Underground cables in operation
Underground cables are still a comparatively young technology in the world of EHV grids. Up to now, the only underground cables in use in the EHV grid are ones that carry comparatively low power levels over relatively short stretches – such as for connecting individual customers or generating facilities. However, the energy transition necessitates that high power levels are transmitted over very long distances. And yet we still don’t have any substantial experience demonstrating how such cables behave in Germany’s meshed transmission grid. That said, one thing is certain: the fact that underground cable systems comprise a large number of operating elements makes the power grid more complex. Integrating all of them safely and reliably into the network demands innovative solutions. This is why we at Amprion are actively promoting research and development – together with our partners in scientific research. We use research projects to test different cable technologies and laying methods.
Warming of underground cables
Wherever current flows, heat arises. Overhead lines release this heat to the surrounding air. Underground cables warm up the soil around them. The temperatures reached as a result of this depend on technical parameters such as the degree of utilisation and the way the cables have been laid. The deeper the cables are in the ground and the closer the individual cables are laid next to one another, the higher the degree of localised warming and the greater the restrictions with respect to power transmission.
For this reason, whenever we plan a new cable system, we calculate the installation depth of and the clearances between the cables very precisely. We bed the cables in materials that dissipate the heat emitted by the cables optimally to the earth surrounding and above the cables.
Reliable grid operation
In today’s meshed power network, the following rule applies: If a link fails owing to, say, a fault, the electricity must always be able to be transmitted via an alternative route – and without this causing another fault or indeed a power failure. This is known as the n-1 criterion, and it essentially guarantees the high level of system reliability and availability in Germany.
Right from the very design phase of an underground cable system, we ensure that we comply with this criterion. We do so by applying the experience gained with underground cable systems operating at lower voltage levels. We know from this that the average availability of underground cables lies far below that of overhead lines. While statistics show that the failure rate of an underground cable system is lower, the length of time it takes to repair a fault is much longer. There are a number of reasons for this: if a fault arises with an overhead line, the line is easily accessible and quick to repair. With underground cables, however, you first have to localise the location of the fault and then excavate and uncover it. The actual repair work and concluding EHV test are also complex and take time.