A high-voltage cable (HV cable), sometimes called a high-tension cable (HT 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 fully rated insulation system that 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 (EG: tree wire). High-voltage cables of differing types have a variety of applications in instruments, ignition systems, and alternating current (AC) and direct current (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 the breakdown of the insulation. The cut lengths of high-voltage cables may vary from several feet to thousands of feet, with relatively short cables used in apparatus and longer cables run within buildings or as buried cables in an industrial plant or for power distribution. The longest cut lengths of cable will often be submarine cables under the ocean for power transmission.
Cable insulation technologies 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 V and above 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 and shielded cable used to be called H-Type Cable. Depending on the grounding scheme, the shields of a cable can be connected to the 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 dielectric 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 to 34 kV. EPR is not commonly used over 35 kV due to losses, however, it can be found in 69 kV 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 of 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 V. 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, especially polyethylene, including cross-linked polyethylene (XLPE) for insulation.
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