IEEE Electrification Magazine - September 2015 - 51

Modeling Verifications
The Electric Power Research Institute, through a nondisclosure agreement under a separate project, was able to obtain
actual field data from an actual BESS unit in operation in the
United States. The battery is a 36-MVA unit capable of
24 MWh for 3 h. The vendor requires that it be always operated in an SOC between 20 and 80%. The measured data
from one operating condition were taken, and we simulated
the sequence of power reference (Pref) changes for which
the actual response of the battery was recorded. The results
are shown in Figure 7 and demonstrate that the fit is quite
good. Also, although not shown here, tests have been done
to confirm similar results can be achieved in the beta implementation of the model in commerical software platforms
that have adopted this model. The model was recently
approved at a meeting of the WECC Modeling and Validation
Working Group and should in due course be released in the
official version of several commercial software platforms
that are widely used in North America.

Summary
The impetus to develop and install renewable generation in
the bulk power system continues and has accelerated in
recent years. In 2014, wind generation capacity increased
worldwide by more than 51 GW, which was a 45% increase
compared the amount installed in 2013. With this continued
increase in renewable energy deployment throughout the
world, it is inevitable that energy storage will soon start to
find a key function to play in the power systems of the
future. One of these technologies is battery energy storage.
Modeling these technologies will be equally important for
transmission planners and operators as they continue to
perform studies at all levels to assess system reliability. This
article discussed a simple model for use in large-scale time
domain stability studies. There is no doubt that more sophisticated models will be needed for other applications and
analysis, and this type of work will continue.

Further Reading
Lithium Ion Technology Status and Directions: 2013 Update. Palo
Alto, CA: EPRI, 2013.
[Online]. Available: http://www.epri.com/abstracts/Pages/
ProductAbstract.aspx?ProductId=000000003002001314
(2014, 23 Jan.). Specification of the second generation
generic models for wind turbine generators. WECC Approved

Real Power (pu)

0.4
0.3
0.2
0.1
0
-0.1

0

10

20

30
40
Time (s)

50

60

70

50

60

70

(a)
Real Power (pu)

There are several clear assumptions inherent in this
model. First and foremost, the battery chemistry is
ignored, as the details are not pertinent to simple stability
studies. Also, the details of the dynamics of the dc current
circuitry are neglected since, again, it is assumed that this
is not particularly relevant to power-system studies. Finally, the model is a positive sequence model, which is meant
for stability analysis and cannot represent a detailed
three-phase response of the equipment as might be needed for design studies.

0.06
0.04
0.02

EPRI Model
Measured
Field Response

0
-0.02

0

10

20

30
40
Time (s)
(b)

Figure 7. Simulation and measurement of an actual 36-MVA BESS
unit's response to a sequence of Pref changes (device in constant
pf control): (a) the real power response per unit, and (b) the reactive
power response per unit.

Document. [Online]. Available: http://www.wecc.biz/Reliability/WECC%20Second%20Generation%20Wind%20Turbine%20
Models%20012314.pdf
(2013). Technical Update-Generic Model for Wind Turbine
Generators and Photovoltaic Generation and Model Validation.
Palo Alto, CA: EPRI. Product ID# 3002001002. [Online]. Available:
http://www.epri.com/abstracts/Pages/ProductAbstract.aspx?
ProductId=000000003002001002&Mode=download.
A. Ellis, P. Pourbeik, J. J. Sanchez-Gasca, J. Senthil, and
J. Weber, "Generic wind turbine generator models for WECC-
A second status report," in Proc. IEEE PES General Meeting 2015,
Denver, CO, paper 15PESGM0126.

Biographies
Pouyan Pourbeik (pouyan@ieee.org) is a senior technical
executive with the Electric Power Research Institute in
Irving, Texas.
Stephen E. Williams is a principal engineer with S&C
Electric Company in Franklin, Wisconsin.
James Weber is the director of software development at
PowerWorld Corporation in Champaign, Illinois.
Juan Sanchez-Gasca is technical director at GE Energy
Inc., in Schenectady, New York.
Jay Senthil is a senior staff engineer at Siemens PTI Inc.,
in Schenectady, New York.
Shengli Huang is a senior engineer at Puget Sound
Energy in Bellevue, Washington.
Kent Bolton is a staff engineer, planning services, at the
Western Electricity Coordinating Council in Salt Lake
City, Utah.

IEEE Electrific ation Magazine / S EP T EM BE R 2 0 1 5

51


http://www.wecc.biz/Reliabili http://www.epri.com/abstracts/Pages/ProductAbstract.aspx http://www.epri.com/abstracts/Pages/

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