Compact transmission line for high DC voltages

Project state

According to the German Federal Network Agency in its Network Development Plan (NDP), the existing transmission grid is not capable of taking on the tasks of the energy transition with the existing infrastructure. When there are high wind energy infeeds, the grid already partially reaches its thermal transmission limits. The expected additional renewable energy can only be transferred safely and reliably by providing additional transmission capacities. The NDP envisages both improvements to existing transmission lines as well as the construction of new ones. With the existing transmission lines, it is intended that three-phase systems at the 400-kV voltage level shall now transmit 3,000 MVA instead of 2,000 MVA. With DC transmission, so-called hybrid overhead line transmission systems are planned where three-phase systems on three-phase masts will be replaced by high-voltage direct current (HVDC) transmission and the capacity increased to four or even five GW. In addition, new HVDC lines are planned. In a densely populated country like Germany it will not be possible to implement both cases – HVDC on existing three-phase masts and new HVDC lines – everywhere as overhead lines. Underground solutions will be required. A technical solution for this is offered by gas-insulated compact transmission lines for transmitting direct current at high voltages (DC CTL).

In the DCCoS project funded by the German Federal Ministry for Economic Affairs and Energy, the basic technology is being jointly supported by the Federal State of Bavaria. In the project, the findings relating to the insulating material, insulator designs and insulating gas are being developed for DC voltage applications that can be generally used in gas-insulated HVDC systems. Their application in a DC CTL still requires, however, more extensive knowledge in regards to the installation and assembly on site and the provision of cost-effective solutions.

Gas as insulation - Space-saving and secure

Gas-insulated pipe

There has already been more than 35 years of operational experience in using gas-insulated pipes in gas-insulated transmission lines (GILs). This form of energy transfer provides the advantages that it takes up very little space compared with overhead lines but has similar positive properties. In addition, GILs can be laid both above and below ground. In terms of their structure, GILs consist of two concentric tubes, whereby the inner one carries the current. Insulating gas (e.g. SF6) is situated between the two tubes, so that the earthed outer pipe is not live. A further advantage is that it remains maintenance-free over several decades. Until now GILs have only been available for AC systems. In the joint DC CTL project, the intention is now to develop this technology for the purpose of transmitting high-voltage direct current.

The main goals of this research project on DC CTL transmission technology:

  • Realising a technology for the particularly efficient underground transmission of very large amounts of energy at high DC voltages (transmission power up to 5 GW per system)

  • Facilitating a reliable technology with intelligent technological solutions to ensure that the new DC CTLs are free of defects by:

    • Using an innovative particle trap for use in gas-insulated DC transmission systems for controlling the movement of conductive particles within the transmission line

    • Designing and applying a new measurement system for monitoring partial discharges in DC transmission systems.

  • Securing long-term stability by using new welding methods and investigating the mechanical and thermal stresses as well as the ageing of components in DC CTLs.

  • Using steel pipes as housing for the DC CTLs to enable a cost-effective solution for the DC transmission.

The project partners at a glance

Salzgitter Mannesmann Line Pipe GmbH

The Salzgitter AG steel and technology group combines its tube companies under the parent company Mannesmannröhren-Werke GmbH. With their high quality products that are internationally recognised, the tube companies predominantly occupy leading positions in the market or are even world market leaders. As part of the DC CTL project, Salzgitter Mannesmann Line Pipe GmbH shall produce the HFI (high frequency induction), longitudinally seam-welded steel pipes, which will be equipped with a particle trap and an anti-corrosion coating. These will then be installed in a test rig and investigated in several tests.

OTH Regensburg

The department of Geotechnical Engineering at the Ostbayerische Technische Hochschule Regensburg (OTH Regensburg) has been scientifically investigating the interaction between soil and structural components such as pipes for over a decade. It has garnered considerable experience in R&D and industrial projects on how district heating pipes behave in regards to the surrounding soil and liquid soils, which is now being extended to DC CTL pipes. As part of the DC CTL project, the department will focus on the following scientific aspects:

  • Calculating the mechanical stresses on the thermally loaded pipes in the ground and establishing the principles for their sizing

  • Investigating and defining suitable ballast materials for laying DC CTLs in the ground

Further project participants at a glance

TU Berlin, Department of High Voltage Engineering

The TU Berlin’s Department of High Voltage Engineering has considerable expertise in the energy cable technology and diagnosis fields, which is now being supplemented by materials research and modern compact lines/compact devices. The following aspects will be scientifically investigated by the TU Berlin:

  • Qualification of insulation components for compact DC applications
  • Developing fundamental principles and concepts for the partial discharge measurement technology for DC voltage
  • Diagnostics and evaluation techniques for DC CTL

HTW Dresden, High-voltage Laboratories department

The High-voltage Laboratories department at HTW Dresden has been scientifically investigating the thermodynamic behaviour of gas-insulated systems for several years. In addition, the department has extensive facilities and laboratories that enable it to conduct experimental investigations on basic samples and test pieces concerned with the thermal stressing and ageing of contact systems. The following topics shall be scientifically investigated by HTW Dresden:

  • Ageing of electrical contacts with DC loading
  • Heating behaviour of buried DC CTLs
  • Electric arcing behaviour with direct current
  • Technology acceptance study

Amprion GmbH

In addition to the aforementioned partners, Amprion GmbH is also acting as a consultant in the project. Amprion GmbH has the longest high-voltage grid in Germany with a total length of 11,300 kilometres. In addition, Amprion GmbH is also responsible for coordinating the interconnected operation in Germany and the northern part of the European extra-high-voltage grid.

Project duration

12/2014 - 02/2018


Dr Tomasz Magier
Project manager
Siemens AG - Energy Sector
Freyeslebenstr. 1
91058 Erlangen

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