IEEE Power & Energy Magazine - January/February 2014 - 104

Chapter 3 by Chow gives an indepth presentation of the theoretical
and analytical basis of slow coherency
and aggregation. The slow coherency
theory arises from the fact that coherent groups of machines swing against
each other at slower oscillatory frequencies, so-called interarea modes,
than the local oscillation modes within
individual groups. The chapter shows
that the slow coherency phenomenon is
attributed to the coherent areas being
weakly coupled in the system. The
singular perturbations theory is introduced to exploit the time-scale separation of the interarea and local modes,
and then eigenvector-based grouping
algorithms are presented for identification of coherent machines. study
results on an nPCC 48-machine system
are presented to illustrate and compare
aggregation algorithms. Chapter 4, by
Chow et al., focuses on exciter aggregation and investigates a trajectory sensitivity method to tune parameters with
aggregated nonlinear exciter models.
in Chapter 5, vittal et al. introduce
a hybrid dynamic equivalent consisting of both a coherency-based conventional equivalent and an Ann-based
equivalent and test it on a system
representing a portion of the weCC
system. The idea behind the hybrid
equivalent is to compensate for the
discrepancy between the full system
model and the reduced equivalent
developed using, e.g., dYnred, by

providing appropriate power injections at all the boundary buses of the
retained area.
Chapter 6 by Liu et al. introduces
two mathematical approaches to model
reduction based on krylov subspace
and balanced truncation methods,
which focus on modeling the input-output behavior at the boundary buses of a
study area. For example, the balanced
truncation method, as an improvement of the krylov subspace method,
takes into account controllability and
observability of the original models
to obtain reduced-order models of the
external system. Thus, it preserves key
system characteristics while removing
redundant information for handling a
very large system. The system model
is updated based on sensitivity analysis
to ensure that the reduced-order model
follows the actual system.
Chapter 8 by Chakrabortty et al.
focuses on constructing simplified
interarea models of large power systems by using dynamic measurements
from synchrophasors installed at
selected points on key power transfer
paths. A measurement-based interarea
model estimation method is introduced
and illustrated using the weCC system, and its potentials in wide-area
monitoring are investigated.
Chapter 9 by rouco et al. introduces
a selective modal analysis framework
for the modeling, analysis, and control of selected dynamics of systems

described by large linear time-invariant models. The roles of participation
factors in the identification of dynamic
patterns, design of damping controllers, and reduced-order eigen analysis
are emphasized and discussed.
Chapter 10 by vanfretti et al.
demonstrates that the analysis on
interarea oscillations observed from
synchrophasor measurements of network variables, such as voltages and
line currents, helps trace how electromechanical oscillations spread
through the power network following
a disturbance. The concept of "dominant inter-area oscillation paths" is
developed to identify the passageways
where the interarea modes of concern
travel the most and is applied in feedback input signal selection for damping controller design.
in summary, i enjoyed reading this
monograph and would highly recommend it. it is an excellent reference
providing an overview of power system coherency, model reduction, and
related problems by integrating analytical bases, engineering practices,
emerging techniques, and insights,
which would benefit both engineers
and researchers in the fields of power
system dynamics.
-Kai Sun

p&e

Moving Filler

104

ieee power & energy magazine

january/february 2014

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