High Voltage Cable

High voltage is defined as voltage over 1000 volts. Normally, 2 to 33 kV is called medium voltage cable, and 50 kV or more is called high voltage cable. A modern HV cable is a simple design consisting of a few parts: conductor, conductor shield, insulator, insulation shield, metal shield and jacket. Other layers may include waterproof tape, ripcords, and armored wire. Copper or aluminum wires carry current. See (1) in Figure 1. (For more information on copper cables, see our main article, Copper Conductors.) Insulators, insulating shields, and conductor shields are typically polymer-based and contain several components. rare exception. Single conductor designs under 2000 KCM are typically concentric. Individual strands are often deformed during the twisting process to provide a smoother overall circumference. These are known to be compact, compressed conductors. The compact reduces the outer diameter of the conductor by 10%, while the compressed version reduces it by only 3%. Choosing a compressed or compact conductor often requires a separate connector when connecting. Transmission cables above 2000 KCM often include a sectorized style design to reduce skin effect losses. Utility power cables are often designed to operate with conductor temperatures up to 75C, 90C, and 105C. This temperature is limited by construction standards and jacket selection. Conductor shields are always permanently bonded to EPR or XLPE cable insulation within solid dielectric cables. The semi-conductive insulating shield can be glued on or removed depending on the purchaser's preference. If the voltage is above 69KV, the insulating shield is usually glued. Purchase a strippable insulating shield to reduce splicing time and skill. It could also be argued that strippable semicon may be less of a manufacturing problem at medium voltages. For paper-insulated cables, the semi-conductive layer consists of a carbon-containing or metallized tape applied over the conductor and paper insulation. The function of these layers is to prevent air-filled cavities and to suppress voltage stress between metal conductors and dielectrics so that small electrical discharges do not endanger the insulating material. is to Insulating shields are covered with copper, aluminum, or lead. "screen. "The metal shield or sheath acts as a ground layer and drains leakage currents. The function of the shield is not to conduct faults, but you can design that function if you wish. Available designs include: These include copper tape, concentric copper wire, longitudinal corrugated shields, copper flat straps, or extruded lead sheaths. Cable jackets are often made of polymers. The jacket's function is to provide mechanical protection as well as to prevent the ingress of moisture and chemicals. The jacket can be semi-conductive or non-conductive depending on soil conditions and required grounding configuration. Semiconductor jackets can also be used on cables to aid in jacket integrity testing. Jacket types include LLDPE, HDPE, Polypropylene, PVC (lowest on the market) and LSZH.

Two properties have proven to be of paramount importance in the nearly half-century development of high-voltage insulation. The first is the introduction of the semiconductor layer. These layers should be perfectly smooth with no protrusions as small as a few microns. Furthermore, the fusion between the insulation and these layers must be absolute. Fissions, air pockets and other defects, even a few microns, are detrimental to cables. Second, the insulator must be free of inclusions, voids, or other defects of similar size. These types of defects reduce the voltage life of cables that should be 30 years or more. A collaboration between cable manufacturers and material manufacturers has resulted in XLPE grades to exacting specifications. Most manufacturers of XLPE compounds specify an "Extra Clean" grade that guarantees the number and size of contaminants. Raw materials must be packed and unloaded within the clean room environment of the cable making machine. The development of extruders for the extrusion and crosslinking of plastics introduced cable manufacturing equipment for the production of pure, defect-free insulation. The final quality control test is a high voltage 50 or 60 Hz partial discharge test with very high sensitivity (5-10 picocoulomb range). This test is run on every reel of cable before shipping.

High voltage cables for high voltage direct current (HVDC) transmission have the same structure as AC cables shown in Figure 1. Physical properties and test requirements are different. In this case, the smoothness of the semiconductor layers (2) and (4) is most important. Insulation cleanliness is still essential. Many HVDC cables are used for DC submarine connections, as AC becomes unusable for distances greater than about 100 km. As of 2021, the longest undersea cable is the North Sea Link cable between Norway and the UK, at 720 km (450 miles) long.

Termination of high voltage cables must control the electric field at the ends. Without such a structure, the electric field would be concentrated at the end of the ground conductor as shown in Figure 2. The equipotential lines are shown here and can be compared with the contour lines on the mountainous map. The closer these lines are to each other, the steeper the slope and the greater the danger. In this case, an electrical accident hazard exists. break down. Isopotential lines can also be compared to isobars on a weather map. The denser the lines, the stronger the wind and the greater the risk of damage. Devices called stress cones are used to control the equipotential lines (and hence the electric field) (see Figure 3). The essence of stress relief is to flare the edge of the shield along a logarithmic curve. Prior to 1960, stress cones were hand-made using tape after the cables were installed. They were protected by potheads, and the potting compound/dielectric was poured around the tape inside the insulation of the metal/porcelain body, hence the name. Around 1960, effective terminations were developed, consisting of a rubber or elastomeric body strung around the end of a cable. A shielding electrode is applied to this rubber-like body R that broadens the equipotential lines and ensures a low electric field. The core of this device, invented by NKF in Delft in 1964, is that the inner diameter of the elastic is narrower than the diameter of the cable. In this way, mechanical pressure is applied to the (blue) interface between the cable and the stress cone, preventing the formation of cavities or air pockets between the cable and the cone. In this way dielectric breakdown is prevented in this area. The structure can be further enclosed in porcelain or silicon insulators for outdoor use, or with devices for the insertion of cables into power transformers under oil pressure or switchgear under gas pressure.

Connecting two high voltage cables together presents two main problems. First, similar to making cable terminals, the outer conductive layers of both cables must be terminated without causing electric field concentrations. Next, we need to create a field-free space that can safely accommodate the cut cable insulation and the two-conductor connector. These problems were solved by the NKF in Delft in 1965 by introducing a device called the Baymanschetcuff. Figure 10 shows a cross-sectional photograph of such a device. A high voltage cable is outlined on one side of this photo. Here, red represents the conductor of the cable and blue represents the insulator of the cable. The black part in this photo is a semi-conductive rubber part. The outer terminal is at ground potential and spreads the electric field in the same way as a cable terminal. The inner one is high voltage and shields the conductor's connector from the electric field. The magnetic field itself is redirected, as shown in Figure 8, and the equipotential lines are directed smoothly from the inside of the cable to the outside of the bimanche (and vice versa on the other side of the device). The crux of the matter here is that the inside diameter of this bimanche is chosen to be smaller than the diameter of the cable insulation, as in the case of cable terminals. In this way a permanent pressure is created between the bimanche and the cable surface, avoiding cavities and electrical weak points. Attaching a terminal or bimanshet cuff is a skilled operation. Technical procedures such as removing the outer semiconductor layer at the end of a cable, placing magnetic field control bodies, and connecting conductors require skill, cleanliness, and precision.

Hand-taped joints are the old-fashioned way to connect and terminate cables. Construction of these joints involves the use of several types of tape and the manual construction of appropriate stress relief. Tapes involved include rubber tapes, semiconductor tapes, friction tapes and varnished cambric tapes. This connection method is incredibly labor-intensive and time-consuming. It is necessary to measure the diameter and length of the layer to be built. Often the tape must be overlapped halfway and pulled tight to prevent windows or voids from forming in the resulting splice. It is very difficult to tape and waterproof by hand.

A premolded joint is an injection molded body that is created in two or more stages. Automation allows Faraday cages to achieve precise shape and placement that cannot be achieved with tape connections. The pre-molded joints are available in a variety of body sizes to match the cable Semicon outer diameter. A tight bonding interface is required to ensure waterproofness. These joints are frequently pressed, which can cause soft tissue injuries among craftsmen.

Heat shrink joints consist of a variety of insulating and conductive heat shrink tubing. These kits are less labor intensive than taping, but less labor intensive than pre-molded ones. There may be concerns about using open flames in manholes and building vaults. Also, if a torch is used, the tube must be completely recovered without burning, and the used mastic must flow into voids to remove air, which can cause finish issues. . You need to give it enough time and heat. There are also many components that need to be placed in the correct order and position with respect to the center of the joint.

Cold Shrink is the newest joint family. The idea is that the polymer tube is formed with a diameter suitable for the cable. It is then factory expanded onto the foam and placed onto the holdout tube. The joint is then easily slipped over the cable end and ready to be installed. After installing the connector, the splicer simply needs to center the joint body and release the holdout. The tube will automatically return to its original size. The only problem is that the cold shrink has a shelf life of about 2-3 years. After that time, the rubber will have shape memory and will not recover to its intended size. Failure to install by the recommended deadline may lead to joint failure. From a utility perspective, this makes it difficult to track inventory or hold emergency reserves for important customers. Cold shrink is the fastest growing segment of electrical distribution splices and is considered to have the fewest installation issues and the shortest installation times.

X-ray cables are used in lengths of several meters to connect HV sources to X-ray tubes or other HV devices in scientific instruments. It carries currents as small as milliamps at DC voltages of 30-200 kV, or even more. Cables are flexible and have an outer sheath of rubber or other elastomeric insulation, stranded conductors, and braided copper wire. The construction features the same elements as other HV power cables.

Considering solid dielectric or paper insulation, there are many possible causes for poor cable insulation. Therefore, there are various test and measurement methods to prove that cables are fully functional or to detect faulty cables. While paper cables are primarily tested with DC insulation resistance tests, the most common test for solid dielectric cable systems is the partial discharge test. A distinction must be made between cable testing and cable diagnostics. While the cable test method gives a go or no go decision, the cable diagnostic method can determine the current state of the cable. Some tests are even able to locate defects in insulation before they fail. In some cases, electrical tree phenomena (water trees) can be detected by tan delta measurements. Interpreting the measurement results may in some cases distinguish new, water-treeed cables. Unfortunately, there are many other problems that can erroneously appear as high tangent delta, and the majority of defects in solid dielectrics cannot be detected by this method. Damage to insulators and electrical trees can be detected and located by partial discharge measurements. Data collected during the measurement procedure are compared with measurements of the same cable collected during acceptance testing. This allows easy and fast classification of the dielectric state of the tested cables. As with Tangente Delta, this method has many caveats, but with good adherence to factory testing standards, results in the field are very reliable.

power transmission high voltage direct current power cable Testing VLF Cables

Kruger, Frederick H. (1991). industrial high voltage. Vol. 1. Delft University Press. ISBN 90-6275-561-5. Kruger, Frederick H. (1991). industrial high voltage. Vol. 2. Delft University Press. ISBN 90-6275-562-3. Kuffel, E. Seungl, WS; Kuffel, J. (2000). High Voltage Engineering (2nd ed.). Butterworth Heinemann/Nuness. ISBN 0-7506-3634-3.

Tan Delta Measurements for Medium and High Voltage Cables Archived in Wayback Machine on 12 April 2013 Detection and localization of electrical trees with partial discharge measurements Field AC proof test of 200kV high voltage cable