What are busbar couplings

Why was there a disruption in the European electricity grid in January?

The triggering of an overcurrent protection in a substation in Croatia apparently triggered the division of the European network on January 8, 2021. This was announced by the association of European transmission system operators ENTSO-E.

Accordingly, the following happened: In the Ernestinovo substation, an overcurrent protection responded and opened a busbar coupling. Substations are nodes in the power grid. Several extra-high voltage lines can meet at them. The current is brought together on short, system-internal extra-high voltage lines, the busbars. For example, so that a power line can be completely switched off, at least two busbars are set up, between which the current can be switched back and forth. The switches for this are the busbar couplings.

The automatic opening of a coupling between these busbars in Ernestinovo separated these two busbars and thus two high-voltage connections that carried electricity from the Balkans to Europe. As a result, the river looked for new ways. This led to further protection shutdowns in other places, so that within less than 50 seconds the European power grid was divided into two areas: the north-west, which lacked electricity, and the south-east, in which too much electricity was generated .

The process is now known, but why this could happen is open. When switching high voltage lines, for example, it is checked in advance whether they are safe. This also includes a so-called N-1 case. This means that if a device fails, the current flow must continue to be guaranteed at maximum load. Busbars, however, are very rarely checked: circuit breakers against short circuits are usually installed in front of them and errors on the busbars themselves rarely occur. Therefore, such a bug would likely have been reported.


Prof. Dr. Christian Rehtanz

Head of Institute, Institute for Energy Systems, Energy Efficiency and Energy Economics (ie3), Technical University of Dortmund

“An unplanned and probably faulty busbar separation, which was not taken into account in advance in the operational network security calculations, is a very large disruption event for the entire network structure. The possibility of busbar separation due to overcurrent should either have been taken into account in the operational planning so that such an event would not have led to subsequent trips, or such an overcurrent protection function should not have been active at this point, especially since the usefulness of this function is questionable at this point. "

“The overcurrent protection for the busbar coupling seems to be a local peculiarity that was not taken into account in the supraregional operational planning. The simultaneous separation of several lines by an overcurrent protection of the busbar coupling appears altogether unusual. "

When asked which points need to be clarified in the announced investigation:
“First of all, it must be clarified why the overcurrent protection disconnected the busbars. Was there a short circuit or an unfavorable network situation? "

“If it was not a fault, then it must be clarified why the system was obviously not operated in an N-1 safe manner. "

"Why an overcurrent protection for the busbar coupling was installed in the switchgear and which cases at an important node point should be protected by this must also be clarified. Why this function was not included in the network security calculation as a potential network disruption (N-1 case) would then also have to be clarified. "

Information on possible conflicts of interest

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