Research & Development Gas-insulated direct current lines instead of power lines for the energy revolution?

For the energy turnaround, the electricity from renewable energies will have to be transported through Germany, in some cases hundreds of kilometers - only the question of how has not yet been clarified. One solution could be completely newly developed direct current lines.

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Renewable energies bring with them the challenge of being location-dependent. Wind energy is mainly generated in the far north, but must be distributed throughout Germany. But how do you bring the energy to the South?

Transporting energy with direct current lines

High-voltage lines cause resistance among the affected residents, and classic alternating current underground cables cannot transport energy economically over several hundred kilometers. Another possibility would be to transport the energy via newly developed direct current cables - but so far this is new technical territory, the planned transmission voltage of up to ± 550,000 volts is still partly in the qualification phase.

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New, gas-insulated DC transmission lines (DC GIL, Direct Current Gas-insulated Transmission Lines) are a sensible addition to the transmission technologies with less space requirements.

This is how the new DC cables with insulating gas work

A metallic inner conductor, supported by insulators, runs inside a metallic outer tube. Direct current of up to 5,000 amperes flows through the high-voltage conductor. Between the inner conductor and the outer tube, there is an insulating gas that electrically insulates the high DC voltage. Due to the compact design of these cables, five gigawatts of power - the power of four to five large power plant units - can be transmitted over a route width of only six meters.

By way of comparison, overhead lines require routes about 60 meters wide for much lower outputs.

First long-term test with new DC lines

For the first time, this technology, which has so far been little researched, but is potentially promising for the future, is now being subjected to at least one year of long-term testing under realistic operating conditions at the TU Darmstadt. In cooperation with other universities and industrial partners, the scientists from the High Voltage Technology department want to find out whether the underground gas-insulated direct-current lines can fulfill the hopes placed in them:

• higher transmission performance

• lower landscape consumption

• lower electrical losses

• high reliability

• higher profitability

This is how the experiment should proceed

Voltages in the range of ± 550,000 volts and currents in the range of 5,000 amperes are required to investigate the lines under real conditions. In terms of calculation, this corresponds roughly to the output of four to five power plant units, which cannot simply be taken from the power grid. The High Voltage Department has therefore developed a new synthetic test method to reduce the power requirement for the long-term test to the power of a 200 hp motor.

The test field is located in Griesheim next to the August-Euler airfield, directly on the outskirts of Darmstadt. For the project, the TU built a 670 square meter test hall, which houses the technology for voltage and power generation. The hall construction and foundations are specially designed for working with high voltages. From here, a total of 250 meters of gas-insulated pipes run through the one-hectare test field, which were laid partly above ground and partly at a depth of around two meters below the earth's surface due to easier accessibility.

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Who stands behind the project

The TU Darmstadt and Siemens coordinate the entire project. The TU Darmstadt is particularly responsible for the implementation and testing of the latest test methods. In addition, thermal and mechanical measurements are carried out at the test road by the East Bavarian Technical University of Regensburg in order to study the soil mechanics and the thermal limit values of the pipeline. Research into the electrical condition of the high-voltage line - which is also new technical territory - is being carried out jointly with Power Diagnostix and TU Berlin. The development and construction of the new gas-insulated line is the responsibility of the cooperation partner Siemens in particular.

The project, with a total financial volume of around 3.2 million euros, is half funded by the Hessian Ministry of Economics through funds from the European Regional Development Fund (ERDF).

This article was first published in German by Elektrotechnik.

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