Questions and answers

Is Swissgrid for or against underground cabling?

In principle, Swissgrid is open to both technologies; however, we want to ensure that the most appropriate technique is used based on the individual project, taking into consideration all relevant aspects (local conditions and special characteristics, spatial development, environmental protection, technology, economy).

Which criteria govern decisions on whether lines will be laid as overhead or underground cables?

At the beginning of each planning operation, Swissgrid also checks possible underground cabling variations. A tool that helps with the decision is the evaluation scheme for transmission lines provided by the Federal Office for Energy (SFOE), which takes account of aspects such as spatial development, technology and systems, environmental protection and economic viability.

What is the purpose of Swissgrid’s testing and monitoring for underground cabling?

Swissgrid gains important experience and insights in the operation and performance of underground cables in the transmission grid through practical tests and monitoring. The tests and monitoring are used, among other things, to keep an eye on parameters calculated during planning and design, such as the dimensions of a line or the heating of the soil under various operating conditions, and to verify them over time. They can thus help assess more precisely what the best solutions are.

Are magnetic fields also a problem for underground cabling?

The transport of electric power with a corresponding current flow always generates magnetic and electric fields. In other words, an underground line causes electric and magnetic fields just like an overhead line. While electric fields are intercepted by the shielding in the cable, magnetic fields occur only on the outside. The strength and extent of the magnetic fields depends on the corresponding cable assembly.

Basically, it is the case that the Swiss regulations regarding electric and magnetic fields are among the strictest in the world. The magnetic field directly beneath an overhead line is lower than directly above an underground cable. On the other hand, the spatial extent of the magnetic fields in underground cabling is significantly lower than in an overhead line.

Are there any differences between overhead lines and underground cabling in regard to disruptions to operations?

To ensure maximum reliability of supply, the time a line is out of order must be kept to a minimum. Lines should therefore break down as little as possible, and where disruptions to operations do occur, the outage must be manageable and as short-lived as possible.

As opposed to a disruption in an overhead line, a disruption in underground cabling is virtually always associated with damage to the cable. In an overhead line, there are a variety of disruptions that do not cause damage (e.g. lightning) and that can be managed by an automatic restart.

Possible causes of cable damage can come from third parties (construction work), manufacturing or assembly faults (couplings, terminals) as well as operational events (overloading). Couplings – the connectors between two cable lengths – are considered to be particularly sensitive elements of a cable system. Assembling them manually requires great care and experience.

Repairing an underground cable normally requires more time than repairing an overhead line, as underground cables cannot be manufactured with reserves – the production of spare cables can take several months. The ability to generate a temporary solution within a short time and at a reasonable cost in the event of large-scale damage is greatly reduced in underground cabling. This means that, in the event of large-scale damage, an underground line may not be available over a prolonged period. This aspect is already taken into account in the design of underground cabling, such as by installing sufficient reserve lines which can be put into operation in an emergency. Nevertheless, the availability of an underground cable system is lower than that of an overhead line.

How can the availability of underground cabling be improved?

To increase availability, spare wires are used; these can be put into operation if a wire fails. However, the system must be switched off briefly to switch wires. However, this only helps if the cause of the disruption is limited to one cable strand and not the entire system.

Which technology is preferable as far as minimising transportation losses is concerned?

The dimensioning of underground lines with given power and terrain conditions leaves relatively little leeway and is highly dependent on the corresponding matching design (conduit block, micro-tunnelling, tunnels, etc.). These conditions play a large role in determining the heat dissipation and thus the necessary cable cross-section. As the losses mainly depend on the cross-section and material of the cable, the losses of the underground line are defined relatively clearly.

For overhead lines, the losses can be minimised more or less using appropriate cable assignment (e.g. bundles of 3 or 4 with 1000 mm2). If it is dimensioned appropriately, an overhead line can therefore have almost equivalent losses as a correspondingly sized cable route. The situation is different if – as in the foundations for the Federal Court decision of Riniken of April 2011 – a non-optimised overhead line is compared with an optimised underground cable.

It must also be considered that a line must be laid down for extreme cases («peak loads»). The underlying maximum currents thus occur only in exceptional cases – accordingly, the actual losses are usually much smaller than in the case of the hypothetical continuous maximum load of a line.

Is underground cabling preferable to overhead lines as far as nature and the environment are concerned?

In contrast to an overhead line, underground cables are invisible because they are under the ground. Installing them nevertheless leaves visible traces, such as cuts through wooded areas or in the form of transitional structures. Underground cables also have a direct impact on nature and the environment. The construction and operation of an underground cable has an effect on the soil in any case. In swamps, on floodplains of national importance, and in certain groundwater protection zones, the construction of underground cables is prohibited. To date, neither comprehensive studies nor assured results exist concerning possible heating of the soil and its consequences for the environment.

Can underground cabling be laid without resistance?

Experience from Germany shows that the landowners affected are particularly vocal in their opposition to underground cabling. Those involved in agriculture predominantly cite concerns about demands on the soil and its structure. So it can be assumed that there will be objections to underground cabling in Switzerland, too.

Why doesn’t Swissgrid use direct current lines?

DC switching technology is not yet sufficiently mature that direct current lines can be simply connected to form a power grid. Especially in terms of distribution, alternating current will therefore continue to be the method for the near future.

DC cables are connected to the 50 hertz supply grid using converter stations. These convert the energy to be transmitted from direct current to alternating current so that the energy can be further transmitted in the alternating current grid.

These stations require a very large area. Due to the extremely high investment costs of several hundred million francs and the losses in conversion stations, a direct current line is only economically viable for transmission distances exceeding several hundred kilometres and is therefore currently only an issue for little Switzerland in connection with the European grid.