TenneT and TU Delft to study liability of underground cables
12 January 2009 by M&CIn the coming years, TenneT and Delft University of Technology will be closely monitoring the behaviour of the underground 380-kV cables that are to be installed in the Randstad region. The two parties have signed an agreement to that effect. The aim of the study is to gain insight into the behaviour of the electricity supply system once the underground cable has been incorporated into it. The study, which will be followed closely by interested parties around the world, is intended to ascertain the long-term feasibility of even longer underground cables. The study will be conducted over a period of six to eight years.
Special circumstances have been taken into account in deciding to construct certain sections of the connection below ground. These may include close proximity to residential areas or locations where the electricity connection would pass through areas of great natural beauty or high ecological value. In 2008, TenneT announced its plans to install up to 20 kilometres of the new 380-kV electricity connection in the Randstad region in the form of underground cables. The cables will be laid in several separate sections along the entire route, which will be 87 kilometres in length. The 20-kilometre-long underground section will consist of 240 kilometres of physical cable (i.e. a bundle of twelve separate cables), a highly innovative conceptthatputtingthat will place the Netherlands at the cutting edge of international cable construction. An initial analysis performed by Delft University of Technology confirmed that a 20-kilometre stretch of underground cable is currently the maximum feasible distance in the complex, meshed 380-kV grid of the Netherlands, particularly with a view to safeguarding the security of supply. The follow-up study to be conducted by Delft University of Technology and TenneT is aimed at providing clarity regarding the technical and operational feasibility of long-distance 380-kV cables. The study will focus mainly on the cables’ reliability. The heat discharge of cables operated at full capacity for long periods of time will also be investigated.
Above-ground construction to remain preferred option for 380-kV connections
The study will be performed over a period of six to eight years, starting when the cables are taken into operation. The long study period is necessary in order to monitor the behaviour of the electricity system and the cables in an effective manner. Although above-ground construction remains the preferred option for 380-kV connections, the government has decided that the total length of all above-ground electricity connections must not increase after 2012. Any new above-ground 380-kV line must replace an existing connection, or a lower-voltage connection must be installed below ground at another location. Underground cables with lower voltages (110 kV or 150 kV) are subject to fewer technical restrictions. In addition, many decades of experience have been gained with such cables.
International collaboration
The experience gained during the construction and subsequent operation of the 20-kilometre cable section in the Randstad region will be shared internationally. TenneT is among the initiators of an international working group which will start its activities in January 2009. Both TenneT and Delft University of Technology will participate in this working group, which will provide a forum for experts to exchange knowledge and practical experiences. TenneT is also planning to collaborate with Energinet.dk, the Transmission System Operator of the Danish electricity grid, as similar plans for the underground construction of 380-kV cables are being developed in Denmark.
Technical information
For technical reasons, TenneT takes a cautious approach to the construction of 380-kV connections below ground. The technical behaviour of an underground cable differs from that of an overhead line. Cables carrying the maximum voltage level (380 kV) require extensive reactive power compensation to make them manageable. In addition, meshed grids consisting of cables as well as overhead lines require capacity control to ensure a balanced distribution of flows across the grid. Capacity control is realised using serial compensators, in order to ensure that the electrical resistance is similar to that of overhead lines. The main challenge for TenneT is to manage the voltage level and the size of the transmissions and to maintain a constant frequency level. This is easier to accomplish when the grid is constructed mainly above ground and has fewer components than when the grid is underground and the required compensation equipment has been added to it.
A grid consisting of above-ground lines and cables with extensive use of compensation equipment results in uncertainty regarding the grid’s voltage stability. Experience shows that voltage instability is the main reason for large-scale black-outs in high-voltage grids, and cables take much longer to repair than overhead lines. Consequently, installing 380-kV cables carries significantly higher risks than constructing overhead lines.
More information, not for publication
Journalists can contact TenneT’s press office: +31 (0)26-3732600 or communicatie@tennet.org. TU Delft: Science Information Officer, Ineke Boneschansker. Tel: +31 (0)15 278 8499, e-mail: i.boneschansker@tudelft.nl. Others can contact Tennet’s Service centre, +31 (0)26-3731717 or servicecentrum@tennet.org. For more details about the new connections: www.randstad380kv.nl
