IEEE Power & Energy Magazine - March/April 2015 - 70

are other serious impacts of concern to the utility, but
these can often be mitigated using relatively straightforward strategies that are usually not overly costly
and follow typical utility practices.
✔✔ Advanced PV impact study processes based on salient
operating periods greatly reduce the data and computational requirements needed to complete a study.
The results from these studies represent the worst-case
possible impacts but can also be used as a proxy for
overall circuit impacts due to a specific PV system
interconnection.
✔✔ Constant-power-factor operation, perhaps the simplest form of advanced PV inverter functionality, is
very effective at mitigating the voltage-related PV
impacts present at the PV system's point of interconnection. In addition, all three high-penetration PV
circuits investigated showed that the optimal power
factor settings (i.e., power factors around -0.95), chosen to reduce voltage-related PV impacts, require a
reasonable amount of reactive power to be absorbed
by the PV system. Such power factor settings can be
implemented with low initial capital cost to the PV
system developer as well as low operating costs over
the life of the PV system.
✔✔ The PV impacts observed on the three study circuits
show that while impacts are measurable, the voltage
swells and positive voltage deviations are of low magnitude. Voltage deviation events that exceed 2.5% pu
are on the order of one per month or fewer for areas
that generally have clear weather (i.e., areas that don't
have notable local weather regimes).
✔✔ PV systems are capable of operating at off-unity
power factor with a control bandwidth higher than the
speed of resource variability for utility-scale systems
interconnected to the distribution system.

Future Work
This study has made good headway in determining how to
increase the amount of PVs integrated onto the distribution
system. Distributed PV deployment at all scales has seen
unprecedented growth since this project began in 2010, however. The continued, if not increased, interest in and realization of high-penetration PV deployment necessitate the
following future work:
✔✔ There must be a determination of the transmission- and subtransmission-level impacts of high penetrations of PVs interconnected on the distribution
system with and without active PV impact mitigation using advanced PV inverter functionality. This
would require detailed, unbalanced models of large
areas of a utility's service territory so as to understand how real and potentially reactive power flows
are modified on the transmission and subtransmission levels by distributed PVs and how to mitigate
these impacts, if necessary.
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✔✔ Dynamic analysis of the transmission and distribution

systems to determine the necessary minimum functionality that would be required of distributed PVs to
maintain bulk system reliability is needed.
✔✔ Further investigation and demonstration of how to
coordinate the operation of advanced PV inverter
functions with each other and with existing utility
equipment are required.
✔✔ Additional high-penetration distributed PV integration
studies on utilities operating under other operating constraints or practices, such as voltage regulation, and in
areas with high or low load growth must be performed.
✔✔ Continued field data acquisition campaigns to quantify
the impact of PV integration at ever higher penetrations and specifically designed for other important PV
impacts, such as system protection and transient overvoltage concerns, must take place.

Acknowledgments
This work was conducted under the dual support of the
Office of Energy Efficiency and Renewable Energy of the
U.S. Department of Energy (DOE) and the Research, Development, Demonstration, and Deployment program of the
California Solar Initiative. The authors wish to acknowledge
the support of Alvin Razon, Ranga Pitchumani, and Kevin
Lynn of the DOE and Stephan Barsun and Ann Peterson of
iTron. NREL's portion of this work was supported by the
DOE under contract number DE-AC36-08-GO28308.

For Further Reading
B. Mather, B. Kroposki, R. Neal, F. Katiraei, A. Yazdani, J. R.
Aguero, T. E. Hoff, B. L. Norris, A. Parkins, R. Seguin, and
C. Schauder, "Southern California Edison High-Penetration
Photovoltaic Project-Year 1 Report," NREL, Golden, CO,
Tech. Rep. TP-5500-50875, 2011.
J. Hambrick and D. Narang, "High-penetration PV
deployment in the Arizona Public Service System, Phase 1 update," in Proc. IEEE PV Specialist Conf., Austin, TX, 2012.
R. Broderick, J. Quiroz, M. Reno, A. Ellis, J. Smith, and
R. Dugan, "Time series power flow analysis for distribution
connected PV generation," SNL, Albuquerque, NM, Tech.
Rep. SAND2013-0537, 2013.
B. Mather, S. Shah, B. Norris, J. Dise, L. Yu, D. Paradis,
F. Katiraei, R. Seguin, D. Costyk, J. Woyak, J. Jung, K. Russell, and R. Broadwater, "NREL/SCE High-Penetration PV
Integration Project: FY13 annual report," NREL, Golden,
CO, Tech. Rep. TP-5D00-61269, 2014.
B. Mather, "NREL/SCE High-Penetration PV Integration Project: Report on field demonstration of advanced inverter functionality in Fontana, CA," NREL, Golden, CO,
Tech. Rep. TP-5D00-62483, pp. 14, 2014.

Biographies
Barry Mather is with NREL, Golden, Colorado.
Sunil Shah is with SCE, Westminster, California.

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