Variable solar generation does not inherently present a barrier to connecting to the grid. While solar output can fluctuate rapidly at a single site, geographical diversity reduces fluctuations across multiple sites. Short-term variability is reduced and has minimal system impacts when forecast and accounted for properly. As solar penetration increases, additional flexibility will be required from generation, transmission, demand response and storage to integrate variable output cost-effectively. With changes to planning and market practices, high levels of solar can be accommodated without overbuilding of conventional generation.

1. Connecting Variable Solar To The Grid
Carl Lenox
Principal Engineer
GW Solar Institute Symposium
April 26th 2011
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2. Does PV Variability Present A Barrier?
Some have used the following argument:
BAD
However, this does beg a few important questions, such as:
• How rapid are these changes, and how often do they occur?
• Does the observed behavior of a single system scale? If so, how?
• What are the impacts of variability on the utility infrastructure and the customer?
• How do these impacts change as penetration increases?
• What mitigations are available for these impacts? What are the best solutions?
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3. Geographical Diversity Is A Crucial Factor
High Irradiance Variability At Single Sites Is Reduced With A Portfolio Of Sites
Single Location
20 Locations
Source: Source:
Weimken Mills et.
et. al. al. 2010
2001
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4. Operational Timeframes & Impacts of Variability
M. Milligan, NREL Timeframe System Local
Impacts Impacts
Seconds to Regulation Voltage
Minutes Fluctuation
10’s of Minutes Load Voltage
to Hours Following Profile
Hours to Days Scheduling
The Electrical System Is Designed To Manage Load Variability
Variable Generation Is Not Fundamentally Different
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5. System Level Impacts Of Short Duration Variability
• System level impact is cost required to provide
incremental frequency regulation due to added sub-
10 minute variability from PV.
• PV integration cost per recent LBNL (Mills &
Wiser) study - comparable to wind, because
• Geographical diversity substantially damps
short duration fluctuations
• Reserves can be scheduled based on
deterministic “clear sky” envelope
• Regulation costs for wind (up to ~ 30%
penetration) across multiple studies are generally
very modest at <$1 / MWh.
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6. Diurnal Variability Must Also Be Considered
• Daily solar cycle adds load following and unit
commitment integration costs; bigger ramps.
• LBNL, NREL (EWIS / WWIS) and others find
modest total integration cost up to ~30%
energy penetration: typically less than $5 /
MWh (for wind and solar).
• Forecast error dominates cost, PV Denholm et al 2008 (% system energy)
forecasting is new, often assumed to be very
inaccurate in integration studies (5-20% error)
• However 4-5% RMSE is achieved in practice
for regional-level PV forecasts in Germany,
comparable to best in class wind forecasting.
• Example: Spain at ~16% VER energy: 14%
wind, 2% PV (3.4 GW), with limited interties CPUC 33% RPS Reference Case:
but world-class operations. Peak of 54% of ~25% energy from VERs ; 11% solar
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system demand served by wind.
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7. Cost Of Integrating PV Is A Function Of Flexibility
As VER penetration increases beyond
20-30% of energy:
• Marginal (offset) generation costs drop.
• Traditional flexible resources may be
displaced or dispatched “out of merit
order”; though flexibility may increase at
some penetrations due to part-load
operation.
Denholm et al 2008
• Eventual conflict with “must run” base
load generation, leading to VER
curtailment (increased cost); though PV
load coincidence helps (compared to
wind).
• Fleet flexibility is key – more flexibility
means more room for VERs. © 2011 SunPower Corporation
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8. What Impacts Flexibility?
Physical
• Available Generator Attributes
• Transmission (diversity)
• Responsive Load (incl. xEVs)
• Energy Storage
Institutional
Denholm 2008
• Use of Forecasts
• Balancing Area Size / Coordination
• Unit Commitment & Dispatch Approaches
• Market Transparency
• Ancillary Service Markets
Existing Physical Flexibility Is Often Inaccessible Due To Institutional Barriers
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9. PV Variability Is Not A Technical Barrier
But PV variability is a real cost allocation challenge!
 The physical and institutional flexibility of a system has a significant impact of the cost
to integrate VERs…
…The same has always been true of the cost to integrate load variability, but no one
has previously considered charging conventional generators (coal, nuclear) for their
inflexibility.
 “High penetration” studies which assume BAU, deterministic planning and operations
practices, do not challenge institutional constraints, and do not adequately consider
flexibility will overstate integration costs (and will show that overbuilding of conventional
generation is ”needed” to “back up” wind and solar)
 Limitations on distribution system penetration of PV are dominated by issues related
to accommodating and managing DG generally, not due to variability. A systematic
approach to distribution upgrades (i.e. smart grid) could greatly increase DG
penetration levels, along with changes to interconnection standards and statutes.
© 2011 SunPower Corporation
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10. Changes Are Coming!
 NERC Integration of Variable Generation Task Force (IVGTF)
– Detailed reports addressing all of these topics and many more are in progress,
or already published.
– Focused on transmission interconnection, but addresses changes needed with
high penetration DG that will impact bulk electrical systems.
 FERC Variable Energy Resource (VER) NOPR
– Directly tackles interplay between operational practices and integration costs;
proposes that TSOs be able to recover integration costs if operations are
modernized.
 IEEE 1547 Updates
– Addresses technical standards changes needed to better accommodate high
penetration DG.
 FERC SGIP (model for California Rule 21 and many others)
– IREC proposing updates to address PV specific issues that can pose
unnecessary barriers to achieving higher penetration of distributed PV.
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11. © 2011 SunPower Corporation

12. Local Impacts Of Variability
Concerns about flicker and voltage regulation are often expressed, but have not been
reported as issues in numerous high penetration circuits being studied:
Location Description Penetration Notes
Ota City, Japan 550 Sites / 2 MW Not Reported Residential energy storage
(2003) residential, one circuit evaluated and removed; no issues
reported post-removal.
Freiburg, 70 Sites / 440 kW multi-unit 110% on capacity (400 Minimal, correctable issues
Germany (2006) residential kVA XFR) reported (phase imbalance)
Kona, HI (2009) 700 kWac commercial 35% on capacity (2 No issues reported
MVA feeder), backfeed
up to 30% in low load
Lanai, HI (2009) 600 kWac commercial (1.2 ~12% on capacity, No issues reported.
MW system, brought online ~25% in low load,
incrementally) weak island system
Anatolia, CA 115 Sites / 238 kW 4% on capacity, 11- No issues reported, PV variability
(2009) residential 13% low load less than AC cycling variability.
Las Vegas, NV > 10 MW commercial, 35 ~ 50% on capacity, No issues reported
(2008) kV interconnection ~100% low load
Atlantic City, NJ 1.9 MW commercial, 23 kV ~24% on capacity, No issues reported
(2009) interconnection ~63% low load © 2011 SunPower Corporation
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13. Local Impacts Of Variability
Field experience in Germany (15.5 GW PV, 99% DG) and Spain (3.4 GW PV, 98%
DG) have not revealed issues with voltage regulation or flicker, even with quite high
penetration levels. Penetrations in excess of 100% (PV production / minimum load)
are commonplace in Germany today.
The “today” scenarios shown here represent two
actual German distribution grids, operating without
Sources: (L) Braun 2010, IEA PVPS Task 14 Workshop; issue. Grid “A” is at 88% penetration*, Grid “B” at
(R) Budenbender et al 2010 37% penetration today.
* Peak PV capacity (kW) divided by transformer rating (kVA)
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14. Mitigation Of Local Voltage Impacts
• Geographical diversity has a substantial (1) Spike in
impact in mitigating variability over small voltage at POI (3) Voltage returns
detected to setpoint
distances, even within a distribution feeder.
• Though uncommon, voltage fluctuations
can result when a single, high penetration
system is interconnected to a circuit with
high impedance (such as a long rural
feeder).
• Reactive power control substantially
reduces the impacts of output variability on
(2) SunPower smart controller
voltage. Simply setting a fixed non-unity commands reactive power
power factor has been demonstrated to be change to reduce voltage
effective.
SunPower has pioneered the
• Active voltage regulation (AVR) is implementation of AVR in large-
particularly effective, if mitigation is needed. scale PV 2011 SunPower Corporation
plants.
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