Methods to Model and Calculate Capacity 

Contributions of Variable Generation for 

Resource Adequacy Planning (IVGTF1-2) 

Michael Milligan

Team Leader, National Renewable Energy Laboratory, NREL

Joint NERC-UWIG Workshop

Kansas City

April 12, 2011

NREL/PR-5500-51485
Scope & Objectives (continued)

 Specific actions, practices and requirements, 

including enhancements to existing or 

development of new reliability standards

• Calculations and metrics, including definitions and their 

applications used to determine capacity contribution and reserve 

adequacy.

• Contribution of variable generation to system capacity for highrisk hours, estimating resource contribution using historical data.

• Probabilistic planning techniques and approaches needed to 

support study of bulk system designs to accommodate large 

amounts of variable generation.
Scope & Objectives

•Consistent and accurate methods are 

needed to calculate capacity values 

attributable to variable generation.

•Technical considerations for integrating 

variable resources into the bulk power 

system 
Report Outline

•Introduction

•Traditional Resource Adequacy Planning

•Data Limitations

•Approximation Methods

•Ongoing Variable Generation Actions

•Conclusion and Recommendation

•Appendix: Results from Western and Eastern 

integration studies
Status

 Draft report presented to NERC Planning 

Committee December, 2010

 Final report approved by NERC Planning 

Committee March 2011

 Task 1.2 initial coordination with NERC 

Resource Issues Subcommittee

 Presentation and comments from NERC’s 

LOLEWG
Traditional Resource Adequacy Planning

 Loss of Load 

Expectation, LOLE

• LOLE analysis is typically 

performed, calculations 

can be done hourly or daily 

on a system to determine 

the amount of capacity that 

needs to be installed to 

meet the desired reliability 

target. Common target is 

0.1 days/year. 

Fundamental metric is 

LOLP; basis of LOLE.

Available margin as a percentage of peak demand

0

Probability 0.02

10 20

Area under curve = 0.1 days/year
Traditional Resource Adequacy Planning

 Loss of Load Hours, LOLH

LOLH is concerned only with the number of hours of 

shortfall, and does not include any dimension for 

persistence of an outage event and therefore there is 

no quantification about how many days the outage is 

spread over.

 Expected unserved energy, EUE

EUE measures cumulative probabilistic energy 

shortfall
Traditional Resource Adequacy Planning

(continued)

 Effective Load Carrying 

Capability, ELCC

 ELCC essentially 

decomposes the 

contribution that an 

individual generator (or 

group of generators) makes 

to overall resource 

adequacy. A generator 

contributes to resource 

adequacy if it reduces the 

LOLP in some or all hours 

or days. Conventional 

generators’ contribution to 

adequacy is typically a 

function of the unit’s 

capacity and forced outage 

rate.

At the same load level, LOLE is reduced

Additional Load that can be served to 

bring the LOLE back to the target

0.12

0.11

0.10

0.09

0.08

0.07

Loss of Load 

Expectations 

days/years 

11.0x103 8.0 8.5 9.0 9.5 10.0 10.5

Load (GW)

Original Reliability Curve

Reliability Curve after Adding New Generation

Target Reliability Level

<-150 MW->
Each generator added to the system 

helps increase the load that can be 

supplied at all reliability levels

Gi Gi+1 Gi+2

Added Generators

Combined 

Resources

With 

Wind

Gn-2
10.0%

11.0%

12.0%

13.0%

14.0%

15.0%

10% Wind 20% Wind 30% Wind

Capacity Value (%)

Penetration

Western Interconnect 2004-2006 2005

2004

2006

Linear 

(Average)

ELCC Variation

 Wind ELCC declines with penetration

 Inter-annual variation implies need for many 

years of data (as with conventional 

generation)
Approximation Methods

 Approximations are less than ideal and often do 

not take LOLP or risk into account

 Approximation to Reliability Analysis:

R’ = Exp{-[(P-L)/m]}

Where:

P = annual peak load, 

L = load for the hour in question, 

R = the risk approximation (LOLP), measured in relative terms (peak hour

risk = 1)

 Time Period Methods

• Define the relevant time period to use

• Calculate the mean output of the variable generation over that period; or 

alternatively calculate a percentile or exceedence level of the variable 

generation over the period
Data Limitations

 Data

• Thermal Generation does exist

 Long-term forced outage rates

 GADS

• Wind and Solar does not have sufficient long term 

data

 Need for data from variable generation

• Collected by NERC’s GADS

• Currently does not satisfy requirements for capacity 

valuation of variable generation
Conclusions and Recommendations

Additional research to equate traditional reliability targets 

(such as 0.1day/year) to alternative metrics is recommended. 

As adequacy studies are performed, we also recommend 

comparisons of results based on these alternative metrics.

Alternative approaches and assumptions regarding the 

treatment of interconnected systems should be transparent to 

the analysis, and the development one or more common 

approaches for handling the impact of interconnected systems 

in the reliability assessments will be useful.

Planning Reserve Margin levels should be benchmarked with, 

or derived from, an LOLP or related approach to resource 

adequacy. This should be done periodically to ensure that any 

correlation between a 0.1days/year target (or other adopted 

target) and a given Planning Reserve Margin do not change 

as a result of an evolving resource mix. 
Conclusions and Recommendations

(continued)

Simplified approaches should be benchmarked and calibrated to 

the rigorous ELCC calculations to ensure the validity of the 

approximation.

NERC should design and implement a way to collect highquality variable generation data that would help inform 

calculations of capacity value. The development of such a 

database should consider defining relevant time periods for 

the variable generation data.
Conclusions and Recommendations

(continued)

NERC should request that government agencies like the DOE, 

working with NOAA/NCAR develop annual high-resolution, modeled 

wind power and solar power data on 10-minute time scales (or faster, 

as technology allows) and 2 km (or smaller) geographic grids. NERC 

should consider collecting 10-minute load data to support reliability 

and other analyses.

NERC should facilitate the dissemination of information on how 

LOLP-related reliability and adequacy calculations perform and 

what they measure.
Related report

 Task Force on the Capacity Value of Wind 

Power, IEEE Power and Energy Society: 

Capacity Value of Wind Power

 In press, IEEE Transactions on Power Systems
Question & Answer