An underground cable is an electrically used cable laid in the ground with a particularly robust insulation to the outside, the cable jacket, which prevents it from being destroyed by chemical influences in the ground or small animals (rodents) living in the ground.
Underground cables have several advantages over overhead lines . They are protected against damage - including from the weather - and do not visually disturb the landscape. Disadvantages are the higher maintenance costs and the more difficult localization of malfunctions, such as those caused by construction work and unintentional damage to laid underground cables, for example. In the case of energy-technical applications in the high-voltage range, the associated higher costs are a disadvantage.
The use of AC systems in the extra high voltage range is only possible to a limited extent according to the Stom network development plan , while several projects are planned in Germany for high voltage direct current transmission .
Underground cables are laid at a safe depth to protect against damage. Laying is done efficiently in open terrain using a cable plow , but in rock and in built-up areas in a previously opened cave . Other laying methods are controlled horizontal drilling , for example for crossing under a street, the lane of which remains untouched, or a small stream. Larger bodies of water can be crossed under with a culvert .
For mechanical protection, underground cables are partly laid in cable ducts and also in a layer of sand surrounding the cable so that sharp-edged stones cannot cause damage when the ground is exposed to loads, for example due to vibrations from nearby rail or road traffic. For voltages below 1 kV, underground cables are usually laid at a depth of 60 cm, and 80 cm in the street area. In addition to warning tape , plastic plates are also used as protection against stinging and digging as cable cover hoods or plates.
There is no legal regulation for depth and cable protection of power cables in the Federal Republic of Germany.
The cable owner specifies the depth at which cables are laid. These are based on DIN standards and current practice.
Larger installation depths are common for higher voltages. Extra high voltage lines with 400 kV are laid, for example, 2.5 to 3.7 m below the surface of the earth, or they are laid in a specially created tunnel or pipe system, which in addition to easier maintenance also serves for cooling.
There are different types of underground cables, which primarily depend on the purpose and area of application.
In Europe and in residential or industrial areas, electrical lines in the area of low- voltage networks with voltages below 1 kV and medium-voltage networks with voltages below 70 kV are usually designed as underground cables. In rural regions and with older installations, overhead lines are also common due to the lower costs .
In electrical power engineering , high-voltage underground cables are designed as high-voltage cables. For electrical voltages below 100 kV, these can be produced in a multi-pole version; for higher voltages, single-pole versions (single-core cables) are used. For the number of conductors, for example with three-phase alternating current, it is then necessary to lay three single-core cables in parallel.
Underground cables in the high voltage range with operating voltages above 200 kV have higher transmission losses compared to the simpler and more reliable overhead lines. The higher losses are due to the comparatively high natural power and the associated higher reactive power requirement of the cable system and the necessary compensation devices . The latter are required after 10 km at the latest and are about the size of a transformer house. Temperatures of around 35 ° C occur directly on the cable, and up to 50 ° C when individual strings are switched off. In the case of larger underground cable systems, the thermal transmission losses are also dissipated by additional indirect cooling devices, for example water pipes laid in parallel and surrounding the cable jacket, or, in the case of lower losses, by forced ventilation. An example of such a cable system is the 380 kV Transversale Berlin .
Maintenance and troubleshooting is also more complex with underground cables: While overhead lines can be checked through periodic optical inspection patrols and optical devices such as corona cameras , this is not possible when they are buried. In the case of underground cables, damage can often not be detected in good time before the failure. Damage to the underground cable, to cable sleeves or to the cable termination often results in damage to the environment. Repairing the damage is also more tedious and expensive. For this reason, regular, complex checks, for example for partial discharge , must be carried out on cable systems .
Telecommunication cables are used to establish fixed network connections in the telephony area or digital subscriber line (DSL). A large number of over 1000 individual cable cores are typical in a cable.
The aim of locating the source of interference is to detect broken or pinched cables and to locate their position. The property of time domain reflectometry is used to detect every change in the medium. This means that the end of the cable, a cable break or a short circuit between the inner and outer conductor can be located.
Underground cables in the high voltage range also have higher initial and operating costs than comparable overhead lines. In the study of the 380 kV Salzburg line, a cost increase for the line section Salzburg – Tauern due to the use of underground cables compared to an overhead line was determined by a factor of 6.2. The length of the line to be constructed is 106 (or 108) km. At the time of the 2007 study, the overhead line would have cost around 190 million euros, whereas full cabling would have cost around 1.176 billion euros.
In October 2014, the transmission system operator Amprion laid a 380 kV underground cable over a length of 3.4 km in the municipality of Raesfeld . At the two cable gardens, each with a space requirement of 60 m × 80 m, the 380 kV line is transferred to the ground. In the 20 m wide cable trench (total building area width 40 m) with a depth of two meters, 14 conduits (two of which are for data lines) are laid. The cost is 1.4 million euros per kilometer of overhead line, while laying the cable is likely to cost 8 million euros per kilometer. The total costs for the 3.4 km should be around 30 million euros. The BMU was even more optimistic about the costs of underground cables - albeit with a low operating voltage - in 2006 and took the view that the total costs of underground cables for the high voltage level of 110 kV and the maximum voltage level of 220 kV are not significantly higher than those of overhead lines.
No data is yet available on the failure rate of 380 kV cables. The repair time should certainly be longer than that of overhead lines (weeks instead of days). Since there is still no long-term experience with 380 kV cables, the service life is calculated at 40 years (based on experience with 110 kV cables), while it is 80 years and more for overhead lines.
One of the first electrical underground cables around 1885 was the so-called Kruesi tube . It consisted of a tube in which three electrical conductors in the form of metallic rods and with insulation wrapped in a spiral were cast separately from one another. At the beginning of the 20th century, structural improvements followed, such as the Höchstädter cable used for high voltage , which ensured an even load due to the electrical field strength of the insulation material and reduced disruptive partial discharges . In the middle of the 20th century, oil cables appeared that compensated for inhomogeneities in a pressure pipe with oil. The insulation material used in today's high-voltage cables for use up to over 500 kV is plastics such as cross-linked polyethylene ( XLPE , abbreviated as XLPE ), which is temperature- resistant up to around 120 ° C and can be attached to the cable structure in homogeneous structures using clean room techniques.
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