The common 65 m high lattice towers have two 380 kV three-phase systems at the top and a 220 kV three-phase system at the bottom. An earth cable is routed at the top of the mast to provide lightning protection. The conductor ropes are insulated by three rods of 1.3 m each arranged in series for 380 kV systems, and by two rods for 220 kV. These rods are made of ceramic, porcelain or glass and have path-extending plate-shaped protrusions to keep creepage currents between the conductor ropes and the mast to a minimum. For safety reasons, two insulators are used next to each other.
The power lines required for 380 kV power transmission are to be kept free of bushes and trees at the base of the pylons over a width of about 35 m, and at the top of about 90 m.
The conductor cables consist of a load-bearing core of interlaced steel wires with a total cross-section of 40 mm² and a current-carrying sheath of interlaced Al wires with a cross-section of 240 mm². Aluminum is used instead of copper, which has a higher resistivity but a much lower density and is less expensive. Dividing the conductor into wires reduces losses due to the "skin effect," which tends to displace the current from the inside of the wire to the outside. At 50 Hz, the so-called skin depth is about 1 cm - at this depth (calculated from the edge), the current in a solid conductor would have dropped to 37% relative to the edge.
Arrangement in a bundle of four
By arranging the conductor wires in a bundle of four, the electric field strength of approx. 50 kV/cm for a single rope is reduced to values below 20 kV/cm, so that corona discharges are reduced.
The arrangement in a bundle of four with a total conductor cross-section of approx. 1,000 mm² for each of the 3 phases enables the transmission of an electrical power of approx. 1.4 GW at voltages of 380 kV (230 kV against ground) and typical currents of up to 2 A/mm² - i.e. a total current of 2 kA per phase.
Maybe amazing is the fact that the transport of the electrical energy does not take place in the conductor cables themselves, but in the electromagnetic field around the cables. However, the electromagnetic energy is quite well concentrated near the conductor ropes: About 90% of it is found within an imaginary tube around the ropes of 30 m radius. The ohmic losses in the conductor cables are covered by the electromagnetic field; this loses power accordingly between the power plant and the consumer.
The electrical resistance of the cables causes the conductor cables to heat up. The colder and more agitated the surrounding air, the better they can be cooled. The maximum temperatures for conventional conductor ropes are 80° C. However, a temperature increase of 10 K results in a resistance increase of approx. 4 %. High-temperature conductor ropes are currently being developed to increase the transmission capacity of existing systems. They allow maximum temperatures of approx. 150° C and higher. Here, however, the ohmic losses are more than 50 % higher than at 20° C. A 12 km pilot line with conductor cables with CFRP cores was installed by RWE in 2012 in the Hunsrück region at the 110 kV level.
Conversion of the transmission grid
The energy turnaround in Germany also requires conversion and expansion of the existing transmission grid. Particularly important for the power supply in Bavaria is the 190 km long Thuringian power bridge. This starts in Lauchstädt/Saxony-Anhalt and runs via Vieselbach/Thuringia, Altenfeld/Thuringian Forest, past Coburg to Redwitz an der Rodach/Bavaria. Full commissioning took place on September 14, 2017, with a total transmission capacity of 5,000 MW. Actually, the line was supposed to be available when the Grafenrheinfeld nuclear power plant was decommissioned in 2015.
More electricity highways are to be used to transmit mainly electricity from off-shore and on-shore wind turbines to the south. Two such systems are already necessary for redundancy reasons. Electricity from the lignite-fired power plants in the north, such as Boxberg, Jänschwalde, Schwarze Pumpe, Grevenbroich-Neurath, Niederaussem will also be needed after the shutdown of the Bavarian nuclear power plants to ensure security of supply during wind lulls and at night. In this regard, discussions are emerging about a route "Südlink" from Wilster, past Hamburg, Hanover and Fulda to Grafenrheinfeld, as well as a route "Süd-Ost" from Wolmirstedt near Magdeburg to Gundremmingen.
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