Grid technologies

Overhead line or cable?

Overhead line and cable

The decision on whether to install a high-voltage line as an overhead line or as an underground cable is based on a wide range of criteria. That's why Swissgrid checks the cable and overhead line options for every grid project.

At the extra-high-voltage level (220 kV and 380 kV), electricity is currently predominantly transported using overhead lines. The air acts as a perfect insulator here, and the heat created by the electricity flow can be transferred well to the surroundings. In the winter, the low temperatures allow much more electricity to be transported than in the summer.

For voltages of up to 150 kV, underground cables are used where possible. Underground cables have been extensively tried and tested in this lower voltage range. By contrast, too few reliable empirical values are available at the extra-high-voltage level for distances of more than one kilometer.

1/5: An overhead line consists of the totality of all phase and earth conductors on a supporting structure and is installed above ground. Overhead lines are used almost exclusively worldwide for energy transmission at high voltage levels, such as 220 and 380 kilovolts. The air surrounding the conductor serves as insulation.
2/5: Concrete tube blocks are laid in open-air at approx. 1 to 2 meters deep. The width of the wiring trench is approx. 5 meters. A minimum width of approx. 25 meters is required for the construction.
3/5: To install underground cables without trenches, tunnels of approx. 3 to 4 meters in diameter are constructed in the form of galleries or microtunnelling. The cables are placed on specific cable holders.
Gesteuerte Bohrung
4/5: A drill head to control the digging direction under the obstacles of the surface. To prevent them from collapsing, a stabilizing liquid is squirted behind the drill head and the empty tubes for the cables are laid. This type of construction is suitable for short distances, for example, passages under rivers or roads.
5/5: Transfer between overhead lines and underground cables: Additional factors have to be taken into account when different technologies are used along a line. Among other things, line routing becomes more challenging, because connecting the underground cable with the overhead line requires the construction of cable transfer stations the size of an ice hockey rink. Ensuring that the cable transfer stations and cable trench route are positioned to avoid existing (traffic) infrastructure poses major challenges for the project planners and engineers.

Switchgears and transformers

Substations are among the most important elements in the extra-high-voltage grid because they represent the nodes of the grid. Substations consist of transformers and switchgears. Swissgrid’s extra-high-voltage grid comprises 125 substations housing 147 switchgears and 21 transformers. This means that every substation has a switchgear, but not all substations have a transformer.

Important elements in the transmission grid

The switchgears in the substations connect the extra-high-voltage lines with each other. The operators in the Swissgrid control centres in Aarau and Prilly can disconnect or connect the lines by means of switching operations. By doing so, they can control energy flows, prevent overloads and switch off lines for maintenance work. The substations also have protective systems installed in them. In the event of a fault (e.g. lightning striking a line), they switch off the affected grid sections in a targeted manner.

Illustration of a substation with transformers
Illustration of a substation with transformers: the substations are controlled from the control centres in Aarau and Prilly. However, if required, the plant managers can take control of the substations and disconnect elements on site.

1 Transformer

The transformer is the heart of the grid. Transformers increase or reduce the voltage of the electrical energy or current. The higher the voltage, the lower the losses along the power lines. The extra-high-voltage grid has a high voltage in order to transport energy over long distances whilst minimising losses as much as possible.


2 Busbar

A busbar is used to connect the extra-high-voltage lines that arrive in the substation. The energy from the various extra-high-voltage lines can be distributed over the busbars via substation feeders to control the power flow in the grid.


3 Disconnecting switch

The disconnecting switch visibly separates the open wire circuit and the switchgear. Following disconnection, the grid element has no voltage, but is actually switched off by the circuit breaker.

Disconnecting switch

4 Circuit breaker

Circuit breakers are used to connect or disconnect elements in the electrical grid. In other words, they can disconnect lines or power plants from the grid, cutting off their voltage, or alternatively connect them to the grid. Circuit breakers and disconnecting switches are controlled from the grid control rooms.

Circuit breaker

5 Instrument transformer

Instrument transformers are among the control instruments of the switchgear. They measure the voltage as well as the intensity of current. The measured values are transmitted to the protection and control technology equipment installed locally as well as to Swissgrid’s grid control rooms in Aarau and Prilly. These values help the Swissgrid experts there to determine the current grid situation. The necessary measures can be taken as appropriate.

Instrument transformer

6 Gantry

A gantry is a portal with a lattice design that the ropes of the overhead lines are attached and anchored to in order to stabilise them. The lattice structure carries the weight of the ropes and absorbs the forces exerted on the ropes by wind and ice.


7 Lightning arrester

A lightning arrester protects the essential components of the switchgear from excessive electrical voltages that can occur, for example, if lightning strikes. In this way, it protects system components such as transformers from damage without interrupting the power supply.

Lightning arrester

8 Earthing switch

An earthing switch grounds part of the system that has been switched off and therefore has no voltage. It prevents hazards from occurring due to charging of any parts that have been switched off. In combination with the line isolating switches, this allows a safe working environment to be created in the switchgear.

Earthing switch

9 Service building / emergency generator

The protection, control and communication technology is located in the service building. The measured values from the entire switchgear are collected here, so that all elements can be managed and checked very quickly. In addition, the service building has its own power supply, including battery systems. These help to ensure the uninterrupted operation of the switchgear at all times.

Service building / emergency generator

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