Oil cable

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Oil cables are a special design of high voltage cables and are pressurized inside with thin mineral oil during operation . They have been used as underground cables in electrical energy technology for operating voltages from 100  kV to 500 kV since the 1930s , primarily in urban areas such as the 380 kV Transversale Berlin and the 380 kV Transversale Vienna . The first developments and installations of oil cables are based on the following essential technologies:

  • Ferranti , 1880: multilayered wrapped paper insulation as a dielectric,
  • Borel , 1879: moisture-proof seal with a seamless lead jacket
  • Höchstädter , 1913: metallized paper as the outer boundary of the field,
  • Emanueli , 1917: Impregnation with low-viscosity oil under pressure during operation.

construction

Section through the cable termination of an oil cable (single conductor) for 220 kV

The primary task of the oil is to compensate for inhomogeneities in the electrical insulation between the inner conductor and the metallic outer shield. The insulation is in the form of oil-soaked paper. Without this compensation, the uneven design of the paper layers, in combination with small dirt or air pockets, would lead to excessive electrical field strengths with partial discharges and, as a result, to breakdowns which would destroy the cable.

In addition to the corresponding electrical devices, there are pressure regulating devices for the oil at the cable end points, which ensure a constant oil pressure inside the cable over the entire route. In the event of a pressure drop, for example due to cable damage, the line must be switched off immediately.

In order to avoid the complex oil control device, which also involves the expense of preventing oil from getting into the soil and groundwater in the event of a leak, underground high-voltage cables have been increasingly used without oil with cross-linked polyethylene ( VPE , abbreviated as XLPE ) as insulation material since the 1990s built up. XLPE high-voltage cables are technically more complex to manufacture because cable production must take place under clean room conditions to avoid imperfections in the insulation material.

There are two primary types of oil cables:

  1. Low-pressure oil cables are constructed with paper insulation, which is soaked in very thin oil during manufacture. These cables are single or multi-core cables with an outer lead or aluminum sheath, pressure sheath and protective sheath to protect against mechanical impact. During operation, the low-viscosity oil is fed into the interior of the cable at the cable end points from pressure equalization tanks at a pressure of 0.5 to 3.5 bar in order to keep the oil pressure inside the cable  within the permissible limits over the full ambient and operating temperature range. For this purpose, the conductor is usually made hollow in single-core cables, while the gussets between the cores serve as an oil channel in two- or three-core cables.
  2. High-pressure oil cables are also designed with paper insulation soaked in very thin oil, but for three-phase alternating current, three conductors with insulation and external electrical shielding are housed together in an external steel tube. This steel pipe is kept under an oil pressure of around 15 bar from the cable end points after it has been laid in the ground.

For higher transmission capacities, the waste heat from the cable can no longer be absorbed by the surrounding soil alone, as the temperature of the cable would be unacceptably high. In these cases, oil cables are supplemented by additional external water cooling. The oil also serves to bring waste heat from the current-carrying inner conductor to the outside area. The water cooling circuit is installed between the cable end points; the distances between two cable end points are a few kilometers, several individual cable sections are to be provided for longer distances.

literature

  • Dietrich Oeding, Bernd Rüdiger Oswald: Electric power plants and networks . 7th edition. Springer, 2011, ISBN 978-3-642-19245-6 .
  • Egon Peschke, Rainer v. Olshausen: Cable systems for high and extra high voltage . Publicis MCD Verlag, 1998, ISBN 3-89578-057-X .

Individual evidence

  1. ↑ 380 kV transmission system in Vienna. , Special issue of the Austrian magazine for the electricity industry, No. 1779, issue 9/10
  2. ↑ L. Hänisch, D. Hecklau, R. Schroth .: Construction of the 380 kV high-performance cable system in Berlin for the composite connection of the BEWAG . Issue 12.Electricity Industry, 1995, pages 680 to 686
  3. ↑ Egon Peschke, Rainer v. Olshausen: Cable systems for high and extra high voltage . Publicis MCD Verlag, 1998, ISBN 3-89578-057-X , p. 20 .