ances in the western U.S. impacted millions
of customers. All of these disturbances caused considerable loss
of generation and loads and had a tremendous impact on cus-
tomers and the economy in general. Typically, these disturbances
take place when power systems are heavily loaded, are operated
outside their intended design limits, and experience multiple out-
ages within a short period of time. These wide-area disturbances
are typically characterized by large power oscillations between
neighboring utility systems, low network voltages, and conse-
quent voltage or angular instability.
The aim of this paper is to explain which relay systems are
most prone to operate during stressed system conditions, and
why relay systems operate, to share experiences and lessons
learned from the past, and to suggest protection system im-
provements to lessen the impact of blackouts and hopefully lead
us toward their prevention in the future.
I. I
NTRODUCTION

Power system blackouts have occurred since the initial de-
velopment of power systems. Most blackouts are caused dur-
ing stressed power system conditions followed by a sequence
of low-probability outages. Power system blackouts are rare
events, however, during the past 15 years and particularly
since 2003 we have experienced an increased frequency and
severity of power system failures. These failures led to major
blackouts with large economic penalties on a society that de-
pends heavily on the availability of high-quality electric
power. The August 14, 2003, major system blackout in the
Midwest and Northeast U.S. and Ontario, Canada, affected
approximately 50 million people in eight states and two Cana-
dian provinces. During the blackout, over 400 transmission
lines and 531 generating units at 263 power plants tripped [1].
Power to New York City and other affected areas was restored
approximately 30 hours later.
Recent regulatory developments, environmental con-
straints, limited power system growth, increased demands on
the electricity supply, and the need for system economic opti-
mization have a significant impact on power system reliabil-
ity. Because of these demands, power system operators are
forced to operate the system closer to its stability limits and
not in its most robust state. In addition, system operators can-
not always anticipate and predict the sequence of events of
low-probability disturbances that render the power system
more vulnerable to blackouts. The power system is designed
to operate reliably under one major contingency (N-1), that is,
loss of one major power system element: a major generation
source, a critical transmission line, or a large transmission
transformer, and not for a sequence of additional low probabil-
ity outages. Most power system blackouts occur following the
loss of successive unscheduled power system element outages
in a very short period of time during which a system operator
cannot respond quickly to prevent the occurrence of the
blackout. In recent blackouts, the unscheduled power system
outages occurred because lines were overloaded and sagged
into trees causing faults in the power system that were cleared
by relay systems, or because of inadequate reactive power
support that caused extremely low voltages, line overloads,
and subsequent operation of distance or other types of protec-
tive relays.
Recent blackouts in the U.S. and Europe over the last few
years have led to much discussion of the role played by pro-
tective relay systems during emergency or extreme power
system operating conditions. Protective relay systems are de-
signed to quickly detect faults and other abnormal conditions
in the power system, take quick action to isolate only the
faulted elements of the power system, and allow continuity of
service to electric utility customers. Protective relay systems
are often involved during major system disturbances, and in
most cases, they prevent further propagation of the distur-
bance. Sometimes, however, unwanted relay system opera-
tions caused by unexpected system loading and emergency
operating conditions during major power system disturbances
have contributed to cascading blackouts that affected millions
of people.
Many power system experts recognize that it is not feasible
to design the power system to completely prevent the occur-
rence of wide-area disturbances and future blackouts. How-
ever, by understanding the phenomena involved during major
disturbances, learning from past incidents, using good design
practices and proper relay settings, and applying new protec-
tion system technologies, we can minimize the impact of fu-
ture disturbances.
The frequency of major blackouts around the world in the
last few years is quite alarming. Adequate analysis of the
events and further research are needed to better understand the
phenomena involved, identify triggering events and mecha-
nisms, and deploy the latest technology to lessen the impacts
and possibly eliminate future blackouts. It is unfortunate and
alarming that three years after the major blackouts of 2003,
the data captured during those blackouts are not available to
researchers and experts in the industry to study the response of 2
protection and control systems to advance the art and science
of protection.
This paper discusses the performance of relaying systems
during major power system disturbances. It presents the rea-
sons why certain relaying systems are prone to operate and
their impact on the system, using relay and digital fault re-
corder data captured during power system disturbances. In
addition, the paper addresses design and setting considerations
to avoid relay misoperations during disturbances and discusses
application of existing and emerging technologies, such as
synchrophasors, to monitor and control power systems and aid
in the mitigation of future wide-area disturbances. An attempt
has been made not to disclose the source of the data used to
generate the plots that illustrate the phenomena discussed in
this paper.
II. C
OMMON
T
HEME OF MAJOR
S
YSTEM
D
ISTURBANCES

Analysis of most recent blackouts, such as the August 14,
2003, U.S. and Canada event, the September 23, 2003, black-
out in Sweden and Denmark, and the September 28, 2003,
event in Italy, indicates a common theme and points to similar
causes and outcomes [1][4]. The common theme in all of the
above-referenced blackouts is summarized below:
1. All of the blackouts occurred when the power system was
stressed the most, i.e., during times of heavy power de-
mand.
2. A number of transmission line and generator outages oc-
curred prior to the disturbances, including equipment be-
ing out-of-service for maintenance reasons, that further
weakened the power systems.
3. Operators did not detect the resulting weakening of the
power system, even though the reasons were different for
each one of the major blackouts
4. The end results were the same in all of the events
Transmission line overloading
Reactive power deficiencies
Low power system voltages
Line overloading and sagging into trees
Line and generator tripping
Relay operations
Voltage instability
Angular instability
Underfrequency load shedding (UFLS)
The primary and common causes of the most recent black-
outs were:
Inadequate level of vegetation management
(tree trimming)
Inadequate understanding of the system
Inadequate level of situational awareness
Inadequate sense of urgency regarding line overloads
and inadequate counter measures
Inadequate coordination of relays and other protective
devices or systems
III. P
HENOMENA
D
URING
S
TRESSED
S
YSTEM
C
ONDITIONS

Before we discuss relay performance during major system
disturbances, we will discuss observed power system phe-
nomena common to the most recent disturbances. One of the
main reasons for this review is to learn from these incidents
and hopefully not repeat the mistakes. Analysis, conclusions,
recommendations, and lessons learned from the 1996 distur-
bances in western North America on July 2 and August 10,
1996, were quickly forgotten and overlooked by many in the
power industry in the U.S. and abroad as if such outages
would not happen at their own power systems [5] and [6].
The two main phenomena observed during wide-area dis-
ruptions were:
Voltage collapse and
Rotor angle instability
These events can occur independently or jointly. Most re-
cent power system disturbances began with reactive power
deficiencies, line overloading, voltage instability and collapse
problems, that later evolved into angle-instability problems
because of a failure to take proper actions to return the system
from the emergency state to alert or normal states.
Fink and Carlsen [7] identified five system-operating states
(Normal, Alert, Emergency, Extreme, and Restoration), as
illustrated in Fig. 1. The power system operates in normal
state when system frequency and voltages are close to nominal
values and there is sufficient generation and transmission re-
serve.
Restoration
Extreme
Emergency
Gen = Load
<V or >V or >I
Normal
Gen = Load
V and I OK
Alert
Gen = Load
V and I OK
Gen = Load
<V or >V or >I
Gen = Load
V and I OK

Fig. 1. Diagram showing possible power system operating states
The system enters an alert state because of a reduction or
elimination of reserve margins, or for a problem with one or
several system components as, for example, when one or sev-
eral lines a