IEEE Power & Energy Magazine - July/August 2017 - 102

CPU 1

CPU 2

EMT Model

EMT Model
G

TCP/IP
Communication
Large
System

Large
System

Transient Stability Model

CPU 1

CPU 3

Pure Transient Stability Model

Hybrid EMT/TS Model

figure 7. A transient stability model running on a single CPU and the same
model shared across multiple CPUs (on the same computer or different computers across a network), with portions of the model run using EMT tools capable
of more detail.

software versions between the subsystems do not have to match. For simulation engineers, the implications of this
time reduction are important. engineering and planning studies employing
emT tools in which model size is currently limited by the speed of a desktop
computer can now increase the size of
the models according to the number of
Cpus across the entire workplace network, provided care is taken to identify
and separate subsystems correctly.

Hybrid and
Cosimulation Tools
The technology that allows multiple
emT simulations to run in parallel also
allows entirely different types of simulation to run in parallel. Figure 7 shows
an example where portions of a conventional transient stability network model
are separated out and run using an emT
tool. The transient stability tool for simple systems and the emT tool capable of
modeling more detail run together using
different simulation time steps.
each portion of the network is run
on a separate Cpu (either on the same
computer or multiple computers across
a Lan). as the simulation runs, the subsystems communicate boundary elec102

ieee power & energy magazine

trical conditions with each other at the
larger simulation time step of the transient stability model. Instantaneous threephase quantities from the more-detailed
emT models are converted to phasor
quantities using discrete Fourier transform algorithms. These new tools are being used in very demanding applications
or in regions where the large adoption of
renewable technologies is straining conventional tools used by planners and operators. as multiple commercial tools are
entering the market and planners begin to
see the advantages of expanding their capability, wider adoption is expected.

Conclusions
as power system planners continue to
strive for a reliable and cost-efficient grid,
developers and equipment designers are
responding to increasing pressure for
growth in renewable energy's share of the
energy supply. HVdc, demanding much
greater sophistication in control than ac,
is growing rapidly in its share of energy
delivery responsibility, and its role in future networks will continue to grow.
Tomorrow's grid will be more complex and sophisticated than today's,
and classical simulation tools must, by
necessity, become more complex and

more closely integrated with detailed
emT tools, all the while allowing human engineering intuition and analysis
to be exercised. Simulation and modeling engineers will continue to respond
to further evolution of our electric power systems with new and more sophisticated simulation methods.

Acknowledgments
The history of power systems simulation
is filled with towering intellectual giants,
brave entrepreneurs, and many stories.
as a (relatively) young engineer writing
about things that have occurred in the living memory of many still-active players,
it is practically guaranteed that I will omit
some important detail or major character
and leave a great story untold. In compiling this article, significant contributions
were supplied by some key people, including Dennis Woodford, Lionel barthold, and John undrill, some of which
have been inserted nearly verbatim. additional support was provided by Ian
grant, garth Irwin, ani gole, and Don
Ramey. any errors or omissions in this
article are purely my own responsibility,
and any correct information is certainly
due to these generous helpers. We encourage further contributions or clarifications on these or other items in the form
of letters to the editor of this magazine.

For Further Reading
R. J. brown and W. F. Tinney, "Digital solutions for large power networks,"
trans. am. Inst. electr. eng. Part III:
Power app. Syst., vol. 76, no. 3, pp. 347-
351, apr. 1957.
W. F. Tinney and J. W. Walker, "Direct solutions of sparse network equations by optimally ordered triangular
factorization," Proc. Ieee, vol. 55,
no. 11, pp. 1801-1809, nov. 1967.
n. Sato and W. F. Tinney, "Techniques for exploiting the sparsity of
the network admittance matrix," Ieee
trans. Power app. Syst., vol. 82, no. 69,
pp. 944-950, Dec. 1963.
H. Dommel, "Digital computer solution of electromagnetic transients in
single and multiphase networks," Ieee
trans. Power app. Syst., vol. 4, pp.
p&e
388-399, apr. 1969.
july/august 2017

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