Conference Paper

NREL/CP-550-46527

September 2009

Large-Scale Wind Integration 

Studies in the United States: 

Preliminary Results 

Preprint 

M. Milligan, D. Lew, and D. Corbus

National Renewable Energy Laboratory

R. Piwko, N. Miller, K. Clark, G. Jordan, and 

L. Freeman

GE Energy 

B. Zavadil

EnerNex Corp. 

M. Schuerger

Energy Systems Consulting Services

To be presented at the 8th International Workshop on Large Scale 

Integration of Wind Power and on Transmission Networks for 

Offshore Wind Farms

Bremen, Germany

October 14–15, 2009
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Abstract-- The National Renewable Energy Laboratory, 

under the sponsorship of the U.S. Department of Energy, is 

managing two large-scale wind integration studies. The 

Western Wind and Solar Integration Study (WWSIS) covers 

the footprint of WestConnect, a group of transmission owners 

that covers most of Colorado, New Mexico, Arizona, Nevada, 

and Wyoming. The Eastern Wind Integration and 

Transmission Study (EWITS) covers a large part of the 

Eastern Interconnection, and leverages a large-scale 

transmission study known as the Joint Coordinated System 

Plan (JCSP). Both studies analyze the impact of 20-30% wind 

energy penetration within the study footprint based on energy. 

This paper discusses key results that have emerged so far from 

each study, focusing primarily on simulation results based on 

hourly production simulations. Results from both studies show 

that high wind penetrations can be successfully integrated into 

the power system, but depend on sufficient transmission and 

significant changes in operations.

Index Terms—Wind Energy, Wind Integration 

I. INTRODUCTION

he National Renewable Energy Laboratory is managing 

two large regional wind integration studies on behalf of 

the United States Department of Energy. These two studies 

are believed to be the largest ever undertaken in the United 

States. Both studies evaluate wind energy penetrations up to 

30% of annual energy demand. Although there are some 

differences in the study objectives and characteristics, a 

common objective of both studies is to perform electric 

system production simulation modeling, using realistic wind 

energy data that covers three years. In addition to the 

operational modeling that is performed on an hourly time 

step, each study also analyzes sub-hourly wind and load data 

to provide insight into the intra- hour impacts and variability 

characteristics. Each study also evaluates alternative wind 

energy build-out scenarios that help to show the impacts of 

developing local wind with lower capacity factors against 

more remote wind resources that require more transmission. 

An evaluation of wind capacity value using reliabilitybased modeling methods is also part of each study. This 

paper provides a broad look at each study and presents 

preliminary results and conclusions.

 

1 National Renewable Energy Laboratory, Golden, CO, USA +1 303-

384-6927, michael.milligan@nrel.gov

2 GE Energy, Schenectady, NY, USA 3 EnerNex Corporation, Knoxville, TN, USA

4 Energy Systems Consulting Services, St. Paul, MN, USA

Both studies have extensive project teams and 

stakeholder groups. Project teams include staff from 3-Tier 

Group, GE Energy, National Renewable Energy Laboratory, 

AWS TrueWind, EnerNex, Ventyx, and the Midwest 

Independent System Operator (MISO). In addition, the 

Eastern Wind Integration and Transmission Study (EWITS) 

is leveraged from the Joint Coordinated System Plan (JCSP) 

and includes representatives from PJM, TVA, and SPP. 

II. STUDY OVERVIEWS AND PRELIMINARY RESULTS

Both the Western Wind and Solar Integration Study 

(WWSIS) and EWITS evaluate the operational impact of 

large-scale wind energy—up to 30% of annual energy 

demand—on the power system. Both studies cover a large 

geographic scope, and collectively cover most of the 

continental United States. Modeling large regions allows 

additional questions to be answered, including: 

• How do local wind resources compare with higher 

capacity-factor wind that requires more 

transmission?

• How does geographic diversity of wind power 

reduce wind integration costs (i.e., spreading the 

wind over a larger region and thereby “smoothing” 

out some of the variability)?

• How does offshore wind compare with onshore 

wind?

• How does balancing area consolidation or 

cooperation affect wind power integration costs?

• How much new transmission is needed to facilitate 

higher penetrations of wind power?

• What is the role and value of wind forecasting?

• What role do shorter scheduling intervals have to 

play? 

• How are wind power integration costs spread over 

large market footprints and regions?

• What additional operating reserves are needed for 

large wind power deployments?

A. WWSIS

The key objectives of WWSIS are to answer questions 

about large-scale wind and solar integration in the 

southwestern part of the United States. This region 

corresponds to the footprint of WestConnect, which is a 

Large-Scale Wind Integration Studies in the 

United States: Preliminary Results

Michael Milligan1, Debbie Lew1, Dave Corbus1, National Renewable Energy Laboratory; Richard Piwko2, Nicholas Miller2, 

Kara Clark2, Gary Jordan2, Lavelle Freeman2, GE Energy; Bob Zavadil3 EnerNex Corporation; Matt Schuerger4

T

, Energy 

Systems Consulting Services

1
group of utilities that operate in Arizona, Colorado, Nevada, 

New Mexico, and Wyoming as shown in Fig. 1. The study 

addresses the following:

• Does geographic diversity of renewable energy 

resource help mitigate variability?

• How do local resources compare to out-of-state 

resources?

• Can balancing area cooperation help mitigate 

variability?

• What is the role and value of energy storage?

• Should reserve requirements be modified?

• What is the benefit of forecasting?

• How can hydro help with integration of 

renewables?

Fig. 1. Study footprint for the WWSIS. 

Several scenarios represent alternative wind build-outs, 

both at different penetration levels and different locations. 

Penetrations are depicted in TABLE I. 

TABLE I. Wind and solar penetrations for the WWSIS. 

Variations in these scenarios include (1) in-area: each 

transmission area meets its target from wind and solar 

resources in that area, (2) mega-project: wind and solar 

projects are concentrated in the best resource areas, (3) local 

priority: a balance of the first two variations. As this project 

is still in process, additional scenarios are yet to be 

determined.

A number of different aspects of system operational 

impacts are investigated in this study. This include

• Impact on hydro operations and alternative hydro 

dispatch

• Comparison of scenario variations to assess 

potential differences in overall variability and 

operational impacts

• Impact of balancing area consolidated operations

• Impact of alternative wind forecasts on operations

• Analysis of displaced generation

• Gas price sensitivities

• Alternative transmission build-out scenarios

The study also examines the capacity value of wind and 

solar power. Many of these results are shown in Lew et al 

[1]. 

GE modeled the Western Electricity Coordinating 

Council (WECC) footprint, but the study focused on 

WestConnect. Including WECC allows the study to take into 

account the flows and interchanges with neighboring 

systems, and provides a more realistic evaluation of the 

study area.

At low wind penetration, the primary impact on 

conventional generation appears on the gas combined-cycle 

generation. As wind penetration increased to 20%, the 

impact on combined cycle units increased and there is an 

additional impact on gas combustion turbines. At 20% 

penetration, there was a small reduction in coal generation, 

but this increased substantially at the 30% wind penetration 

rate along with an additional 5% solar penetration. This can 

be seen in Figs. 3-5 that are selected from a particularly 

challenging week in April using the 2006 load and 

renewable profiles. During this time of the year, the load is 

significantly less than during the summer and winter 

seasons, and there is much more wind generation in the 

spring than during high-demand periods. At the 10% wind 

penetration (for brevity, we refer to the penetrations in terms 

of wind only, but solar’s penetration is implicit as shown in 

TABLE I) there is a significant impact on gas combinedcycle generation, along with a small impact on coal. As the 

penetration of wind and solar increase, the impact on baseload coal increases, becoming very challenging at the 30% 

penetration. We note that the modeling of the power system 

included details on coal ramping capability and minimum 

up- and down-times from publically-available data. 

Fig. 2. Example of dispatch from April; no wind. 

In Footprint Rest of WECC

Wind Solar Wind Solar

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20% 3% 10% 1%

30% 5% 20% 3%

Study Area Dispatch - Week of April 10th - No Wind

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Fig. 3. Example of dispatch from April; 10% penetration. 

Fig. 4. Example of dispatch from April; 20% penetration. 

Fig. 5. Example of dispatch from April, 30% penetration. 

The impact of the displaced generation on production 

costs at higher penetrations of wind and solar power can be 

seen in Fig. 6. The scenarios represented on the graph 

represent (a) pre-selected sites with perfect forecast 

(approximately the current installed capacity), (b) in-area, 

10%, 20%, and 30% wind penetration respectively with 

perfect forecast, with similar scenarios for representative 

forecast errors. The I2020R case in Fig. 6shows the impact 

of a reduction in forecast errors compared to the 

representative forecast error cases.

Fig. 6 shows that as renewable penetration increases, 

production cost savings also increase because of the 

reduction in fuel burned. In addition, we can see that larger 

forecast errors reduce the savings from wind and solar. As 

renewable penetration increases, the incremental saving in 

production cost decreases. This is consistent with the 

findings of the change in economic dispatch: at relatively 

low penetrations, the primary displaced fuel is natural gas. 

But as renewable penetrations increase enough to displace 

coal generation, the production savings from coal are less 

than the production savings from gas.

Fig. 6. Impact of renewable energy on production cost 

savings for the WWSIS. 

 

Corresponding to the reduction in fuel and operating cost, 

the reduction in emissions was calculated. Fig. 7 shows this 

impact for the cases that include representative forecast 

errors (the perfect forecast cases do not appear here). As 

would be expected, the incremental decrease in coal 

generation at higher penetrations results in increasing perunit reductions in carbon and other emissions.

Fig. 7. Total emission reductions per MWh of renewable 

generation..

This study is scheduled to be completed in early 2010, 

and additional results and insights are forthcoming.

However, conclusions to date indicate: 

• Up to 20% penetration of wind power can be 

accommodated with no significant operational issues. 

• 30% wind power is more challenging. 

• Production costs and emissions decline with 

increasing renewable penetration. 

Study Area Dispatch - Week of April 10th - 10%R

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Incremental Renewable Energy ($/MWh) - WECC - 2006

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• Based on the value of unserved energy, responsive 

load would appear to be easily justified as an 

economic option to help manage variability. 

• The need for additional storage is not apparent from 

study results. 

• Little differences in operational impacts have been 

found between alternative geographic cases at given 

penetration rates. 

• Forecasts have value and forecast accuracy has an 

influence on operational savings. 

• Operational impacts appear to be sensitive to the 

penetration in the rest of the interconnection. 

B. EWITS

The EWITS expands work done by the JCSP by 

evaluating additional wind scenarios and developing 

supporting high-voltage transmission buildout scenarios. 

The study footprint is shown in Fig. 8. 

Fig. 8. EWITS study footprint (excludes IESO). 

The study considers four scenarios (all penetrations are 

relative to annual energy in the study footprint):

1. 20% high-capacity factor wind, all onshore, results in 

significant wind use in the western portion of MISO 

and more transmission development than other 20% 

cases

2. 20% hybrid with some offshore. This scenario is the 

closest match to the JCSP and therefore provides a 

common reference

3. 20% wind with aggressive offshore and less onshore 

wind

4. 30% wind, aggressive onshore and offshore. 

A key objective of this study was to evaluate the wind 

integration cost, similar to other studies done in the United 

States. This approach compares production cost with wind 

to production cost with a flat-block daily energy-equivalent 

to wind. However, it was determined that this approach does 

not work in studies with large wind penetrations because of 

the large, artificial ramp that occurs at midnight between the 

daily flat blocks. At low wind penetrations and in small 

footprints, this impact is not believed to be significant, but 

the EWITS analysis found such large impacts as to judge the 

method as invalid for the study.

Separate work [2] characterized these impacts and also 

showed that using the daily block proxy resource also 

introduces differences in value that are an artifact of the 

proxy resource. Fig. 9 shows the correlation of wind 

production and locational marginal prices (LMP) (see [2]). 

Using fixed LMPs and the wind data from EWITS, the value 

differential was shown to improve with alternative flat block 

proxies, but was still an issue as shown in Fig. 10

Fig. 9. Example of LMP profiles over 3 years and negative 

correlation with wind [2]. 

Fig. 10. Using different rolling average and fixed block 

definitions yield a differential in value of wind compared to 

the proxy. [2]

As a result of these and other discussions, the decision 

was made to explore a set of alternative cases to estimate the 

integration cost of wind, as follows:

• Ideal wind case. No day-ahead wind forecast error, 

and no incremental operating reserve driven by 

wind. Only load forecast error and uncertainty have 

any impact on operating reserve. 

• Intermediate wind case. Uses day-ahead forecast 

errors, but has no incremental reserve requirement 

because of wind. This case allows us to estimate a 

cost of uncertainty. 

• Actual wind case. This adds an incremental reserve 

requirement that is driven by wind power needs.

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Although the wind penetration of the various scenarios is 

expressed relative to annual energy of the footprint, the 

various regions do not have the same penetration because of 

the way the individual sites were ranked and selected (best 

wind with various constraints on onshore, offshore, etc.; see 

Fig. 11). 

Fig. 11. Wind penetration varies by region. 

The impact that wind has on market prices is similar to 

what has been found so far in the WWSIS. As the wind 

penetration increases, LMPs decline generally as a function 

of wind penetration, as shown in Fig. 12. It is clear that

LMPs follow a similar downward trend but in some cases 

increase significantly at the 30% penetration case. At this 

penetration level it becomes somewhat difficult to trace all 

of the impacts of wind’s variability and uncertainty because 

generators may respond to wind (and load) variations that 

occur in another region, subject to relative prices and costs.

Fig. 12. Relative change in locational marginal prices by 

region and wind penetration level in the EWITS 

(preliminary). 

EWITS also is performing an extensive analysis of wind 

capacity value, using the effective load carrying capability 

(ELCC) metric. Because EWITS also contains a 

transmission expansion component, the sensitivity of system 

adequacy and wind ELCC to the transmission assumptions 

will be explored in some detail. Cases under evaluation 

include: 

• Stand-alone, isolated transmission zones

• Without transmission overlay (existing 

transmission)

• Infinite tie limits (copper sheet)

Preliminary results show a significant sensitivity of wind 

capacity value to transmission limits—as the transmission 

capability expands, the overall system is better able to take 

advantage of the geographic and time diversity of a larger 

footprint, increasing the relative wind ELCC. Of at least

equal importance is the conclusion that a well-developed 

transmission system can defer or avoid new generation 

additions, resulting in a more efficient and cost-effective 

power system.

III. SUMMARY

Both of the large-scale wind integration studies that are 

currently underway in the United States will be completed 

by early 2010. Emerging conclusions indicate that 20% 

wind energy penetration can be managed, but the role of 

wind forecasting is important to meet this objective. 

Additional transmission will be needed to deliver wind 

power to market, and storage appears to be unnecessary to 

achieve this penetration. Results for the 30% case appear to 

be more challenging, and as the studies move forward we 

anticipate more analysis will be done to achieve a better 

understanding of the operational challenges and potential 

solutions. High penetrations of wind reduce spot energy 

prices and production costs. Both analyses also show that 

carbon emissions are reduced, but that the level of reduction 

is a function of the displaced generation. The higher wind 

penetration cases tend to offset more coal than the lower 

penetration rates, consistent with an economically rational 

dispatch process.

IV. REFERENCES

[1] D. Lew, M. Milligan, G. Jordan, L. Freeman, N. Miller, K. Clark, R. 

Piwko, “How do Wind and Solar Power Affect Grid Operations: The 

Western Wind and Solar Integration Study,” in Proc. 8th International 

Workshop on Large Scale Integration of Wind Power and on 

Transmission Networks for Offshore Wind Farms, to appear.

[2] M. Milligan and B. Kirby, “Calculating Wind Integration Costs: 

Separating Wind Energy Value from Integration Cost Impacts,” in 

Proc. WindPower 2009. Available online at 

http://www.nrel.gov/docs/fy09osti/46275.pdf

[3] D. Corbus, M. Schuerger, and B. Zavadil, Eastern Wind Integration 

and Transmission Study – Preliminary Findings” in Proc. 8th

International Workshop on Large Scale Integration of Wind Power 

and on Transmission Networks for Offshore Wind Farms, to appear.

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Large-Scale Wind Integration Studies in the United States: 

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G. Jordan, L. Freeman, B. Zavadil, and M. Schuerger

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14. ABSTRACT (Maximum 200 Words)

The National Renewable Energy Laboratory is managing two large regional wind integration studies on behalf of the 

United States Department of Energy. These two studies are believed to be the largest ever undertaken in the United 

States. 

15. SUBJECT TERMS

Wind; solar; resource data; integration; grid studies; WWSIS; EWITS; transmission; power system operation; reserve 

requirements.

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