IEEE Power & Energy Magazine - March/April 2021 - 58

as well as the natural energy sources. Thus, the consumption
centers are located in the north, together with run-of-river
hydropower plants and nuclear stations, whereas the storage
and pumped-storage plants (in other words, the more flexible power plants) are in the Alps in the south. The strongest transmission path runs from the north to the south, or
rather southeast, from where it extends to Italy. Historical
flows naturally followed the structure of the transmission
lines, with the cross-border exchange joining in dominantly with the imports from France and Germany and
exports toward Italy.
The Swiss power system features another characteristic that is not common in most other continental European
systems: the subtransmission system and some distribution
systems do not have a radial structure but are rather strongly
meshed and connected to transmission nodes in several locations. Thus, they accommodate part of the power flows in
parallel to the transmission lines.
The installed production capacity exceeds 20 GW, with
the peak consumption being approximately 11.5 GW, so the
traditional adequacy assessment does not reveal the weakness of the system, which is the energy scarcity in particular months due to the seasonality in water inflows. This is
caused by the utilization of water in the reservoirs of hydropower plants during winter, which only start to refill again
in the summer when snow in the Alps melts.
Switzerland has adopted a self-scheduling market design,
with the vast majority of transactions taking place " over-thecounter " on a typically bilateral basis between the market
participants. Only a small portion of transactions are " visible " in the power exchange. There is no explicit connection between transactions and the underlying physical assets.
All technical constraints of the power plants are embedded
into the transactions or bids. Although electricity transactions have to respect the net transfer capacities among countries, the transmission constraints on the level of individual
transmission lines are not explicitly included in the market
activities-the assumption of a " copper plate " is applied.
If congestion in network elements occurs as a result of the
market activities, it is the responsibility of the TSO to apply
remedial actions to mitigate them.
From an organizational point of view, the Swiss electricity landscape is quite fragmented for a system with an annual
consumption of about 65 TWh. There are about 80 power
plant companies, even though they often have similar shareholders, and more than 600 distribution system operators
(DSOs). About 100 balance responsible parties group consumers and producers for scheduling and electricity transit
purposes. Switzerland is surrounded by member countries of
the European Union subjected to its legislation, which is not
directly adopted by Switzerland.

Operation and Operational Planning
The factors described in the previous section imply that the
operation of the Swiss transmission system is strongly affected
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by annual seasonality, short-term volatility, the availability of
timely and accurate information/data from other stakeholders,
and limited enforceability or intervention possibilities. These
challenges are addressed in a set of procedures spanning various geographical ranges and time scales.
✔ Day-ahead regional (cross-border) coordinated operational planning: To capture the effect of various parts
of the interconnected system on other parts, TSOs of
continental Europe exchange their data for the coming
day and carry out a coordinated security assessment.
If they recognize a risk, which can be mitigated only
by an action in another country, they place the corresponding request. First, topological measures (e.g., reconfiguring busbars in a substation) are checked, and
if the computed effect is not sufficient, a preventive
redispatch (or, more precisely expressed, countertrading) is triggered among TSOs. Alternatively, transmission capacities between the countries are restricted
for the trading activities of the following day.
✔ Day-ahead operational planning in Switzerland: The
data from the stakeholders are collected first, including expected power plant schedules, consumption
forecasts, and planned cross-border exchanges. An
N−1 security assessment in the form of contingency
screening is applied for the expected deterministic
scenario. In the case of congestions in the production
regions, congestion warnings are issued to the producers. Some topological measures may be applied. No
redispatch actions take place as further trading activities may take place in the intraday market.
✔ Intraday and real-time operation: Switzerland is not
included in the electricity market coupling of the European Union member states. Thus, the Swiss system
is occasionally subjected to a sudden change of the
transmission flows with minimal previous awareness.
Also, stakeholders within Switzerland may change
their positions and actions communicated in the dayahead market on very short notice due to a variety
of reasons. Therefore, a rolling security assessment
forecast is carried out every 15 min for the period of
the subsequent 4 h. Operators may then decide which
countermeasures to employ and in which sequence
(preventive, corrective) to guarantee security. Again,
typically topological measures are applied first, when
available, which are mostly determined by operators
based on experience or simulation-based methodologies. If no suitable topological measure is available,
redispatch is employed within Switzerland or if no
national redispatch measure is available, cross-border
redispatch with the neighboring TSOs is applied.

The Maintenance of Transmission Assets
The maintenance of transmission assets naturally takes
place in the summer months as environmental conditions are
most favorable at that time, especially in the Alps, and the
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IEEE Power & Energy Magazine - March/April 2021

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