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.
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.
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.
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.
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.
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.
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.
