IEEE Power & Energy Magazine - May/June 2015 - 61

This article discusses a conceptual framework
of power system resilience, its key features, and potential
enhancement measures.

before, during, and after the external shock. It is interesting
to notice how some measures might make the system more
resilient operationally but less resilient from an infrastructure
perspective. For instance, moving an overhead corridor underground might improve the capability of the system to withstand events, but then if the cable is damaged it may take much
longer to repair it than an overhead line. This might become a
critical issue if a new event were to happen relatively soon (for
instance, settling waves following a major earthquake wave).
It is important to highlight that for a full understanding
and assessment of system resilience, which is by definition a multidimensional concept, both the resilience levels
of and the transition times between the power system states
associated with an event are needed. Referring to Figure 1,
the system resilience is not only characterized by the levels
R o, R pe, and R pr associated with different states but also by
the transition time between states (i.e., t pe - t e , t pr - t r , and
t pir - t ir , respectively). In particular, actions to increase resilience should aim at 1) reducing the resilience level degradation during the event (R o - R pe) , 2) achieving a relatively
"slow" and possibly controlled degradation (t pe - t e) thus
also mitigating the degree of cascading; and 3) reducing the
recovery time (both from operational point of view, t pr - t r
and infrastructure point of view, t pir - t pr ). As also indicated
in Table 1, this "time dimension"
is an important feature that distinguishes resilience from reliability.

A Conceptual Long-Term
Resilience Framework
The resilience definition by the
U.S. National Infrastructure Advisory Council takes the infrastructure resilience framework a step
further, as it additionally considers
the long-term adaptation as a key
feature for achieving resilience.
This feature refers to the ongoing process of resilience building
using the information and experiences from past events to evaluate
existing resilience measures and
regularly update resilience planning and decision making. Figure 2
shows the framework for conceptualizing this infinite procedure of
may/june 2015

Power
System State

evaluating and improving power systems resilience, which is
depicted by the resilience enhancement circle.
The adaptation capacity, which enables the long-term
resilience planning, is thus a critical resilience feature as it
provides the capacity to deal with unforeseeable and continuously changing conditions. As can be seen in Figure 2,
the first step toward this goal is to perform vulnerability and
adaptation studies using the input from past experiences
and/or simulations. This would help detect the vulnerabilities
of a power system at the different stages associated to an
event, i.e., before, during, and after, and develop the adaptation strategies necessary for improving the key resilience
features and enhancing the response of the power system to
the evolving conditions during a similar event that were to
occur in the future.
Based on this analysis, the resilience enhancement
measures are identified and prioritized depending on the
criticality and contribution of each measure for improving
resilience. These may refer to operational and/or reinforcement measures, as will be discussed later in this article.
However, some of these measures are more resilience efficient than others, and some measures are more cost efficient
than others. Therefore, a cost/benefit analysis would help
gain insights on the benefits of implementing each measure

Resilient
State

Degraded
State

Restorative
State

Vulnerability/
Adaptability Studies

Application of
Identify and Prioritize
ResilienceResilience-Enhancement Enhancement Circle Resilience-Enhancement
Measures
Measures

Cost/Benefit Analysis

figure 2. A conceptual long-term resilience framework.
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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - May/June 2015