IEEE Power & Energy Magazine - March/April 2017 - 73

inverter-based distributed generation. These techniques monitor grid conditions and actively try to push grid voltage and
frequency out of the normal operating parameter so the units
trip quickly. This works well when the grid is stiff, but the
basic principle of this anti-islanding technique actually tries
to destabilize the grid. In a future grid with much more distributed generation and potentially weaker grids, new antiislanding techniques will need to be used that ensure overall
grid stability. These may be communications based, wherein
a permissive signal is sent to generators or phasor measurements are compared across the grid.

Black Start
One consideration for operating inverter-dominated ac
power systems is the need to start grids once they have
gone down, known as "black start." This ability to restart
a grid is critical to overall system reliability. To accomplish this, the generation on the system needs to be able to
both act as a voltage source and provide adequate power to
start electrical equipment with high in-rush currents, such
as transformers and motors. Synchronous generators are
able to do both of these tasks when the load is properly
sized to their capabilities. Inverter-dominated systems will
need to be able to provide sufficient starting current, or
the loads must be segregated in such a manner as to enable
controlled repowering of the grid. These special characteristics of inverter-based sources must be considered by reliability organizations in their plans to restore power supply
after blackouts or natural disasters.

Conclusions
Achieving 100% VRE grids will require
✔✔ better ways of matching supply and demand over multiple timescales
✔✔ significant curtailment
✔✔ proper operations with very high instantaneous penetrations of VRE.
As ac power systems evolve from synchronous generatordominated systems to inverter-dominated ones, we must
ensure that these technologies operate in a compatible
manner. This incudes designing inverter-based systems to
provide system stability and additional grid services necessary for proper ac power system operations. These requirements for grid stability should be incorporated at all sizes of
inverters because a future grid might have extremely large
numbers of small, highly distributed VRE systems. Initial
studies have shown that with proper control considerations,
inverter-based systems can not only maintain or improve
grid stability under a variety of contingencies but also dramatically improve the response characteristics of power systems and increase operational stability.

Acknowledgments
The authors would like to acknowledge the U.S. Department of Energy's Solar Energy Technology Office and
march/april 2017

Wind Energy Technology Office for their support funding
several of the studies described in this article.

For Further Reading
A. Bloom, A. Townsend, D. Palchak, J. Novacheck, J. King,
C. Barrows, E. Ibanez, M. O'Connell, G. Jordan, B. Roberts,
C. Draxl, and K. Gruchalla, "Eastern renewable generation
integration study," NREL, Golden, CO, Tech. Rep., NREL/
TP-6A20-64472, Aug. 2016.
M. M. Hand, S. Baldwin, E. DeMeo, J. Reilly, T. Mai,
D. Arent, G. Porro, M. Meshek, and D. Sandor, "Renewable
electricity futures study," NREL, Golden, CO, Tech. Rep.,
NREL/TP-6A20-52409, June 2012.
P. Denholm and R. Margolis, "Energy storage requirements for achieving 50% solar photovoltaic energy penetration in California," NREL, Golden, CO, NREL/TP6A20-66595, Aug. 2016.
J. Taylor, S. Dhople, and D. Calloway, "Power systems
without fuel," Renew. Sustain. Energy Rev., vol. 57, pp.
1322-1336, May 2016.
J. O'Sullivan, Y. Coughlan, S. Rourke, and N. Kamaluddin, "Achieving the highest levels of wind integration: A
system operators perspective," IEEE Trans. Sustain. Energy,
vol. 3, no. 4, pp. 819-825 Oct. 2012.
V. Gevorgian, Y. Zhang, and E. Ela, "Investigating the
impacts of wind generation participation in interconnection
frequency response," IEEE Trans. Sustain. Energy, vol. 6,
no. 3, pp. 1004-1012 July 2015.
N. Miller, B. Leonardi, R. D'Aquila, and K. Clark, "Western wind and solar integration study phase 3A: Low levels of
synchronous generation," NREL, Golden, CO, Tech. Rep.,
NREL/TP-5D00-64822, Nov. 2015.
D. Ramasubramanian, V. Vittal, and J. Undrill, "Transient stability analysis of an all converter interfaced generation WECC system," in Proc. Power System Computation
Conf., Genoa, Italy, 2016.

Biographies
Benjamin Kroposki is with the National Renewable Energy
Laboratory, Golden, Colorado.
Brian Johnson is with the National Renewable Energy
Laboratory, Golden, Colorado.
Yingchen Zhang is with the National Renewable Energy
Laboratory, Golden, Colorado.
Vahan Gevorgian is with the National Renewable Energy Laboratory, Golden, Colorado.
Paul Denholm is with the National Renewable Energy
Laboratory, Golden, Colorado.
Bri-Mathias Hodge is with the National Renewable
Energy Laboratory, Golden, Colorado.
Bryan Hannegan is with the National Renewable Energy
Laboratory, Golden, Colorado.
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