The high voltage cable ( HV cable ) is the cable used for transmitting power at high voltage. Cables include conductors and insulation, and are suitable to run underground or under water. This is different from the air duct, which has no isolation. High voltage cables of various types have various applications in instruments, ignition systems, and AC and DC power transmissions. In all applications, cable insulation should not be damaged due to high voltage voltage, ozone generated by an electric charge in the air, or tracking. Cable systems should prevent contacts of high-voltage conductors with objects or others, and must contain and control leakage currents. Cable and terminal connections should be designed to control high voltage voltage to prevent insulation damage. Often the high voltage cable will have a metal shield layer above the insulation, connected to the ground and designed to equalize the dielectric stress on the insulating layer.
High voltage cables can be long, with relatively short cables used in apparatus, longer cables working in buildings or like cables grown in industrial plants or for power distribution, and the longest cable is often run as undersea submarine cables for power transmission.
Video High-voltage cable
Construction
Like other power cables, the high-voltage cable has a structural element of one or more conductors, insulation, and a protective jacket. High voltage cables differ from low voltage cables because they have an additional internal layer in the insulation jacket to control the electric field around the conductor.
For circuits operating at or above 2,000 volts between conductors, conductive protectors may surround each insulated conductor. This equates the electrical voltage to the cable insulation. This technique was patented by Martin Hochstadter in 1916; The shield is sometimes called the Hochstadter shield. The protective individual conductor of the cable is connected to the ground at the tip of the shield, and in splices. The stress relief cone is applied to the shield end.
Cables for power distribution of 10 kV or higher may be isolated with oil and paper, and run in stiff steel pipes, semi-rigid aluminum or lead sheath. For higher voltages, oil can be stored under pressure to prevent the formation of voids that will allow partial removal in cable insulation.
Sebastian Ziani de Ferranti was the first to show in 1887 that carefully drained and prepared paper can form satisfactory cable insulation at 11,000 volts. Previously isolated paper-wires have only been applied to low-voltage telegraph and telephone circuits. The main sheath extruded on top of the paper cord is required to ensure that the paper remains completely dry.
The vulcanized rubber was patented by Charles Goodyear in 1844, but was not applied to cable insulation until the 1880s, when used for lighting circuits. The rubber-insulated cable used for 11,000 volt circuits in 1897 was installed for the Niagara Falls Power Plant project.
Medium-isolated commercial medium-insulated tension cables were practical in 1895. During World War II several types of synthetic rubber and polyethylene insulation were applied to cables. Modern high-voltage cables use polymers or polyethylene, including (XLPE) for insulation.
Maps High-voltage cable
AC power cord
High voltage is defined as a voltage greater than 1000 volts. The cables for 3000 and 6000 volt are available, but most cables are used from 10 kV and above. The 10 to 33 kV are usually called medium voltage cables, cables above 50 kV high voltage .
Modern HV cable has a simple design consisting of several parts. A copper or aluminum wire conductor transports a current, see (1) in Fig. 1. (For a detailed discussion of copper wires, see main article: Copper wire and cable. )
Parts of conductors up to 2000 mm 2 can carry current up to 2000 amperes. Individual strands are often formed to provide a smoother overall circumference. Isolation (3) may comprise crosslinked polyethylene, also called XLPE. It's flexible enough and tolerates operating temperatures up to 120 ° C.
On the inside (2) and outside (4) side of this insulation, the semi-conductive layer integrates with the insulation. The function of this layer is to prevent air-filled cavities between metal conductors and dielectrics so that little electrical charge can not arise and harm the insulating material.
The outer conductor or sheath (5) functions as an earthed layer and will conduct a leakage current if necessary.
Most high voltage cables for power transmission that are currently sold on the market are isolated by XLPE sheath. Some cables may have lead or aluminum jackets along with XLPE insulation to enable optical fiber. Prior to 1960, underground electrical cables were insulated with oil and paper and used stiff steel pipes, or semi-rigid aluminum or jackets or lead sheets. The oil is kept under pressure to prevent the formation of voids that will allow partial removal in cable insulation. There are still many isolated wires from oil and paper used around the world. Between 1960 and 1990, polymers became more widely used in distribution voltages, mostly EPDM (ethylene propylene diene M-class); However, relatively unreliable, especially early XLPE, results in a slow uptake in the transmission voltage. While 330 kV cables are generally built using XLPE, this has happened only in the last few decades.
Quality
During the development of HV insulation, which has taken about half a century, two characteristics proved to be paramount. First, the introduction of semiconductor layers. These layers must be absolutely smooth, without bulge as small as a few Âμm. Further the fusion between insulation and these layers must be absolute; any fission, airbag or other defects - of the same micro-dimensions as above - are detrimental to the characteristics of cable damage.
Second, insulation must be free of inclusions, cavities or other defects of the same size. Each of these types of defects shortens the cable voltage lifespan which should be in the order of 30 years or more.
Cooperation between cable makers and materials manufacturers has produced XLPE values ​​with strict specifications. Most manufacturers of XLPE compounds set the "extra net" class in which the number and size of foreign particles is guaranteed. Packing raw materials and dismantling them in a clean environment on a cable-making machine is required. The development of extruders for plastic extrusion and cross-linking has resulted in cabling to make the insulation defect-free and pure. The final quality control test is a high voltage 50 or 60 Hz partial discharge test with very high sensitivity (in the 5 to 10 picoCoulomb range). This test is performed on every cable roll before it is sent.
HVDC cable
The high voltage cable for HVDC transmission has the same construction as the AC cable shown in Figure 1. Physics and test requirements are different. In this case the smoothness of semiconductor layers (2) and (4) is the most important. Cleanliness of insulation remains important.
Many HVDC cables are used for DC submarine connections, because at a distance of more than 30 km AC can not be used anymore. The longest submarine cable currently is the NorNed cable between Norway and the Netherlands, which is nearly 600 km long and carries 700 megawatts, a capacity equivalent to a large power plant. Most of these long deep-sea cables are made in older construction, using oil-impregnated paper as an insulator.
Cable terminal
The high-voltage cable terminals must manage the electric field at the ends. Without such constructions the electric field will concentrate on the end-earth conductor as shown in figure 2.
The equipotential lines shown here are comparable to the contour lines on the map of the mountains: the closer these lines to each other, the steeper the slope and the greater the danger, in this case the danger of electrical interference. The equipotential line can also be compared with isobar on the weather map: the thicker the line, the more wind and the greater the danger of damage.
To control the equipotential line (ie to control the electric field), a device is used called cone-stress , see figure 3. The core of the stress reliever is to ignite the shield tip. along the logarithmic curve. Prior to 1960, stress cones were made using ribbons - after the cables were installed. It is protected by potheads, so named because the pot/dielectric compound is poured around the tape inside the metal/porcelain insulator of the body. Around 1960, a preformed break was developed which consisted of a rubber or elastomer body that stretched over the end of the cable. On the body like this rubber R shield electrode is applied which spreads the equipotential line to guarantee the low electric field.
The core of this device, invented by NKF in Delft in 1964, is that the holes of the elastic body are narrower than the diameter of the cable. In this way the (blue) interface between the cable and cone is pressed under mechanical pressure, so that no cavities or airbags can be formed between the cable and the cone. Electrical damage in this region is prevented in this way.
This construction can then be surrounded by porcelain or silicon insulators for outdoor use, or with a means to plug the wires into a power transformer under oil, or switchgear under gas pressure.
Cable connection
Connecting two high voltage cables with each other raises two major problems. First, the outer conductor layer on both cables must be terminated without causing field concentration, similar to the manufacture of cable terminals. Second, the free space of the field shall be made in which the insulation of the cut-down cable and connector of the two conductors can safely be accommodated. These problems were solved by NKF in Delft in 1965 by introducing a device called bi-manchet .
Figure 4 shows the cross section of the tool. On one side of this photo, high-voltage cable contours are taken. Here red represents the cable's conductor and blue cable insulation. The black part in this image is a semi-conductive rubber part. The outermost is at the earth's potential and spreading the electric field in the same way as in the cable terminals. The inside is at high voltage and protects the conductor connector from the electric field The field itself is diverted as shown in figure 5, where the equipotential line is smoothly directed from the inside of the cable to the outside of the bi-manchet (and vice versa on the other side of the device).
The crux of the problem is here, as in the cable terminal, that the inner hole of the bi-manchet is chosen to be smaller than the diameter over the cable-isolation. In this way a permanent pressure is made between the bi-manchet and the surface of the cable and the cavities or weak electrical points are avoided.
Installing a terminal or bi-manchet is a skilled job. Removing the outer semiconductor layer at the end of the cable, placing the field control body, connecting the conductor, etc., Requires skill, cleanliness and precision.
X-ray cable
X-ray cables are used within a few meters to connect HV sources with X-ray tube or other HV devices in scientific equipment. They transmit a small current, in the order of milliamperes at a DC voltage of 30 to 200 kV, or sometimes higher. Flexible cable, with rubber or other elastomeric insulation, stranded conductor, and outer sheath of braided copper wire. Construction has the same elements as other HV power lines.
High voltage cable test
There are different causes for faulty cable insulation when considering strong dielectric or paper insulation. Therefore, there are various testing and measurement methods to prove the cable is fully functional or to detect the wrong. While paper cables are primarily tested with the most common insulation resistance test DC tests for solid dielectric cable systems are partial discharge tests. Someone needs to distinguish between cable testing and cable diagnosis . While the cable testing method produces a cable diagnostic method the go/no go statement allows assessment of current cable conditions. With some tests, it is even possible to find a defective position in isolation before failure.
In some cases, the water tree can be detected by tan delta measurement. Interpretation of measurement results in some cases may result in the possibility of distinguishing strongly planted new cables. Unfortunately there are many other problems that can mistakenly present themselves as high tangent delta and most of the solid dielectric defects can not be detected by this method. Damage to insulation and electric circumference can be detected and placed by partial discharge measurements. The data collected during the measurement procedure is compared to the measurement values ​​of the same cable collected during acceptance tests. This allows a simple and rapid classification of the dielectric conditions of the cable under test. Just as with the delta tangent, this method has many warnings but with good adherence to the factory test standards, field results can be very reliable.
See also
- Power transmission
- High voltage electric current
- Power cord
- VLF cable testing
References
Source
Kreuger, Frederik H. (1991). Industrial High Voltage . Volume 1. Delft University Press. ISBN: 90-6275-561-5. Kreuger, Frederik H. (1991). Industrial High Voltage . Volume 2. Delft University Press. ISBN: 90-6275-562-3. Kuffel, E.; Zaengl, W.S.; Kuffel, J. (2000). High Voltage Engineering (2 ed.). Butterworth-Heinemann/Newnes. ISBN 0-7506-3634-3.Note
External links
- Measurement of delta Tan medium and high voltage cable
- Partial discharge measurements for detecting and finding electric trees
- Test Hold AC in place for 200kV High Voltage Cable
Source of the article : Wikipedia
0 Comments