IEEE Power & Energy Magazine - May/June 2015 - 59

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is evidenced by several catastrophes that occurred in the last decade or so. For example, the U.S.
northeastern states were struck by Hurricane Sandy in 2012, which destroyed over 100,000 primary
electrical wires; in addition, several substation transformers exploded, and numerous substations
were flooded. This led to the disconnection of approximately 7 million people. Over the 2010-2011
summer, Australia's second largest state, Queensland, was affected by widespread flooding that
resulted in significant damage to six zone substations and numerous poles, transformers, and overhead wires. Approximately 150,000 customers experienced power disruptions. In 2008, China was
hit by a severe ice storm, which resulted in the failure of 2,000 substations and in the collapse of
8,500 towers leading to power interruptions in 13 provinces and 170 cities.
These are only a few examples of the effect of weather-driven, high-impact, low-probability
events that a power infrastructure can experience. These events also illustrate that we need to distinguish blackouts from disasters. A blackout occurs when a large proportion of a power grid is
disabled by a combination of unplanned contingencies, resulting in a temporary power interruption.
A reliable and well-designed power system should be capable of minimizing the amount of power
disruption and of recovering very quickly from a blackout. On the other hand, a disaster, which usually includes a blackout, refers to severe and rapidly changing circumstances possibly never before
experienced. A disaster can cause the incapacitation of several and often large parts of a power grid,
which may last for a long period depending on the extent of the disaster. Hence, a power infrastructure that can maintain high levels of performance under any condition should be reliable to the most
"common" blackouts but also resilient to much less frequent disasters.
Resilience (or resiliency) comes from the Latin word "resilio," which literally refers to the ability of an object to rebound or return to its original shape or position after being stressed (e.g., bent,
compressed, or stretched). In the context of power systems, it refers to the ability of a power system
to recover quickly following a disaster or, more generally, to the ability of anticipating extraordinary and high-impact, low-probability events, rapidly recovering from these disruptive events, and
absorbing lessons for adapting its operation and structure for preventing or mitigating the impact of
similar events in the future. Adaptation thus refers to the long-term planning and operational measures taken to reduce the vulnerability to external sudden shocks.
As power engineers, how can we build a network that is both reliable and resilient? The most
obvious way is building a bigger and stronger (more redundant and robust) network. However, how
cost efficient is this approach? A more cost-efficient solution could be investing more into "smart"
operational measures. But how robust is this approach? More insights into the concept of resilience
can help address this issue.

Conceptualizing Power Systems Resilience
C.S. Holling first defined resilience in 1973 as a measure of "the persistence of systems and of
their ability to absorb change and disturbance and still maintain the same relationships between
populations or state variables." Since this foundational definition, the concept of resilience has
evolved remarkably in several systems, such as safety management, organizational, social-ecological, and economic ones. After Holling, numerous
interpretations of resilience have been developed,
resulting in many different definitions and a lack of a
universal understanding of what resilience really is.
In the context of power systems as critical infrastructures, the picture is even blurrier, as the concept
of resilience has only emerged in the last decade or
so. There have been several attempts by organizations
worldwide in the power and energy engineering communities, such as the U.K. Energy Research Centre
and the U.S. Power Systems Engineering Research
Center, to define resilience and distinguish it from the concept of reliability. According to the U.K.
Cabinet Office, resilience encompasses reliability and it further includes resistance, redundancy,
response, and recovery as key features. Another pioneer definition comes from the Multidisciplinary
and National Center for Earthquake Engineering Research, where a generic resilience framework
has been developed that is applicable to any critical infrastructure, including power systems. This

Presenting a Conceptual
Framework of Power
System Resilience

may/june 2015

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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - May/June 2015

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