of the
Sammis-Star 345-kV line in Ohio, following the
loss of other transmission lines and weak voltages
within Ohio, triggered many subsequent line trips.
Second, many of the key lines which tripped
between 16:05:57 and 16:10:38 EDT operated on
zone 3 impedance relays (or zone 2 relays set to
operate like zone 3s) which responded to over-
loads rather than true faults on the grid. The speed
at which they tripped spread the reach and accel-
erated the spread of the cascade beyond the Cleve-
land-Akron area. Third, the evidence collected
indicates that the relay protection settings for the
transmission lines, generators and under-fre-
quency load-shedding in the northeast may not be
entirely appropriate and are certainly not coordi-
nated and integrated to reduce the likelihood and
consequences of a cascadenor were they
intended to do so. These issues are discussed in
depth below.
This analysis is based on close examination of the
events in the cascade, supplemented by complex,
detailed mathematical modeling of the electrical
phenomena that occurred. At the completion of
this report, the modeling had progressed through
16:10:40 EDT, and was continuing. Thus this
chapter is informed and validated by modeling
(explained below) up until that time. Explanations
after that time reflect the investigation teams best
hypotheses given the available data, and may be
confirmed or modified when the modeling is com-
plete. However, simulation of these events is so
complex that it may be impossible to ever com-
pletely prove these or other theories about the
fast-moving events of August 14. Final modeling
results will be published by NERC as a technical
report in several months.
Why Does a Blackout Cascade?
Major blackouts are rare, and no two blackout sce-
narios are the same. The initiating events will
vary, including human actions or inactions, sys-
tem topology, and load/generation balances. Other
factors that will vary include the distance between
generating stations and major load centers, voltage
profiles across the grid, and the types and settings
of protective relays in use.
Some wide-area blackouts start with short circuits
(faults) on several transmission lines in short suc-
cessionsometimes resulting from natural causes
such as lightning or wind or, as on August 14,
resulting from inadequate tree management in
right-of-way areas. A fault causes a high current
and low voltage on the line containing the fault. A
protective relay for that line detects the high cur-
rent and low voltage and quickly trips the circuit
breakers to isolate that line from the rest of the
power system.
A cascade is a dynamic phenomenon that cannot
be stopped by human intervention once started. It
occurs when there is a sequential tripping of
numerous transmission lines and generators in a
widening geographic area. A cascade can be trig-
gered by just a few initiating events, as was seen
on August 14. Power swings and voltage fluctua-
tions caused by these initial events can cause
other lines to detect high currents and low volt-
ages that appear to be faults, even if faults do not
actually exist on those other lines. Generators are
tripped off during a cascade to protect them from
severe power and voltage swings. Protective relay
systems work well to protect lines and generators
from damage and to isolate them from the system
under normal and abnormal system conditions.
But when power system operating and design cri-
teria are violated because several outages occur
G U.S.-Canada Power System Outage Task Force G August 14th Blackout: Causes and Recommendations G
73 simultaneously, commonly used protective relays
that measure low voltage and high current cannot
distinguish between the currents and voltages
seen in a system cascade from those caused by a
fault. This leads to more and more lines and gener-
ators being tripped, widening the blackout area.
How Did the Cascade Evolve on
August 14?
A series of line outages in northeast Ohio starting
at 15:05 EDT caused heavy loadings on parallel
circuits, leading to the trip and lock-out of FEs
Sammis-Star 345-kV line at 16:05:57 Eastern Day-
light Time. This was the event that triggered a cas-
cade of interruptions on the high voltage system,
causing electrical fluctuations and facility trips
such that within seven minutes the blackout rip-
pled from the Cleveland-Akron area across much
of the northeast United States and Canada. By
16:13 EDT, more than 508 generating units at 265
power plants had been lost, and tens of millions of
people in the United States and Canada were with-
out electric power.
The events in the cascade started relatively
slowly. Figure 6.1 illustrates how the number of
lines and generation lost stayed relatively low dur-
ing the Ohio phase of the blackout, but then
picked up speed after 16:08:59 EDT. The cascade
was complete only three minutes later.
Chapter 5 described the four phases that led to the
initiation of the cascade at about 16:06 EDT. After
16:06 EDT, the cascade evolved in three distinct
phases:
u
Phase 5. The collapse of FEs transmission sys-
tem induced unplanned shifts of power across
the region. Shortly before the collapse, large
(but normal) electricity flows were moving
across FEs system from generators in the south
(Tennessee and Kentucky) and west (Illinois
and Missouri) to load centers in northern Ohio,
eastern Michigan, and Ontario. A series of lines
within northern Ohio tripped under the high
74
G U.S.-Canada Power System Outage Task Force G August 14th Blackout: Causes and Recommendations G
Impedance Relays
The most common protective device for trans-
mission lines is the impedance (Z) relay (also
known as a distance relay). It detects changes in
currents (I) and voltages (V) to determine the
apparent impedance (Z=V/I) of the line. A relay
is installed at each end of a transmission line.
Each relay is actually three relays within one,
with each element looking at a particular zone
or length of the line being protected.
u
The first zone looks for faults over 80% of the
line next to the relay, with no time delay before
the trip.
u
The second zone is set to look at the entire line
and slightly beyond the end of the line with a
slight time delay. The slight delay on the zone
2 relay is useful when a fault occurs near one
end of the line. The zone 1 relay near that end
operates quickly to trip the circuit breakers on
that end. However, the zone 1 relay on the
other end may not be able to tell if the fault is
just inside the line or just beyond the line. In
this case, the zone 2 relay on the far end trips
the breakers after a short delay, after the zone 1
relay near the fault opens the line on that end
first.
u
The third zone is slower acting and looks for
line faults and faults well beyond the length of
the line. It can be thought of as a remote relay
or breaker backup, but should not trip the
breakers under typical emergency conditions.
An impedance relay operates when the apparent
impedance, as measured by the current and volt-
age seen by the relay, falls within any one of the
operating zones for the appropriate amount of
time for that zone. The relay will trip and cause
circuit breakers to operate and isolate the line.
All three relay zone operations protect lines from
faults and may trip from apparent faults caused
by large swings in voltages and currents.
Figure 6.1. Rate of Line and Generator Trips During
the Cascade loads, hastened by the impact of Zone 3 imped-
ance relays. This caused a series of shifts in
power flows and loadings, but the grid stabi-
lized after each.
u
Phase 6. After 16:10:36 EDT, the power surges
resulting from the FE system failures caused
lines in neighboring areas to see overloads that
caused impedance relays to operate. The result
was a wave of line trips through western Ohio
that separated AEP from FE. Then the line trips
progressed northward into Michigan separating
western and eastern Michigan, causing a power
flow reversal within Michigan toward Cleve-
land. Many of these line trips were from Zone 3
impedance relay actions that accelerated the
speed of the line trips and reduced the potential
time in which grid operators might have identi-
fied the growing problem and acted construc-
tively to contain it.
With paths cut from the west, a massive power
surge flowed from PJM into New York and
Ontario in a counter-clockwise flow around
Lake Erie to serve the load still connected in
eastern Michigan and northern Ohio. Relays on
the lines between PJM and New York saw this
massive power surge as faults and tripped those
lines. Ontarios east-west tie line also became
overloaded and tripped, leaving northwest
Ontario connected to Manitoba and Minnesota.
The entire northeastern United States and east-
ern Ontario then became a large electrical
island separated from the rest of the Eastern
Interconnection. This large area, which had
been importing power prior to the cascade,
quickly became unstable after 16:10:38 as there
was not sufficient generation on-line within the
island to meet electricity demand. Systems to
the south and west of the split, such as PJM,
AEP and others further away, remained intact
and were mostly unaffected by the outage. Once
the northeast split from the rest of the Eastern
Interconnection, the cascade was isolated.
u
Phase 7. In the final phase, after 16:10:46 EDT,
the large electrical island in the northeast