High-voltage cable
A high-voltage cable, sometimes called a high-tension 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. 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 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 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 the 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 V 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 made of steel. With pipe-type cables, the pipes are constructed first, and then at a later date, the cable is pulled through. The cable may feature skid wires to prevent damage during the pulling process. The cross-sectional volume of oil in a pipe-type cable is significantly higher than in an oil-filled cable. These pipe-type cables are oil-filled at nominal low, medium, and high pressures. Higher voltages require higher oil pressures to prevent the formation of voids that would allow partial discharges within the cable insulation. Pipe-type cables will typically have a cathodic protection system driven off voltage where an oil-filled cable circuit would not. Pipe-type cable systems are often protected from forming through an asphaltic coating. There are still many of these pipe-type circuits in operation today. However, they have fallen out of favor due to the high front-end cost and massive O+M budget needed to maintain the fleet of pumping plants.
Cable insulation components
is defined as any voltage over 1000 volts. Those of 2 to 33 kV are usually called medium voltage cables, those over 50 kV high voltage cables.Modern HV cables have a simple design consisting of a few parts: the conductor, the conductor shield, the insulation, the insulation shield, the metallic shield, and the jacket. Other layers can include water blocking tapes, ripcords, and armor wires. Copper or aluminum wires transports the current, see in figure 1. The insulation, insulation shield, and conductor shield are generally polymer-based with a few rare exceptions.
Single conductor designs under 2000 KCM are generally concentric. The individual strands are often deformed during the stranding process to provide a smoother overall circumference. These are known as compact and compressed conductors. Compact offers a 10% reduction in conductor outer diameter while the compressed version only offers a 3% decrease. The selection of a compressed or compact conductor will often require a different connector during splicing. 2000 KCM and larger transmission cables often include a sectored style design to reduce skin effect losses. Utility power cables are often designed to run at up to 75°C, 90°C, and 105°C conductor temperatures. This temperature is limited by the construction standard and jacket selection.
The conductor shield is always permanently bonded to the EPR or XLPE cable insulation in the solid dielectric cable. The semi-conductive insulation shield can be bonded or removable depending on the desires of the purchaser. For voltages 69KV and up the insulation shield is generally bonded. A strippable insulation shield is purchased to reduce splicing time and skill. It can be argued that strippable Semicon can lead to fewer workmanship issues at medium voltage. With paper insulated cables the semiconducting layers consist of carbon-bearing or metalized tapes applied over the conductor and paper insulation. The function of these layers is to prevent air-filled cavities and suppress voltage stress between the metal conductors and the dielectric so that little electric discharges cannot arise and endanger the insulation material.
The insulation shield is covered by a copper, aluminum, or lead "screen." The metallic shield or sheath serves as an earthed layer and will drain leakage currents. The shield's function is not to conduct faults but that functionality can be designed if desired. Some designs that could be used are copper tape, concentric copper wires, longitudinally corrugated shields, copper flat straps, or extruded lead sheath.
The cable jacket is often polymeric. The function of the jacket is to provide mechanical protection as well as prevent moisture & chemical intrusion. Jackets can be semiconducting or non-conducting depending on soil conditions and desired grounding configuration. Semiconducting jackets can also be employed on cables to help with a jacket integrity test. Some types of jackets are LLDPE, HDPE, polypropylene, PVC, LSZH, etc.
Quality
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 This test is performed on every reel of cable before it is shipped.