page_title: High-voltage cable
section_title: Quality
hierarchical_section_title: High-voltage cable / Quality
caption_attribution_description: English: Plastic extruder cut in half and labeled.
context_page_description: A high-voltage cable is a cable used for electric power transmission at high voltage. A cable includes a conductor and insulation. Cables are considered to be fully insulated. This means that they have a full rated insulation system which will consist of insulation, semi-con layers, and a metallic shield. This is in contrast to an overhead line, which may include insulation but not fully rated for operating voltage. High-voltage cables of differing types have a variety of applications in instruments, ignition systems, and alternating current and direct current 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.
context_section_description: During the development of high voltage 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 — again, even of a few µm — is detrimental to the cable. Second, 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.
caption: Figure 7: An extruder machine for making insulated cable
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