IEEE Power & Energy Magazine - March/April 2021 - 62

network decreased significantly. Figure 3 illustrates the
country-wise aggregated load profile right before and after
the lockdown, where the sudden reduction in consumption is
apparent. The subsequent higher injection of reactive power
from distribution grids (because feeder cables are capacitive
at light loading conditions) and the lower inductive absorption of reactive power in transmission lines increased the risk
of local overvoltages. To mitigate this risk, additional reactive
power reserves were activated. Also, topological measures
(e.g., line switching and transformer tapping) were implemented to decrease or redistribute reactive power flows.
As mentioned in the " Operation and Operational Planning " section, preventive remedial actions are typically

Load Relative
to Peak Load in March 2019

Overall Trends in System Operation

0.9
0.8
0.7
0.6
0.5
0.4
0.3

20 40 60 80 100 120 140 160 180
Hour of the Week, Starting Sunday 00:00
Average Consumption in March 2019
Week Before Lockdown
First Week of Lockdown
Second Week of Lockdown

figure 3. The impact of the lockdown measures on Swiss
electrical consumption.

Number of Topological Measures

2,000
Load Flow Optimization
Voltage Optimization
Total

1,800
1,600
1,400
1,200
1,000
800
600
400
200
0

2015

2016

2017
Year

2018

2019

figure 4. The trend in topological measures as remedial
actions.
62

determined by the operators based on experience or computed using simulation-based approaches. Due to the
advantages of optimization-based methods, such methods
are being investigated by some TSOs for determining the
best remedial actions. Optimization models are mostly
based on dc network models as they target avoiding line
overloadings. Nevertheless, the COVID-19 situation
showed the significance of an operational philosophy
allowing system operators to implement preventive measures to circumvent voltage violations in addition to typical curative measures. To efficiently and systematically
compute preventive measures against voltage violations,
optimization models that can handle ac network models
would be required together with the associated computational and algorithmic innovations.

ieee power & energy magazine

The previous analysis of recent grid events revealed specific
aspects of system operation that would benefit from a new
operational philosophy. To evaluate the need for a new operational paradigm, one can also observe the overall trends in
system operation in recent years. In this section, we present a simple analysis of the historical remedial actions in
the Swiss transmission system taken against N−1 or voltage
security violations. The remedial actions include topological measures to optimize either power flows or voltage profiles as well as generation redispatch. Furthermore, we distinguish between two types of redispatch actions: national
and international redispatch. National redispatch actions
are on Swiss power plants in response to internal issues
in the Swiss transmission grid. On the other hand, international redispatch actions implemented by Swiss power
plants are part of an international coordinated redispatch
effort to resolve problems in either the Swiss or neighboring
power grids.
Figure 4 displays the annual number of topological measures from 2015 to 2019 that were implemented either to
optimize the power flows and relieve congestions or to optimize the voltage profile across the grid. A linear increasing
trend is visible in both cases.
Figure 5 demonstrates the trend in redispatched energy
from 2014 to 2019. Besides the total energy values, we
present the breakdown into Swiss and international
redispatches as well as generation decrease and increase
actions. Even though it is hard to identify clear patterns
for the various combinations, there is a general increasing trend for the total redispatched energy. A clearer trend
exists in the frequency of redispatch actions, which is
quantified by the number of days when at least one redispatch action was carried out (Figure 6). Furthermore, it is
relevant to mention that the majority of redispatch actions
was international (79% in terms of redispatch duration).
This observation underlines the significance of TSO-TSO
coordination as part of today's operational procedures, as
explained previously.
march/april 2021

IEEE Power & Energy Magazine - March/April 2021

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