The human circulatory system and the electricity grid both have an indicator that shows how well they are working. In the human body, it is blood pressure, while in the grid, it is voltage. Blood pressure is the measurement of the force with which blood circulates through the bloodstream to deliver oxygen to all the organs in the body. When it is too high, the force of the blood can damage the blood vessels and the heart. Low blood pressure on the other hand can lead to weakness, dizziness and loss of consciousness. There are parallels with electrical voltage. If the voltage is too low, itcan affect the electricity system, i.e. it can suffer a malfunction or outage. In the same way that blood pressure pushes blood through the body, electrical voltage pushes electrons around the grid. If the voltage is too high, it can damage electrical systems. If it is too low, it can lead to faults in the grid.

Electrical voltage is therefore an important indicator of the state of the grid and is closely monitored by Swissgrid. If the voltage falls, reactive power must be fed into the grid, and if the voltage is too high, power must be withdrawn from the grid to ensure its stability. One way of doing this is by targeted switching, which enables power to be redirected to other lines with capacity.

Markus Imhof, Head of the System Operations System Management team, explains what causes overvoltage or undervoltage in the grid, and how the Swissgrid specialists avoid it in day-to-day grid operations.

What happens if the voltage suddenly falls or rises?

We operate our extra-high-voltage grid at either between 220 kV and 245 kV or between 390 kV and 420 kV. If the voltage becomes too high, the insulating distance – i.e. the size of the insulators – is no longer sufficient, which can cause an electric arc. This arc can damage our systems or, in the worst-case scenario, cause personal injury. It can also trigger a chain reaction, bringing about simultaneous outages of multiple lines, which could lead to a power system failure.

If the voltage is too low, it can result in a voltage collapse. This type of collapse can be localised or regional, causing a supply disruption.

How does Swissgrid monitor the voltage of its grid?

We have two control centres, one in Aarau and the other in Prilly. Our specialists in these centres continuously monitor  compliance with the voltage limits using our control system, which enables us to view the voltage values of our 151 substations. If the voltage becomes too high or too low, it triggers an alert. We then analyse which measures are most appropriate.

How does Swissgrid ensure that the voltage remains within the acceptable limits?

Voltage maintenance in Switzerland is governed by the voltage maintenance concept. In Switzerland, power plants are required to help maintain the voltage. This means that they have to feed reactive power into the grid or absorb it from the grid.

What is reactive power?

Reactive power is a type of electrical energy that flows along our lines but does no direct work – so you can’t use it to switch on LEDs or charge batteries. It is nevertheless required to operate many electrical appliances, such as motors and transformers. Too much reactive power can overload the grid. That is why it is constantly monitored and, if necessary, compensated for by means of targeted measures to guarantee the efficiency and stability of the grid.

Who is responsible for compensating this reactive power?

This is one of the tasks of the power plants, which receive compensation for the reactive energy they generate correctly. Swissgrid calculates what are known as optimum voltage values to ensure that the power plants know how to adjust the voltage properly. These values are based on an optimisation process called Optimal Power Flow. This keeps the loss of power in the grid as low as possible and minimises reactive power costs at the same time.

Swissgrid sends the voltage plan it has calculated to the power plants. The power plants adjust their voltage by exchanging more or less reactive power with the transmission grid as required.

If the reactive power generated by the power plants is not sufficient, Swissgrid can use additional synchronous condensers. In this case, a generator in the power plant functions like a motor: it does not generate electrical energy but can feed in or absorb reactive power in a targeted manner, helping to keep the voltage in the grid stable.

Do the distribution grids have an impact on voltage maintenance?

Yes, the distribution grids, i.e. the grids of the companies that distribute electricity to households and businesses, have a major impact on voltage maintenance. They exchange large volumes of reactive power with the transmission grid. However, they have little or no control over this exchange of reactive power. The main reason for the fact that they exchange so much reactive power is that they have extensive underground cable grids. An underground cable grid generates much more reactive power than an overhead grid. If the distribution grids are unable to compensate for this reactive power locally, it is fed into the transmission grid. This can lead to a rise in the voltage on our grid, and even to violations of voltage limits.

What are the current and future challenges facing Swissgrid with regard to voltage maintenance?

There are a number of challenges associated with voltage maintenance. As a result of the progressive burying of distribution grids, more and more reactive power (or reactive energy) is being generated. If this reactive power is not compensated for in the distribution grid, it leads to high voltages in the transmission grid. We then have to compensate for this reactive power via the power plants. The increase in underground cable lines in our grid also generates more reactive power, which we must compensate for.  As the number of renewable energy systems rises, particularly photovoltaic systems, the load on both the distribution grid and the transmission grid decreases. Lines with light loads or no loads at all generate a lot of reactive power, which causes the voltage to rise. It is becoming increasingly difficult for power plants to absorb sufficient reactive power. Swissgrid is therefore planning to build compensation systems to ensure that enough reactive power resources are available in Switzerland in the future.

Swissgrid’s transmission grid is one of the safest and most reliable in the world, and every effort is made to ensure that it remains so, including in terms of maintaining voltage.