IEEE Power & Energy Magazine - July/August 2020 - 55

The overall resilience is the aggregation of system performance over all possible adverse events. Obviously, it cannot
be calculated as a precise number given the lack of knowledge of all relevant types of events and their probabilities.
However, analyzing specific events allows for a qualitative
comparison of the relative resilience impact of certain measures and approaches for improvement.

Application of Resilience Metrics
for Power System Restoration
In the autumn of 2005, there was a major outage in a part
of the German distribution system the like of which had not
occurred for decades. It was caused by an extreme weather
situation. Beginning in the morning of 25 November and
increasing throughout the day, heavy snow and ice cover
accumulated on overhead lines. In combination with unfavorable wind, multiple pylons of the 110-kV high-voltage
(HV) network and hundreds of pylons within the mediumvoltage (MV) network failed, causing an outage that affected
up to 250,000 people in 25 municipalities. It took more than
four days until all customers were reconnected to the power
system. Due to the high number of destroyed lines, it was
not possible to reconnect all loads by means of the existing
infrastructure. Mobile emergency supply diesel generators
as well as provisional lines had to be brought into the region.
Figure 1 shows a schematic of the incident relating the
events to common resilience terms. The timestamp te marks
the onset of the adverse event. After te, in the first hours of
the extreme weather event, when ice acumulated on the overhead lines, the power system was able to "resist" due to the
designed mechanical robustness. However, at t1 the first overhead line failed. Therefore, the performance of the power
system from the operators' point of view decreased. But due
to N−1 contingency design (defined absorption capacity), the
system was able to "absorb" this contingency. Consequently,

System Performance

Resist

Absorb

the hard measure of system performance-supplied/unsupplied customers-was not yet reduced. Subsequent line failures exceeded the available absorption capacity, resulting in
unsupplied customers. To resupply customers, emergency
generators and provisional lines were brought to the region
to "adapt" the damaged infrastructure, and after five days all
customers were provisionally resupplied (t = t4 in Figure 1).
The system performance from the customers' point of view
was restored as soon as they were resupplied.
In the terminology of power system operation, parts of
the adaptive as well as the operation restoration phase are
called power system restoration because it aims at resupplying all customers. However, from the operators' point of
view, the provisional measures represented a still-impaired
state of the system, and it took much longer to "restore"
the infrastructure and end the emergency service arrangements. After the complete reconstruction, on the other
hand, the performance from this perspective was even better than before the adverse event. The components were
not just replaced by equivalents but in some cases by more
robust versions. Therefore, the infrastructure restoration and
improvement phases overlap in the example. The indicator power system performance from an infrastructure perspective does not only include the indicator of "supplied
customers" from a system operational perspective but also
accounts for addtional criteria such as observability and operational reserves.
Today, this region contains a large amount of distributed
generators (DGs), such as wind energy, photovoltaic (PV), and
biomass plants. At many times, the available primary energy
and distributed energy resources (DERs) could serve a significant share of the load. However, there is almost no islanding capability from the technical or the regulatory points of
view. Additional communication infrastructure, grid-forming
devices/controllers, and operational procedures would be

Restore
Operation

Adapt

Restore
Infrastructure

Improve

Improved SP

1

Restored SP
Adverse
Event
Power System Performance
Supplied Customers

0
te

t1

tf

t2

t3

t4

t5

t6

Time

Power System Restoration

figure 1. The changes in system performance and supplied customers over time during an outage event, naming the
resilience categories resist, absorb, adapt, restore, and improve. SP: system performance.
july/august 2020

ieee power & energy magazine

55



IEEE Power & Energy Magazine - July/August 2020

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - July/August 2020

Contents
IEEE Power & Energy Magazine - July/August 2020 - Cover1
IEEE Power & Energy Magazine - July/August 2020 - Cover2
IEEE Power & Energy Magazine - July/August 2020 - Contents
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IEEE Power & Energy Magazine - July/August 2020 - Cover3
IEEE Power & Energy Magazine - July/August 2020 - Cover4
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