IEEE Power & Energy Magazine - November/December 2015 - 32

Transient Stability in a Nutshell
Lots of power engineers spend their entire careers immersed

of whether or not the node to which they are connected

in the complexities of studying transient stability. For every-

is moving.

one else, the topic can be opaque. In essence, transient sta-

The scissors represent a disturbance, which might cut a line

bility analysis answers the question "Does the system return

or disconnect a generator. The rubbery mass-spring system

to an acceptable condition within the first minute of being

bounces around. If the event is too severe or some of the lines are

perturbed by a generation or transmission line outage?" Fig-

stretched too taught (too much loading), more lines will break.

ure S1 shows a widely used visualization of the problem,

It is easy to imagine a cascading failure in which each succes-

updated to include wind and photovoltaic (PV) power. The

sive break leads to another failure. A substantial part of system

round masses represent generators, with the tension on

planning is aimed at avoiding such unacceptable consequences.

the various springy lines representing power transfer. The

This article examines differences in behavior suggested by

board at the top represents the academic fiction of an infi-

the weight versus the hand. The article "Emergency Response"

nite bus-a real, finite power system is floating. The level at

listed under "For Further Reading" explores different ways to

which it is floating is a proxy for frequency, which must stay

make the controls of wind and PV generators (the hands in

very close to 60 Hz. The hands represent wind and PV. They

the drawing) smarter for maintaining frequency. That work

put tension (inject power) into the system, but they are all

shows that available controls for wind plants can be consid-

control and no weight. The mission of these devices, unless

erably more effective than the initially dead weight (inertia) of

taught to do otherwise, is to pull uniformly, regardless

synchronous machines.

figure S1. Visualization of the transient stability problem.

Transient stability is dominated by the dynamic behavior
of the essential elements of the power system during the first
minute following a system disturbance. The primary concern
is that the power system return to a near equilibrium state that
is acceptable to customers and equipment. Engineers worry
about a wide range of events, but the quintessential disturbance is a lightning strike causing a short circuit on a critical
(usually heavily loaded) transmission line, which is then isolated by circuit breakers that trip the faulted line. The period
during which the short circuit is detected and isolated (the
"fault duration") is typically on the order of 1/20th to 1/5th
of a second: a seemingly short time, but one that can inflict a
great deal of turmoil on the grid.
32

ieee power & energy magazine

During the first century of interconnected AC power systems, the dynamics of the system were dominated by the
behavior of synchronous generators. By now, the behavior
of synchronous machines is well understood, and models of
acceptable fidelity reflecting that behavior are widely available.
A body of experience, practice, and supporting tools evolved to
allow planning and operations decisions that assure acceptable
levels of transient stability. All the North American grids shown
in Figure 1 have substantial operational limitations that hinge
on transient stability. Often, transient stability considerations
limit the amount of power that can be pushed across individual
power lines and across interfaces between different portions of
the grid. Because transient stability limits bound the operational
november/december 2015



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