Go to Content | Go to Page Updated Information | Go to Footer
Welcome to the California Energy Commission

Public Interest Energy Research Program: Final Project Report

cover of report Intermittency Analysis Project

Publication Number: CEC-500-2007-081
Publication Date: July 2007
PIER Program Area: Renewable Energy Technologies Research

The executive summary, abstract and table of contents for this report are available below. This publication is available as an Adobe Acrobat Portable Document Format File. In order to download, read and print PDF files, you will need a copy of the free Acrobat Reader software installed in and configured for your computer. The software can be downloaded from Adobe Systems Incorporated's website.


The California Energy Commission's Public Interest Energy Research Program assembled an industry team to tackle the challenges of integrating renewables into a future 2020 electricity transmission system. The Intermittency Analysis Project conducted a series of scenario-based studies to examine the statewide system impacts of higher levels of intermittent renewables on the California electricity and transmission infrastructure. Based on the analysis, technical and operational strategies and mitigation measures are recommended for consideration by California's utilities and system integrator. The analysis also provides a framework for system operators, utilities, and infrastructure planners to gauge the needs of the future 2020 system. Working with various agencies and California utilities to ensure coordination and to review results and findings, the Intermittency Analysis Project team also incorporated recent findings and input from a number of regional study groups in California, as well as lessons learned from the international perspective. Results include providing a detailed technical analysis of existing and future infrastructure needs, addressing potential operational strategies, developing a set of utility "best practices," and tools for integrating intermittent renewables, and as problems were encountered, assessing potential mitigation options to ensure sustainable operation.

This final project report summarizes the results and recommendations of a series of reports and presentations produced by the project team.

Keywords: Aggregate megawatt contingency overload, intermittency analysis, renewable integration, remedial action schemes, renewable portfolio standards, renewable transmission benefit ratio, transmission impacts, wind energy, solar energy

Executive Summary


This report serves as the final project report for the Intermittency Analysis Project. The Intermittency Analysis Project was organized to:

  1. Consider the impacts on the electricity grid of higher levels of intermittent renewables from a scenario basis;

  2. Trace the historical evolution of wind technologies in California and their changing impact on the grid;

  3. Determine how other international regions have integrated intermittent renewables generation.

The Intermittency Analysis Project team produced five major reports, and the project resulted in recommendations for California to successfully integrate higher levels of variable renewable energy generation per state energy policy targets, which are discussed in this final report and associated appendices.


The Intermittency Analysis Project is a comprehensive project aimed at addressing multiple aspects of the potential impact of more intermittent renewables. The project team, through transmission load flows, statistical analysis, and production cost modeling, examined the statewide impacts of more intermittent renewables on the California electricity and transmission infrastructure. These higher levels are in response to meeting the Renewables Portfolio Standard of 20 percent renewable energy by 2010 and the accelerated target of 33 percent renewable energy by 2020. The project quantified impacts on the grid as a result of increasing renewable penetration by analyzing transmission infrastructure needs as well as operational flexibility though a series of scenarios. Mitigation options as well as operational response strategies were demonstrated using production cost modeling and load-flow simulation tools. The results and recommendations based on the statewide analysis provide a framework for system operators, utilities, and infrastructure planners to gauge transmission and future grid needs for their service areas and the region as more renewable energy generation is installed in California.

The Intermittency Analysis Project assessed the following four scenarios considering renewables within California:

  • 2006 Base - Baseline for the existing California grid.

  • 2010T - 2010 Tehachapi case with 20 percent renewables and 3,000 megawatts (MW) of new wind capacity at Tehachapi.

  • 2010X - 2010 accelerated case planning toward 33 percent renewables.

  • 2020 - 2020 case with 33 percent renewables.

The effort resulted in seven reports and various presentations for two public workshops. All reports and presentation material along with study datasets are available from the Energy Commission's website (www.energy.ca.gov).

BEW Engineering was commissioned to produce a report on the historical evolution of wind turbine technology in California, while Exeter Associates, Inc., prepared a report on the international experience with integrating variable renewable energy generation. AWS TrueWind prepared the wind profiles and wind forecasts; Davis Power Consultants compiled the renewable resource mixes, load flows, and transmission expansion build out for the four scenarios; and General Electric Energy Consulting conducted statistical analysis, quasi-steady-state simulations, and production cost modeling.

The Davis Power Consultants' and General Electric Energy Consulting reports are included as part of the Intermittency Analysis Project Final Report. The analysis and results of both reports are interdependent and should be viewed as such. Davis Power Consultants prepared the renewable resource mixes and the load flows for the four assessed scenarios, and their efforts provided an illustration of the "future grid" that could accommodate more intermittent renewable energy generation. General Electric Energy Consulting, in turn, relied upon the renewable resource mixes, transmission infrastructure, and the load flows to conduct statistical analysis and production simulation analysis of the potential effects of intermittent renewable energy generation on operations and scheduling.

By conducting power flow and production cost simulations, the Intermittency Analysis Project team, with the Intermittency Analysis Project industry advisory team, established a 2006 baseline and renewable resource portfolios and infrastructure for 2010 and 2020 study cases. Load flows were prepared using PowerWorld software. Production costs were modeled using General Electric's Multi-Area Production Simulation™ modeling software to evaluate grid operation with increasing levels of wind and solar generation in the generation mix. All datasets were prepared with utility stakeholders and will be provided to the Western Electricity Coordinating Council and utilities for ongoing study needs.

Project Objectives

The Intermittency Analysis Project focuses on five objectives:

  1. Statewide transmission options to integrate in-state renewables to meet policy targets;

  2. Identifying the positive and negative quantitative effects of various options on transmission reliability, congestion, and mix of renewable technologies;

  3. Developing the tools and analysis methods to evaluate renewables along with conventional generation;

  4. Providing a common perspective for evaluating different technologies competing for limited system resources, and

  5. Providing a common forum for commissions, utilities, and developers to examine the location and timing of new generation/transmission projects and the public benefits of these resources.

Project Outcomes and Conclusions

California can incorporate the amount of renewables based on the Intermittency Analysis Project scenarios, provided appropriate infrastructure, technology, and policies are in place. Specifically, this successful integration will require:

  • Investment in transmission, generation, and operations infrastructure to support the renewable additions.

  • Appropriate changes in operations practice, policy and market structure.

  • Cooperation among all participants, for example, the California Independent System Operator, investor-owned utilities, renewable generation developers and owners, non-Federal Energy Regulatory Commission jurisdictional power suppliers, and regulatory bodies.

Numerous findings and recommendations are discussed in further detail in each of these project-related reports.

  1. Impact on Past, Present and Future Wind Turbine Technologies on California Grid by BEW Engineering, publication # CEC 500-2006-050, May 2006.

  2. BEW reviewed past, present, and future wind turbine technologies in terms of performance and grid impact. State-of-the-art technologies are becoming more grid-friendly and incorporate advance power electronics for added electrical control during intermittent periods.

  3. Intermittency Analysis Project: Characterizing New Wind Resources in California, (PDF file, 540 kb) by AWS TrueWind LLC, publication # CEC 500-2007-014, March 2007.

  4. AWS Truewind provided wind energy forecasts in geographically diverse areas in the state and simulated generation data to support the California Energy Commission's Intermittency Analysis Project. An objective of the Intermittency Analysis Project is to assess the effects of a substantial expansion of wind generation on the reliability, operation, and economic performance of the California Bulk Power System. At the same time, the forecasts and generation took into account performance of new wind turbine technologies tailored for operation in varying wind regimes. The scenarios of wind generation include more than 10 gigawatts (GW) of new wind production capacity, in addition to the existing capacity, of approximately 2 GW. AWS Truewind's role was to:

    • Identify and characterize a number of sites for prospective new wind energy projects to satisfy the expansion scenarios in 11 focus areas of the state.

    • Simulate three years of hourly wind generation from both the existing and proposed new wind plants using power curves representative of current and future wind turbines.

    • Simulate next-day hourly wind generation forecasts for the same plants.

    • Produce samples of one-minute output data for significant or representative times.

    These data were used by General Electric Energy Consulting to assess the effects of wind generation on various aspects of the California Bulk Power System's operations and costs. This report describes the methods, assumptions, and results of AWS Truewind's analysis.

  5. Summary of Preliminary Results for the 2006 Base and 2010 Tehachapi Cases by Intermittency Analysis Project Team, (PDF file, 752 kb) publication # CEC 500-2007-009, February 2007.

  6. This interim report focuses on the assessment methods, scenarios, and explanation of assumptions and highlights some of the preliminary findings presented at a Commission staff workshop on August 15, 2006.

  7. Review of International Experience Integrating Variable Renewable Energy Generation by Exeter Associates Inc., publication # CEC 500-2007-029, April 2007.

  8. This report summarizes the experience in the United States and internationally through 2006 with integrating variable renewable energy generation, primarily wind generation, and discusses potential operating and mitigation strategies for incorporating variable renewable energy generation. Initially, wind development in Europe, particularly in Denmark and Germany, consisted of smaller but numerous wind projects interconnected to the distribution grid, in contrast with larger, utility-scale wind projects interconnected to the transmission grid in the United States. The differences between Europe and the United States are starting to narrow as development of variable renewable energy generation (for example, wind and solar) increases and as wind development takes place in more countries. In addition, as more utility-scale wind projects emerge, more countries are relying on common strategies, such as grid codes, to help integrate variable renewable energy generation. A review of the international experience provides perspective and insight to the Intermittency Analysis Project analysis team on various techniques for managing intermittency. Detailed country profiles are also provided for Germany, Denmark, India, and Spain.

  9. Intermittency Impacts of Wind and Solar Resources on Transmission Reliability by Davis Power Consultants.

  10. Intermittency Analysis Project by General Electric Energy Consulting.

  11. Intermittency Analysis Project: Final Report by Intermittency Analysis Project Team.

Reports 5 and 6 are contained in this Final Report as Appendices A and B (See above to download). The final report is Report 7. Highlights of the transmission expansion study by Davis Power Consultants and the operational impact study by General Electric Energy Consulting are provided below. Findings are recommendations are organized and presented in three topical areas: Generation Resource Adequacy, Transmission Infrastructure, and Renewable Generation Technology, Policy and Practice.

Generation Resource Adequacy

  • A combination of in-state generating resources and power exchange agreements or capability should be pursued to allow operation to a minimum net load of between 18,000 to 20,000 MW.

  • Pursuing generating resources with greater minimum turndown and diurnal start/stop capabilities, ensuring greater participation by loads, and optimizing use of pumped storage hydro will also aid with integrating variable renewable energy generation.

  • In-state generating resources should also be targeted for providing scheduling flexibility hourly. For light load conditions, total hourly scheduling flexibility requirements are smaller, but the relative impact of variable renewables is greater. Maintaining or improving hydro flexibility and accessing generating resources with faster start and stop capabilities will aid with hourly scheduling flexibility.

  • For sustained, multi-hourly load increases and decreases, the California grid should have the capability to meet a maximum morning load increase of 12,000 MW over three hours and a maximum evening load decrease of 14,000 MW over three hours.

  • An increase of 10 MW/minute of load following is necessary to incorporate the levels of renewables studied as compared to the requirements for load alone. About 70 MW/minute of down load-following requirements is necessary during light load periods.

  • California should consider allowing import and export scheduling to occur more frequently and at other times than on the hour.

  • The effect of variable renewables on regulation is relatively modest (20 MW). Still, California should, at least, maintain current level of regulation capability and consider other means of providing regulation besides conventional generation, such as flywheels or variable speed pumped hydro.

  • While operational flexibility is valuable to the grid, it can impose significant costs and revenue reductions on generation providers. Expanded ancillary service markets, incentives, and requirements may be necessary to overcome this problem.

  • The Intermittency Analysis Project analysis did not include historical constraints such as long-term contractual obligations. New proposed contracts and existing long-term contracts up for renewal or subject to renegotiation should be reviewed to increase grid flexibility and adequacy.

  • Increased competition from new resources, renewables or otherwise, may push marginally profitable generating resources out of business. Plant retirements should be projected, monitored, and evaluated.

  • California should measure, verify, and catalogue the flexibility characteristics of individual generating resources.

Transmission Infrastructure

  • Transmission planning to accommodate multiple wind plants should consider the spatial diversity of these plants. Wind plants close together will typically require transmission capability to the aggregate rating of the plants, while wind plants further apart may require less transmission capability.

  • Significant transmission investments are necessary to meet the 2010 and 2020 renewable targets. For the 2010 Tehachapi case, 74 new or upgraded transmission line segments are needed at a first-order estimated cost of $1.2 billion. Most of these line segments (63) are needed to serve growing load. In addition, 31 new or improved transformers would be needed for an additional cost of $161 million (excluding detailed land use and right of way costs).

  • The 2020 case would require 128 new or upgraded transmission line segments, with just over half (66) needed to serve increasing load requirements. For just the 500-kilovolt (kV) and 230-kV additions, a first-order estimated cost would be $5.7 billion. In addition, 40 new or improved transformers would be needed at an estimated cost of $655 million (excluding detailed land use and right-of-way costs).

  • The transmission analysis suggests that wind variability may contribute to transmission congestion under certain renewable energy dispatch scenarios, and that transmission congestion patterns are more difficult to predict as the penetration of variable renewable energy resources increases.

Renewable Generation Technology, Policy, and Practice

  • Policy and regulatory and contractual practices to maximize existing use of transmission should be encouraged, such as real-time line ratings, local short-term forecasting, and controls that manage output from multiple variable renewable energy resources.

  • Under rare circumstances of coincident minimum load, high wind generation, and low conventional hydro flexibility, curtailment of variable renewable energy generation may be necessary.

  • Regulatory and contractual arrangements for intermittent renewables should be designed to allow and compensate for the provision of ancillary services such as frequency regulation.

  • Wind and solar forecasting offers significant benefits in the multi-day, unit commitment, and short-term (hours and minutes) time frames. Through policy or investment, high fidelity forecasting for all variable renewable energy generation in California should be conducted.


This project provides a piece of the larger transmission planning and system operations picture. As renewable penetration levels increase, continuing timely and routine long-term planning and analysis of the statewide system as well as local system by utilities is needed to determine the existence of and the magnitude of potential problems. Technology, policy, and the environment (market and infrastructure) need to be assessed holistically with periodic reassessments using complete and quality data. As much as possible, experiences, data, and results from this effort should be leveraged for other state, multi-state, and western region studies. The Intermittency Analysis Project as designed is limited to the in-state renewable resource perspective, defined technical scope, and a number of assumptions defined by the scenario analysis. For a more complete description of the scenario assumption, see Summary of Preliminary Results for the 2006 Base and 2010 Tehachapi Cases by Intermittency Analysis Project Team, CEC 500-2007-009, February 2007. It is recognized that the portfolio implemented in the future will be a balance of in-state and out-of-state resources, thus defining in actual impacts on the grid. This feasibility analysis identifies some of the issues seen from a statewide integrated perspective using in-state renewables. However, it is limited in resolution and data. The results and findings supplement and support continuing transmission and renewable integration studies being conducted by the California Independent System Operator, the California Public Utilities Commission, and the utilities.

Benefits to California

The Intermittency Analysis Project should provide significant information, recommendations, and benefits in support of attaining the state's accelerated Renewables Portfolio Standard targets. These include:

  1. A vision of the "in-state future transmission grid" (infrastructure and operating services) and the mix of renewable resources (wind, solar, geothermal, biomass) needed to accommodate the Renewables Portfolio Standard penetration levels.

  2. Recommendations for a portfolio of renewable resources to meet the 20 percent Renewables Portfolio Standard target by 2010 and the 33 percent goal by 2020.

  3. Unified transmission infrastructure solutions and intermittency mitigation measures that transcend utility service boundaries to achieve an economically robust and reliable grid.

  4. Quantified system performance and results based on the "future grid solutions" that can later be converted into integration cost adders.

  5. Integrated transmission expertise from various California utilities, industries, state agencies, and consultants to form a consolidated statewide system of solutions, mitigation measures, and intermittency management strategies.

  6. Delineation of the physical transmission limits from policy and contract limits to push intermittent renewable resource penetration levels and to provide future market structure recommendations.

  7. Estimates on emission benefits for the state based on study scenarios for oxides of sulfur (SOX), oxides of nitrogen (NOX), and carbon dioxide (CO2).

  8. Identification of the technology, policy, and market gaps that may be barriers to meeting Renewables Portfolio Standard goals.

Page Updated:
Go to Content | Go to Page Updated Information | Go to Footer