IEEE Power & Energy Magazine - November/December 2015 - 87

Integrating additional wind and solar generation
is often aided by broadening the power system operator's suite
of flexible resources.

table 1. Evolving characteristics of VG technologies.
Characteristic

Old

New

Dispatchability

Uncontrollable, "must take"

Dispatchable through participation in economic dispatch

Forecast/uncertainty

Unpredictable

Increasingly forecastable

Variability

Highly variable over multiple timescales

Very short-term variability largely mitigated via spatial
diversity

Reserve requirements

Requires dramatic increase in operating
reserves from thermal units

Relatively small increase in regulation required. Can selfprovide multiple reserves across multiple timescales with
selective/economic curtailment

Grid support

Provides no grid support/decreases grid
stability

Can provide multiple grid support services

Summary and Conclusions

For Further Reading

Wind and PV generation have emerged as mainstream
energy resources that have increasing economic competitiveness in many power systems around the world. With
increased deployment, there is also a more fundamental
understanding of VG's characteristics, impacts, and benefits.
The traditional "old" model of VG, shown in Table 1, has
been replaced by a "new" model based on detailed operational simulation as well as years of real-world experience in
systems throughout North America and Europe.
This more mature approach to assessing the impacts and
benefits of VG recognizes the ongoing challenges to integration at increased penetration levels. VG increases the
net variability and uncertainty on the system, and therefore
more creativity and flexibility is required to maintain reliable operation. In response, system operators and planners
have discovered and developed a larger set of flexibility
options both in methods to operate existing grid assets and
to deploy new technology options. On the generation side,
these options range from reexamining historical operating
practices for operating traditional thermal generation to
exploiting advanced capabilities of VG itself. On the demand
side, new markets may tap significant flexibility from dispatchable loads. Developing these flexibility resources in
the most cost-effective manner requires ongoing assessments of the various options. With proper incentives and
market designs, all flexibility options, including provisions
of multiple flexibility services from wind and solar, can be
deployed to minimize the overall costs of a clean, reliable
power system.

EirGrid, Delivering a secure sustainable electricity system.
[Online]. Available: http://www.eirgrid.com/operations/ds3/
N. W. Miller, M. Shao, S. Pajic, and R. D'Aquila, "Western wind and solar integration study phase 3-Frequency
response and transient stability, NREL/SR-5D00-62906.
Golden, CO: National Renewable Energy Lab., 2014.
M. Milligan, K. Porter, E. DeMeo, P. Denholm, H. Holttinen,
B. Kirby, N. Miller, A. Mills, M. O'Malley, M. Schuerger, and
L. Soder, "Wind power myths debunked," IEEE Power Energy
Mag., vol. 7, no. 6, pp. 89-99. Nov./Dec. 2009.
N. Miller, C. Loutan, M. Shao, and K. Clark, "Emergency
response," IEEE Power Energy Mag., Nov./vol. 11, no. 6, pp.
63-71, Dec. 2013.

november/december 2015

Biographies
Michael Milligan is with NREL, Golden, Colorado.
Bethany Frew is with NREL, Golden, Colorado.
Brendan Kirby is with Consult Kirby, Palm City, Florida.
Matt Schuerger is with Energy Systems Consulting Services, Minneapolis, Minnesota.
Kara Clark is with NREL, Golden, Colorado.
Debbie Lew is with GE Energy Consulting, Golden, Colorado.
Paul Denholm is with NREL, Golden, Colorado.
Bob Zavadil is with EnerNex.
Mark O'Malley is with University College Dublin,
Ireland.
Bruce Tsuchida is with The Brattle Group, Boston, Massachusetts.
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