In the coming years TenneT will install more than a quarter of new 380-kV connections through the Randstad underground along an 80-km route. Since the underground installation of a 380-kV cable along such a long distance has never be done before, not even on international scale, the Netherlands will be a trendsetter in the field of innovative installations of high-voltage cable. By means of this decision TenneT chooses an optimal balance between innovation and power supply reliability. The Ranstad comprises the cities of Amsterdam, Rotterdam, The Hague and Utrecht in the west of the Netherlands.
At the beginning of this year the members of the Upper House of the Dutch Parliament (Eerste Kamer) and Lower House (Tweede Kamer) approved the installation of the new high-voltage connection through the Randstad, running from Wateringen, via Zoetermeer, to Beverwijk. This new electricity infrastructure is required in order to meet the strongly increased demand for electricity transmission. TenneT, the national grid manager, will install and manage the connection. On the 23rd of May the cabinet decided on the exact route of the connection in the southern section of the Randstad, what is referred to as the zuidring. It was indicated where the connection will be situated, and which section will be installed underground. Furthermore – on advice given by TenneT – a decision was taken with regard to the remaining part which could be installed underground in the northern connection.
There is a broad social and political call to install new high-voltage connections underground as much as possible. However, the technical restrictions of installing high-voltage cables and their impact on the reliability of the power supply must be taken into account. First and foremost, underground cables ensure a less stable electricity grid when compared with overhead lines. Experience has proven that instability is the main reason for large-scale blackouts of high-voltage grids. The technical and operational feasibility of underground cable connections over long distances in the connected 380-kV transmission grids has not yet been proven.
Secondly, the repair times of failures will be negatively influenced. Due to the crucial function of the transmission grid in the Randstad region, the repair time should be limited to a minimum. The social economical damages of failures will be significant. A study revealed that one single hour of power failure in the Randstad will cost Euro 72 million. Experience with cable connections shows that the duration in which a circuit will be out of order after a failure, varies between 2 and 20 days for each failure. The repair time of overhead lines is between 8 and 48 hours.
World-wide the crucial 380-kV transmission grid is almost fully installed as overhead lines. The 'cabling' of the 380-kV grid is an exception for short, special trajectories. In the Randstad, TenneT is seeking the limits of what is responsible in the technical sense of the word. Here TenneT brings further innovation of cable technology into action. For the implementation, TenneT is working in close cooperation with science and international grid managers to gain further experience. TenneT has already developed a new innovative pylon for the section which will be installed as overhead lines. This pylon ensures a space saving of more than sixty percent when compared to existing pylons.