Topic of the month May 2011

The nuclear disaster of Fukushima has led to a rethinking in Germany. As recently as last year the life span of the nuclear power plants was prolonged. Now a fast nuclear phase out is within eyeshot. However, a transition to renewable energy is not possible without large-scale investments in the German power network. Big challenges await the network operators and the cable manufactures during the upcoming years. In doing so, they reinforce the testing of new transmission technologies.

At least 3600 kilometers power network have to be built

The dena-II-study “Integration of renewable energies into the German power supply in the period 2015 – 2020 with forecast 2025”, which was commissioned by the Federal Network Agency in 2010, provides a basis for the calculation of how many kilometers power network has to be build. According to the study, the share of renewable energies at the current consumption will be about 39 per Cent in 2020. Wind energy will be most important with 37.000 megawatt installed electrical power onshore and about 14.000 megawatt offshore. The other renewable energies will be approximately 25.000 megawatt. Based on this calculation 3600 kilometers of new power supply lines have to been built, the study says. According to the German Economy Minister Rainer Brüderle time is short. “Without an accelerated network expansion the expansion of renewable energy will not move forward”, he says when presenting a law concerning the speeding-up of network expansion.

01: A network expansion with exclusively overhead lines would cost about 10 billion Euro, the Cologne Institute for Economic Research says. According to the Institute this would be the cheapest solution. (Photo: Amprion GmbH)

For cable producing companies and network operating companies the costs and the acceptance of new power lines by the population are the biggest problems concerning the network expansion. Because of that, they consider more and more alternative solutions as for instance high-voltage underground lines, high temperature conductors and overhead line monitoring.

Underground cable instead of overhead lines

In Germany almost every high-voltage transmission line is overhead. This lines transport power with 380 kilovolt over big distances. A dislocation of high-voltage transmission lines underground would be a good solution – especially with regard to the aspect, that new overhead lines are often disliked by residents, says Andreas Preuß from the network operating company Amprion. But at the moment, high-voltage underground lines are a huge technical challenge, he explains: “A high-voltage underground line, which is integrated in the network, does not exist by now. We do not know if it works.” The biggest problems are costs and maintenance. Due to the fact that the transfer of the same amount of power needs twice as many underground cables compared to overhead lines, the technology is much more expensive, Preuß says. Furthermore, if there is any disruption in the underground network, it is much more difficult to locate the source of it.

Anyway, according to Preuß Amprion is testing the technology: “We are planning three cables with a total length of 12 kilometers on the route Meppen-Wesel.” The network operator 50Hertz, which is responsible for Northern and Eastern Germany, is testing underground cables as well. According to press-spokesman Volker Kamm there are four pilot projects. Anyway, underground cables are an intervention into the countryside as well, he warns: “The trace has to have a breadth of at least 25 till 30 metres and you need a kind of coupling every 1, 5 kilometers at least.” Finally, underground lines are very expensive at the moment. The costs for underground lines through mountains are 15 times higher than overhead lines in the same area, Kamm explains.

Upgrading of existing power lines as alternative?

In Germany the upgrading and reinforcement of existing power lines is discussed as an alternative to the building of new lines too. A promising technology seems to be Overhead Line Monitoring: The operating temperature of the conductor rope is monitored and – if necessary or possible – the amount of transferred power is changed. In case the temperature decreases because of for instance strong wind the ropes can be loaded more. Especially in combination with the transfer of power from wind energy plants the technology is advantageous, the network operator Tennet TSO states: “In case there is a strong wind the transfer capacity of the power lines in Northern Germany can be enhanced about 50 per Cent. […] The enhanced transfer capacity – due to the cooling effect of the wind – is available at times of high input of wind energy and thus a high transfer demand.” Tennet TSO converted 15 exiting lines for using Overhead Line Monitoring last year, spokeswoman Cornelia Junge says. This corresponds to lines with a total length of 1800 kilometers and 22 relay stations. Anyway, the technology is not an alternative for the building of new lines, Junge says: “Considering the increasing input of wind energy, it is not possible to transfer the produced energy with the existing lines, even if the whole power network would be upgraded with Overhead Line Monitoring.”

02: A 380-kilovolt underground cable with a copper conductor. According to the the
Cologne Institute for Economic Research the costs of the network expansion would increase to 14 billion Euro if 800 of the 3600 kilometers of new lines would be dislocated underground. (Photo: TenneT TSO GmbH)

According to the dena-II-study another technology is already state of the art: high temperature conductors. Normally, conductor ropes can only be loaded till an operating temperature of maximum 80 degrees Celsius. The use of special aluminium, which tolerates high temperatures, the maximum operating temperature can be increased to 150 degrees Celsius and thus the load capacity of the lines is 50 per Cent higher, the dena-II-study says. Amprion and 50 Hertz are testing the technology at the moment. The cable producing company Nexans is also involved in upgrading-projects with High Temperature Conductors, spokeswoman Hannelore Glotzbach says. According to her, the conductors are only recommendable for short periods of high loadings: “If you use them under high temperatures for a long time, on the one hand you can transfer much more energy, but on the other hand the loss of energy increases disproportionately high.

That’s why high temperature conductors can enhance the network security and help to avoid supply shortfalls, but they are not an alternative to the building of new power lines in an efficient manner. Anyway, high temperature conductors – as well as other upgrading technologies – will be used more and more in future, especially to get along with the opposition against the building of new overhead lines. “We are doing a lot of research, in order to build as little as possible new lines in future”, 50 Hertz-spokesman Kamm says.


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