In the early afternoon on 8 January 2021, the grid frequency in the European interconnected grid dropped to 49.745 Hz for a short time due to a grid separation. The tripping of a 400 kV busbar in the Ernestinovo substation in Croatia is the suspected trigger, leading to a cascade of trips on other lines in Croatia, Hungary, Bosnia-Herzegovina and Serbia, causing the grid separation in Southeast Europe. The European transmission system operators worked closely together to stabilise the grid, which they managed to resynchronise after an hour. ENTSO-E published an intermediate report on the incident on 26 February in which they described the condition of the system during the episode and outlined the synchronisation process. In an interview, Markus Imhof, Head of Balancing & Scheduling, explains the role that Swissgrid played in this event.
The press wrote about a virtual power system failure. How serious was the situation on 8 January 2021 really?
It can basically be said that the Swiss transmission grid is one of the most stable and secure in the world. The close meshing with the European grid – Switzerland has 41 international interconnection lines – also contributes to this stability: the more closely the grid is connected, the smaller the impact on its stability if a node fails. Such incidents are very rare. However, when major failures occur, everyone is affected, and Switzerland cannot be viewed in isolation from the overall European transmission grid. Accordingly, Swissgrid works closely with the European transmission system operators. The grids work together to prepare for disturbances and there are corresponding procedures in place for the reinstatement of the grid frequency of 50 Hertz.
On 8 January 2021, the systems and processes worked correctly and therefore prevented further consequences arising from the fault. The automatic response and the coordinated measures by the transmission system operators enabled normal operating conditions to be reinstated very quickly.
As Coordination Centre South, Swissgrid assumes an important role in the European interconnected grid.
How did Swissgrid respond to the situation?
The Swiss extra-high-voltage grid is very closely linked with the European transmission grid. This meant that Swissgrid also detected the frequency deviation.
Swissgrid, in its role as Coordination Centre South, and Amprion, in its role as Coordination Centre North, managed to initiate the necessary coordination via teleconference to reinstate normal operating conditions, and worked closely with the other European transmission system operators to stabilise the grid.
What exactly does this coordination entail?
As Coordination Centre South, Swissgrid assumes an important role in the European interconnected grid. It monitors the frequency of the European extra-high-voltage grid together with Amprion, which oversees the Coordination Centre North. Swissgrid and Amprion are the guardians of grid time. Swissgrid is responsible for this in the even months of the year, and Amprion in the odd months. Grid time is an important indicator of whether too much or too little energy is present in the interconnected grid, in other words whether the balance between production and consumption is being observed. In the event of deviations in grid time, the relevant Coordination Centre initiates pre-defined procedures to find the fault and reinstate a stable state, or changes the target frequency so that power plants work faster or slower to ensure that the grid frequency corresponds to the correct time again. The last major deviation in grid time was in January 2018 when oven clocks were running more than six minutes behind.
In the event of a disturbance such as the one on 8 January, Swissgrid is responsible for introducing, coordinating and monitoring the prearranged processes with southern partners such as France, Spain, Portugal and Italy, as well as countries ranging from the Balkan states to Turkey.
What means are available to transmission system operators to secure the grid stability of the European transmission grid?
Swissgrid and the other European transmission system operators also hold permanent energy for stabilising the grid frequency, known as control power, which can be used to balance unforeseen oscillations in production, caused for example by the outage of a major power plant, or unexpected variations in consumption. The overall European grid has automatic inbuilt safeguards to stabilise the grid frequency. When the grid frequency falls short of defined limits, pre-defined loads are automatically separated from the grid on a rolling basis until a balance between production and consumption is once more achieved.
The transmission system operators are prepared for disturbances affecting the grid. The European interconnected grid has pre-defined, practised processes in place to minimise the impact of system disturbances and, in particular, to prevent and equalise major frequency deviations. Well-rehearsed stabilisation procedures have been prepared.
Switzerland is a highly connected country that is dependent on the circumstances in its neighbouring countries. Without an electricity agreement with the EU, import capability from the EU and the export willingness of the EU are put at risk.
International cooperation plays an important part in grid stability. Why is that?
The Swiss transmission grid is connected to other countries in 41 places. It is therefore an integral part of the continental European interconnected grid. Exchanging energy helps to overcome bottlenecks and to prevent overloads in individual countries. This international cooperation allows power plant outages or overproduction to be compensated for.
Switzerland is a highly connected country that is dependent on the circumstances in its neighbouring countries. Without an electricity agreement with the EU, import capability from the EU and the export willingness of the EU are put at risk. Thanks to close cooperation with its European partners, Switzerland has so far been able to compensate for power plant failures and overproduction. But the implementation of the EU's Third Energy Package and the Clean Energy Package is expected to lead to an increase in unplanned flows and to a reduction in Switzerland's ability to import, which will have a negative impact on system security in both Switzerland and Europe. It is to be expected that the intensity of the challenges will rise significantly by 2025.
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