IEEE Power & Energy Magazine - March/April 2021 - 59

In general, a comprehensive medium-term energy security planning
process is needed, which includes planned grid and power plant
outages, hydro storage levels, and market conditions.
electricity demand is lowest in summer, not necessitating the
availability of all power plants.
The process follows a bottom-up approach: transmission
assets are grouped according to region, with the regional
manager submitting the maintenance requests for approval.
The requests are checked by the operational planning team
in order of submission, applying rough assumptions about
the expected system conditions as the future system state is
not accurately known or forecasted. This means there is also
little information about the assumed availability of possible
corrective actions.
Power plants similarly announce their maintenance
schedules on an individual basis. Their unavailability is considered for the system security assessment, but the TSO has
only an advisory role in rescheduling, with the only exception being the power plants contracted for black start and
islanding operation capability.
Note that data exchange with DSOs takes place only to
a limited extent in the planning stage. Thus, when maintenance takes place in the distribution grid, the transmission
system may be affected in real time without the detailed
awareness on the side of the TSO.

Historical Grid Events
in the Swiss System
Although today's operational philosophy has been largely
successful in maintaining the security of the power supply,
some recent grid events have demonstrated growing potential risks due to the evolution of the changes to which the
system is subjected. This section discusses several recent
events and attempts to identify relevant aspects of the operational philosophy that could be improved to better handle
such situations in the future.

The Winter 2015-2016 Situation
A problematic supply and grid situation was identified in
December 2015 due to a combination of circumstances. Dry
weather conditions during summer and autumn led to lower
power generation from run-of-river plants. The filling levels of hydro storage plants published by the Swiss Office
of Energy were below the long-term annual average. Moreover, nuclear power reactors Beznau 1 and 2 on the 220-kV
level were out of operation, totaling 720 MW of missing
power in-feed.
The missing base production from run-of-river and
nuclear plants had to be compensated by imports and production from hydro storage plants. While the supply of Swiss
march/april 2021

consumers is mostly via the 220-kV level, up to 85% of
imported power comes in via the 380-kV level during wintertime. Since the transformation capacity between the
380- and 220-kV level was limited, the risk of premature
depletion of the hydro storage dams (most of them are connected to the 220-kV network) followed by a lack of energy
supply increased. The problem was amplified due to the
rather high energy prices at the beginning of the winter,
which resulted in many Swiss hydro storage power plants
often exporting energy. An illustration of the problem
related to the limited production resources in the 220-kV
network is given in Figure 1.
To mitigate these problems, a temporary connection of
the transformer in Laufenburg that allowed transformation
between the 380- and 220-kV networks was carried out,
and a reserve transformer was commissioned in Tierfehd,
as depicted in Figure 2. Moreover, topological measures in
Werben and Bürs increased the import on the 220-kV level
from Austria via the Rhine valley. The outcome of these
measures was to increase the net transfer capacity to France,
Germany, and Austria during peak times. To allow for a
higher import to cover the domestic consumption in Switzerland, monthly export products (cross-border capacity) to
neighboring countries were made unavailable during winter.
The measures taken by Swissgrid, together with the unusually mild, rainy winter weather and the recommissioning
of block 2 of the Beznau nuclear power plant on 23 and
24 December 2015, helped improve the grid situation from
the Christmas holidays 2015-2016 onwards.
This event demonstrated the challenges in maintaining
the overall adequacy and security of supply in an electricity system with unbundled production and grid operation.
Swissgrid is responsible for maintaining network security, but no entity in Switzerland is in charge of ensuring
the overall security of supply. In general, a comprehensive
medium-term energy security planning process is needed,
which includes planned grid and power plant outages, hydro
storage levels, and market conditions.

Grid Security Violation on 20 May 2019
Due to attractive market conditions, there was an unusually high generation of electrical power in Switzerland (up
to 12 GW) on 20 May 2019 as well as a high net export to
neighboring countries (up to 4.5 GW). In particular, there
was high planned export to Germany (up to 4 GW export,
whereas Switzerland typically imports from Germany), very
low export to Italy (1.3 GW, which is quite uncommon), and
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