IEEE Power & Energy Magazine - May/June 2015 - 60

Infrastructure
Resilience

Operational Resilience
Robustness/
Resistance
Ro

Resourcefulness/Redundancy/
Adaptive Self-Organization

Response/
Recovery

Robustness/
Resistance

Infrastructure
Recovery

Resilient
State

Rpr

Event
Progress

Rpe

Restorative
State

Postrestoration
State

Postevent Degraded State
to

tpe

Time

tpr

tir

tpir

figure 1. A conceptual resilience curve associated with an event.

framework consists of the "4Rs": robustness, redundancy,
resourcefulness, and rapidity.
The list of power system resilience definitions is endless,
but the majority of these definitions focus on the ability to
anticipate, absorb, and rapidly recover from an external,
high-impact, low-probability shock. Although a full comparison is outside the scope of this article, some key resilience characteristics that differentiate it from the concept of
reliability are shown in Table 1.

A Conceptual Resilience Curve
Associated with an Event
The illustrative conceptual resilience curve of Figure 1 shows
the resilience level as a function of time with respect to a disturbance event. This figure demonstrates the key resilience
features that a power system must possess for coping effectively with the evolving conditions associated to an event, for
instance, a heavy storm moving across the system.
table 1. Reliability versus resilience.

Reliability

Resilience

High probability, low
impact

Low probability, high impact

Static

Adaptive, ongoing, short and long
term

Evaluates the power
system states

Evaluates the power system states
and transition times between
states

Concerned with
customer interruption
time

Concerned with customer
interruption time and the
infrastructure recovery time

ieee power & energy magazine

Before the event occurs at t e a power system must be
robust and resistant to withstand the initial shock. A welldesigned and -operated power system should demonstrate
sufficient resilience (indicated here with R o where R is a
suitable metric associated to the resilience level of the system) to cope with extreme events. The capability of preventive operational flexibility is highly critical here, as it
provides the operators with the assets to configure the system in a resilient state.
Following the event, the system enters the postevent
degraded state, where the resilience of the system is significantly compromised (R pe) . Resourcefulness, redundancy,
and adaptive self-organization are key resilience features at
this stage of the event, as they provide the corrective operational flexibility necessary to adapt to and deal with the
evolving conditions (which possibly were never experienced
before). This helps minimize the impact of the event and
the resilience degradation (i.e., R o - R pe) before the restoration procedure is initiated at t r. The system then enters the
restorative state, where it should demonstrate the restorative
capacity necessary for enabling the fast response and recovery to a resilient state as quickly as possible.
Once the restoration is completed, the system enters the
postrestoration state. The postrestoration resilience level R pr
may or may not be as high as the pre-event resilience level
R o , i.e., R pr 1 R o. In particular, while the system may have
recovered from the point of view of fully returning to its preevent operational state (thus showing a certain degree of operational resilience), the infrastructure may take longer to fully
recover (infrastructure resilience), i.e., (t pir - t pr) 2 (t pr - t r) .
This would depend on the severity of the event as well as on
the resilience features that the power system will demonstrate
may/june 2015

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