High voltage cables
A high voltage cable is an electrical cable that is designed to operate with high voltage (that is, electrical voltages above 1 kV ). This type of cable is used, among other things, for the transmission of high power (up to over 1 GW and voltage up to the order of 500 kV) in power networks for electrical energy supply as an alternative to overhead lines and gas-insulated pipelines . Further applications are in the area of high-voltage direct current transmissions (HVDC) and submarine cables .
Like other cables, high-voltage cables are characterized by an insulating sheath around the live electrical conductor , with high-voltage cables generally only having one conductor per cable. For the three-phase alternating current common in electrical energy networks , three individual high-voltage cables laid in parallel are required. Due to the higher voltages, the insulation material is made thicker in order to withstand the high electrical field strengths and it includes an outer shield which defines the course of the electrical field strength in the insulation material.
construction
High-voltage cables, in particular cables for operating voltages above 100 kV, have an axially symmetrical cross-section with a cylinder-symmetrical electrical field and consist of only one electrical conductor inside. They are built up in several layers, as shown in the adjacent sectional view.
In the center is the actual conductor, which is made of copper or aluminum and can have a cross-section of up to 3500 mm 2 , labeled (1) in the picture. This is followed by an electrically weakly conductive layer (2), followed by the actual insulation material (3). This is followed by a weakly conductive layer (4), followed by the external electrical shielding (5) and the external insulation, which protects against environmental influences, moisture and mechanical damage.
The weakly conductive layers on both sides of the insulation material are used for field control. They ensure a uniform and smooth surface between the electrical conductor and the insulation material. Without this weakly conductive layer, local field strength increases in the border area would occur due to unevenness, which favor partial discharges and thus trigger voltage breakdowns and, as a result, cause thermal destruction of the cable. They also serve to avoid air inclusions.
Insulation materials
The type of high-voltage cable is differentiated depending on the insulation material (3) used.
Ground cable
Earth cables are the oldest type of high-voltage cables, some of which are still used in medium-voltage networks. The insulation consists of oil-soaked cable paper tapes, which are wound around the conductor in a spiral shape and offset in layers. The gaps between the paper edges allow a certain bending radius. The paper is impregnated with various resins and mineral oil and thus forms a coherent and tough bond, which is called mass and is named after. A technological improvement is the Höchstädter cable (H cable), which uses a metallization layer applied to the outside of each conductor for electrical field control in the insulator. Changes in temperature can lead to undesirable cavities in ground cables and, as a result, partial discharges, which is why these cable types are mostly only used in the lower high-voltage range, for example at medium voltage .
Oil cable
The insulation of oil cables is made up of layers of paper soaked in oil, similar to that of earth cables; however, the paper is only impregnated with low-viscosity mineral oil and during operation oil is continuously pressed into the cable insulation by an external oil pressure control system. A distinction is made between low-pressure and high-pressure oil cables. Thanks to the oil insulation ensured during operation, no cavities can form even with temperature fluctuations, so oil cables can be used up to the maximum voltage range of around 500 kV. Disadvantages are the complex oil pressure control and the structural safeguarding so that no oil can get into the groundwater in the event of leaks.
plastic
The latest development is high-voltage cables with plastic insulation. As early as 1971, the insulation of high-voltage cables with fiber paper made from poly (2,6-diphenyl-p-phenylene oxide), which is resistant to 175 ° C., was proposed.
However, only insulation made of cross-linked polyethylene ( VPE , XLPE , PE-X or XPE for short), which is temperature- resistant up to approx. 120 ° C, was successful . It differs from normal PE in that it has a chemical composition or radiation treatment that creates additional internal bonds. It is applied to the inner conductor in homogeneous structures under clean room conditions . The VPE must be applied very evenly (homogeneously) in the structure and must not have any air pockets, foreign bodies or dirt. Inclusions in the insulation body would also lead to an uneven field strength curve, resulting in a voltage breakdown. Correspondingly designed XLPE cables can be used up to the maximum voltage range of 500 kV.
In addition to low voltage , polyvinyl chloride (PVC) is sometimes used in the lower medium voltage range. The disadvantage of PVC as an insulator is the high dielectric losses and the associated low thermal stability of the cable.
Other plastics used to insulate high-voltage cables include cross-linked ethylene-propylene polymer (EPR) and silicone rubber .
Cable ends
At the ends of high-voltage cables, special attention must be paid to the field strength curve on and in the insulation material. At the point where the outer shielding ends, there is an increase in the field strength, which can be above the dielectric strength of the air or even of the insulation material.
Special cable terminations provide a remedy, as shown in the illustration below on the right. Their geometry results in almost uniform field strength gradients. These elements are used at the cable ends, for example in cable transfer stations between underground cables and overhead lines or at cable ends in substations .
- Cable end with and without field control
Critical field strength curve at the end of the screen (black line), the inner conductor is red
Optimized field strength at the end of the cable through cable end sleeves (R = rubber; red = inner conductor, black line: trumpet-like, extended shield)
See also
literature
- Andreas Küchler: High voltage technology: Basics - Technology - Applications . 3. Edition. Springer, 2009, ISBN 978-3-540-78412-8 .
Individual evidence
- ↑ E. Kuffel, WS Zängl: High Voltage Engineering Fundamentals . 2nd Edition. Newnes, ISBN 0-7506-3634-3 .
- ↑ Dirk Willem van Krevelen : Development tendencies in the chemical fibers . In: Lenzinger reports . No. 32 , December 1971, p. 10-20 ( PDF ).
Web links
- High voltage cable , construction and techn. Data, company publication Tele-Fonika Kable , 2007