IEEE Power & Energy Magazine - January/February 2014 - 102

book review

coherency theory
an excellent reference on the subject

The book reviewed in This
issue, Power System Coherency and
Model Reduction, is an excellent reference on the subject, the reviewer
states. The theme of coherency was
also a topic of an earlier monohraph
by the author, Time-Scale Modeling of
Dynamic Networks with Applications
to Power Systems.

Power System Coherency
and Model Reduction
Joe H. Chow, editor
This is a timely monograph edited by
dr. Chow on an increasingly important
topic. i learned a lot on the slow coherency theory of power systems from his
previous book, Time-Scale Modeling
of Dynamic Networks with Applications to Power Systems, published in
1982 by springer. i was excited to read
this new book.
dr. Chow states at the beginning,
"in the simulation of large power system dynamics for stability analysis on a
digital computer, there is always a tug of
war between capturing as much detail
as possible in the simulation and completing the simulation in a reasonable
amount of time so that the results can
be useful." For instance, power system
engineers with the owners or operators
of north American power transmission
grids build their system models based
on interconnection-wide models with
50,000 or more buses. The portions in
their territories are modeled in greater
Digital Object Identifier 10.1109/MPE.2013.2276202
Date of publication: 17 December 2013

102

ieee power & energy magazine

detail while the portions from other
territories or of lesser interest are usually represented by simplified equivalents to result in a customized overall
equivalent model for daily use. Thus,
an immediate challenge is to obtain an
authentic equivalent model at a manageable scale.
Coherency, as a nature of power
systems and other dynamic network
systems, is found as key for model
equivalencing and reduction, which is
one of the themes
of the 1982 monograph and of this
new book as well.
Today, coherency is
becoming an even
more attractive topic
in power systems
thanks to the increasing need for realtime power system
situational awareness
using wide-area synchrophasor measurements.Understanding
the phenomena and
nature of coherency is an approach
to gain insights on
power system dynamic patterns, oscillation modes, and potential stability issues.
This book covers: 1) a summary of the
original coherency and aggregation
procedure and slow coherency theory,
2) recent applications of linear model
reduction methods for power systems,
3) other methods for obtaining nonlinear reduced power system models, e.g.,
by artificial neural networks (Anns),

4) application of synchrophasor measurements to create reduced-order models, and 5) tracing of interarea modes,
i.e., phenomena of coherency.
The first chapter by Chow gives a
brief literature review on power system
coherency and model reduction. Model
reduction techniques are introduced in
two broad categories: 1) coherency and
aggregation-based nonlinear power system model reduction and 2) input-output
properties-based linear or nonlinear
model reduction for
external or less relevant parts of the
system.
in Chapter 2,
Podmore provides an
overview of coherency and aggregation ideas, including
key algorithms and
engineering considerations for the identification of coherent
generators, reduction
of generator and load
buses, and aggregation of generating
unit models. ePri's
dYnred computer
program is also introduced, which is one
of the most widely used software tools
for power system dynamic aggregation
and reduction. Later in Chapter 7 by
Morison et al., a practical case study is
presented to illustrate how dYnred is
applied to create a reduced-order 7,000bus model from a full weCC model to
be used by bC hydro's eMs for online
dynamic security assessment.
january/february 2014

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - January/February 2014