IEEE Power & Energy Magazine - March/April 2021 - 60

import from France (600 MW). This export situation to the
north, unusual in comparison to the past, is potentially problematic for the Swiss transmission grid and might result in
congestion because production has to be transported to the
north via the 220-kV grid within Switzerland.
On top of this unusual export pattern, the concurrence
of several other factors led to a security violation. In the
distribution grid, several outages planned by a DSO were
executed, and these were not known to the TSO. In the same
region of the distribution grid, transit flows from the transmission grid through the distribution grid caused the overloading of a distribution line and the need for an unplanned
outage of the affected line, of which the TSO was not
aware. These topological changes in the distribution grid
due to the execution of planned outages and actions to
avoid overloading the line, which were unexpected for the
TSO, required topological measures (transformer phase
shifts) in the transmission grid. These, in turn, led to load
redistribution. Furthermore, there were short-term production increases planned and communicated only shortly
before real time. The combination of these factors led to

overloading a 220-kV network element and multiple N−1
violations on the 220-kV level of the transmission grid.
The following aspects were identified as the main causes
of this event:
✔ the uncommon export situation and high energy
production in Switzerland together with short-term
changes in power production schedules
✔ the effect of topological changes in the distribution
grid on the transmission grid as well as the effect of
topological measures in the transmission grid on the
load allocation in the lower-voltage grids.
Whereas the first point is mostly related to the expansion
planning of the Swiss power grid, the second one is related to
today's operational philosophy. Specifically, the congestion
forecast tool was unable to predict the violations of transmission grid elements in time because the subtransmission and distribution networks are not included in the model. Therefore, the
main lesson from this grid event is the importance of having an
operational philosophy that considers the subtransmission and
distribution networks (or at least important networked parts of
them) both in outage planning and real-time operations.

Germany,
France, Austria
Import/
Export

Import/
Export

Nuclear
Storage
Power Plants Power Plants
Leibstadt
Gösgen
MW

Peak
Energy

t

Nuclear
Storage
Power Plants
Power Plants
Mühleberg Beznau 1/
Beznau 2
MW

MW

Exports

75-85%

5%

80-90%

15-5%

95%

10-20%

GL 1

MW
t
Peak Energy

GL 1

Run-of-River
Power Plant

Storage
Power Plants

GL 2

MW

Distribution Grid

GL 1

t
Base Load

220-kV Grid

Supply

MW

t
Base Load

380-kV Grid

Imports

MW

t
Base Load

Transformation

t
Peak Energy

GL 3- 7

Power Plants

figure 1. An illustration of the cause of the winter 2015-2016 situation with most of the load, but only limited power production and import, in the 220-kV grid. Import was mainly via the 380-kV level, but the transformation capacity between
the 380- and 220-kV levels was limited.
60

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