11,000 volts. Previously paper-insulated cable had only been applied for low-voltage telegraph and telephone circuits. An extruded lead sheath over the paper cable was required to ensure that the paper remained absolutely dry.
Vulcanized rubber was patented by Charles Goodyear in 1844, but it was not applied to cable insulation until the 1880s, when it was used for lighting circuits. Rubber-insulated cable was used for 11,000 volt circuits in 1897 installed for the Niagara Falls Power Generation project.
Mass-impregnated paper-insulated medium voltage cables were commercially practical by 1895. During World War II several varieties of synthetic rubber and polyethylene insulation were applied to cables. Modern high-voltage cables use polymers or polyethylene, including (XLPE) for insulation.

AC power cable

High voltage is defined as any voltage over 1000 volts. Cables for 3000 and 6000 volts exist, but the majority of cables are used from 10 kV and upward. Those of 10 to 33 kV are usually called medium voltage cables, those over 50 kV high voltage cables.

Figure 1, cross section of a high-voltage cable, (1) conductor (3) insulation (2) and (4) semiconducting layers (5) outer conductor and outer coat.
Modern HV cables have a simple design consisting of few parts. A conductor of copper or aluminum wires transports the current, see (1) in figure 1. (For a detailed discussion on copper cables, see main article: Copper wire and cable.)
Conductor sections up to 2000 mm2 may transport currents up to 2000 amperes. The individual strands are often preshaped to provide a smoother overall circumference. The insulation (3) may consist of cross-linked polyethylene, also called XLPE. It is reasonably flexible and tolerates operating temperatures up to 120 °C. EPDM is also an insulation.
At the inner (2) and outer (4) sides of this insulation, semi-conducting layers are fused to the insulation. The function of these layers is to prevent air-filled cavities between the metal conductors and the dielectric so that little electric discharges cannot arise and endanger the insulation material.
The outer conductor or sheath (5) serves as an earthed layer and will conduct leakage currents if needed.
Most high-voltage cables for power transmission that are currently sold on the market are insulated by a sheath of cross-linked polyethylene (XLPE). Some cables may have a lead or aluminium jacket in conjunction with XLPE insulation to allow for fiber optics. Before 1960, underground power cables were insulated with oil and paper and ran in a rigid steel pipe, or a semi-rigid aluminium or lead jacket or sheath. The oil was kept under pressure to prevent formation of voids that would allow partial discharges within the cable insulation. There are still many of these oil-and-paper insulated cables in use worldwide. Between 1960 and 1990, polymers became more widely used at distribution voltages, mostly EPDM (ethylene propylene diene M-class); however, their relative unreliability, particularly early XLPE, resulted in a slow uptake at transmission voltages. While cables of 330 kV are commonly constructed using XLPE, this has occurred only in recent decades.

Quality

During the development of HV insulation, which has taken about half a century, two characteristics proved to be paramount. First, the introduction of the semiconducting layers. These layers must be absolutely smooth, without even protrusions as small as a few µm. Further the fusion between the insulation and these layers must be absolute; any fission, air-pocket or other defect - of the same micro-dimensions as above - is detrimental for the breakdown characteristics of the cable.
Secondly, the insulation must be free of inclusions, cavities or other defects of the same sort of size. Any defect of these types shortens the voltage life of the cable which is supposed to be in the order of 30 years or more.
Cooperation between cable-makers and manufacturers of materials has resulted in grades of XLPE with tight specifications. Most producers of XLPE-compound specify an "extra clean" grade where the number and size of foreign particles are guaranteed. Packing the raw material and unloading it within a cleanroom environment in the cable-making machines is required. The development of extruders for plastics extrusion and cross-linking has resulted in cable-making installations for making defect-free and pure insulations. The final quality control test is an elevated voltage 50 or 60 Hz partial discharge test with very high sensitivity (in the range of 5 to 10 picoCoulombs) This test is performed on every reel of cable before it is shipped.

High Tension cables:

Single Core Copper Conductor, XLPE insulated, Copper Wire Screen and HDPE Sheathed 38 / 66 (72.5) kV
Single Core Copper Conductor, XLPE insulated, Lead Sheathed and HDPE Sheathed 38 / 66 (72.5) kV
Single Core Copper Conductor, XLPE insulated, Copper Wire Screen and HDPE Sheathed 76 / 132 (145) kV
Single Core Copper Conductor, XLPE insulated, Lead Sheathed and HDPE Sheathed 76 / 132 (145) kV
Single Core Copper Conductor, XLPE insulated, Copper Wire Screen and HDPE Sheathed 127 / 220 (245) kV
Single Core Copper Conductor, XLPE insulated, Lead Sheathed and HDPE Sheathed 127 / 220 (245) kV

Medium Tension cables:

Single & Three Cores Aluminum Conductors, XLPE Insulated and PVC Sheathed 6/10 (12) kV
Three Cores Copper or Aluminum Conductors, XLPE Insulated, Steel Tape Armored and PVC Sheathed 6/10 (12) kV
Three Cores Copper or Aluminum Conductors, XLPE Insulated, Steel Wire Armored and PVC Sheathed 6/10 (12) kV
Single & Three Cores Copper Conductors, XLPE Insulated and PVC Sheathed 8.7/15 (17.5) kV
Single & Three Cores Aluminum Conductors, XLPE Insulated and PVC Sheathed 8.7/15 (17.5) kV
Three Cores Copper or Aluminum Conductors, XLPE Insulated, Steel Tape Armored and PVC Sheathed 8.7/15 (17.5) kV
Three Cores Copper or Aluminum XLPE Insulated, Steel Wire Armored and PVC Sheathed 8.7/15 (17.5) kV
Single & Three Cores Copper Conductors, XLPE Insulated and PVC Sheathed 12/20 (24) kV
Single & Three Cores Aluminum Conductors, XLPE Insulated and PVC Sheathed 12/20 (24) kV
Three Cores Copper or Aluminum Conductors, XLPE Insulated, Steel Tape Armored and PVC Sheathed 12/20 (24) kV
Three Cores Copper or Aluminum Conductors, XLPE Insulated, Steel Wire Armored and PVC Sheathed 12/20 (24) kV
Single & Three Cores Copper Conductors, XLPE Insulated and PVC Sheathed 18/30 (36) kV
Single & Three Cores Aluminum Conductors, XLPE Insulated and PVC Sheathed 18/30 (36) kV
Three Cores Copper or Aluminum Conductors, XLPE Insulated, Steel Tape Armored and PVC Sheathed 18/30 (36) kV
Three Cores Copper or Aluminum Conductors, XLPE Insulated, Steel Wire Armored and PVC Sheathed 18/30 (36) kV
Single & Three Cores Copper Conductors XLPE Insulated and PVC Sheathed