IEEE Power & Energy Magazine - May/June 2015 - 96

in my view

Chen-Ching Liu

distribution systems
reliable but not resilient?

O

OuTAges ReguLARLY OccuR On
distribution systems due to events such
as weather, maintenance, and equipment failures. Most outages are caused
by a single event, such as a short circuit
fault on a feeder or failure of a piece of
equipment. In a typical scenario, fault
indicators identify the section of the
feeder where the fault occurs. Remotecontrolled (or manually operated) sectionalizing switches are used to isolate
the fault, and service to customers is
restored in a matter of minutes.
An extreme event is not typical,
however. Hurricane sandy, "superstorm sandy," swept through seven
countries, causing at least 286 deaths
and an estimated damage of us$68 billion. Twenty-four states in the united
states were affected; thousands of
homes were destroyed with millions
left without power, particularly in new
Jersey and new York. In the united
states, about 87% of the outages
affecting 50,000 or more customers
since 2002 have been caused by severe
weather.

The reliability of
distribution systems  is
measured essentially by
the frequency and duration of power outages.
Indeed, two commonly
used reliability indexes
are the  system Average Interruption Frequency Index (sAIFI)
and  the system Average Interruption Duration  Index (sAIDI).
since most outage scenarios  involve simple
events, service restoration is straightforward and outage durations are short.
In a highly automated distribution system, service restoration can be achieved
by remote-controlled devices. If the
outage durations are sufficiently short,
say, fewer than 1-5 min, they are not
included in the outages for calculating
reliability indices. In other words, in a
highly automated distribution system,
the frequency and duration of outages
will be low, and, hence, the reliability is

high based on existing
reliability indices.
The resiliency of a
distribution system is
defined with respect to a
system's ability to withstand rare and extreme
events. As shown in
Table 1, numerous components may be damaged, power sources may
become unavailable, and
transmission/distribution
system facilities may not
be ready for service restoration. In addition, the communication
and remote-control capabilities may be
lost, and numerous priority tasks compete for the limited availability of field
crews and facilities. As a result, by existing reliability metrics, a highly reliable
distribution system is not necessarily
resilient. There is presently no standard
for the design of resilient distribution systems. several elements of the strategy are
well known: the hardening of distribution system components, the deployment

The reliability
of distribution
systems is
measured
essentially by
the frequency
and duration of
power outages.

table 1. Typical and catastrophic outages.
Typical Outages

Catastrophic Outages

Most cases involve a single fault/failure.
A small number of customers are affected.
Power sources are available and accessible.
Transmission and distribution system facilities
are available.
Repair and restoration are straightforward.

Multiple or numerous components are damaged.
A large number of customers are affected.
Power sources may not be available or accessible.
Transmission and distribution system facilities may be damaged
or unavailable.
Repair and restoration are complex.

Digital Object Identifier 10.1109/MPE.2015.2397332
Date of publication: 17 April 2015

96

ieee power & energy magazine

(continued on p. 93)
may/june 2015



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