page_title: High-voltage cable
section_title: Cable Insulation Technologies
hierarchical_section_title: High-voltage cable / Cable Insulation Technologies
caption_attribution_description: English: cut-away view of typical urd style cable used by power utilities for MV operation.
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: Like other power cables, high-voltage cables have the structural elements of one or more conductors, an insulation system, and a protective jacket. High-voltage cables differ from lower-voltage cables in that they have additional internal layers in the insulation system to control the electric field around the conductor. These additional layers are required at 2,000 volts between conductors. Without these semi-conducting layers the cable will fail due to electrical stress within minutes.  This technique was patented by Martin Hochstadter in 1916; the shield is sometimes called a Hochstadter shield Shielded cable used to be called H-Type Cable..Depending on the grounding scheme tThe shields of a cabl can be connected to ground at one end or both ends of the cable. Splices in the middle of the cable can be also grounded depending on the length of the circuit and if a semiconducting jacket is employed on direct buried circuits..
     
  Since 1960 solid dielectic extruded cables have taken dominance in the distribution market. These medium voltage cables are generally insulated with EPR or XLPE polymeric insulation. EPR insulation is common on cables from 4-34KV. EPR is not commonly used over 35KV due to losses however it can be found in 69KV cables. XLPE is used at all voltage levels from the 600V class and up. Sometimes EAM insulation is marketed, however, market penetration remains fairly low. Solid, extruded insulation cables such as EPR and XLPE account for the majority of distribution and transmission cables produced today. However the relative unreliability early XLPE resulted in a slow adoption at transmission voltages. Cables of 330, 400, and 500 kV are commonly constructed using XLPE today, but this has occurred only in recent decades.      
  An increasingly uncommon insulation type is PILC or paper insulation lead covered cable. Some utilities still install this for distribution circuits as new construction or replacement. Sebastian Ziani de Ferranti was the first to demonstrate in 1887 that carefully dried and prepared kraft 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 moisture free. 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 cross-linked polyethylene (XLPE) for insulation. The demise of PILC could be considered to be in the 1980s and 1990s as urban utilities started to install more EPR and XLPE insulated cables. The factors for the decreased use of PILC are the high level of craftsmanship needed to splice lead, longer splicing times, reduced availability of the product domestically, and pressure to stop using lead for environmental and safety reasons. It should also be noted that rubber insulated lead covered cable enjoyed a short period of popularity prior to 1960 in the low and medium voltage markets but was not widely used by most utilities. Existing PILC feeders are often considered to be near end of life by most utilities and subject to replacement programs.
  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.
  Oil filled, gas filled, and pipe type cables have been largely considered obsolete since the 1960s. Such cables are designed to have significant oil flow through the cable. Standard PILC cables are impregnated with oil but the oil is not designed to flow or cool the cable. Oil filled cables are typically lead insulated and can be purchased on reels. Pipe type cables differ from oil filled cables in that they are installed in a rigid pipe usually 
caption: Figure 3, Cross section of typical 15KV #2 copper medium voltage EPR cable. Suitable for URD installation, direct buried or in duct. All layers of the cable construction are marked and identified.
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