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Winds of Change Freshen Resource Adequacy

Intermittent and interruptible resources increasingly are being considered in regional resource adequacy calculations—but the approaches differ.

Fortnightly Magazine - May 2007

considered, ERCOT currently counts only 2.9 percent of a wind farm’s capacity toward reserve margin because the maximum wind output from West Texas does not correspond to peak demand (4 p.m. to 6 p.m. in July and August). 10 In its 2006 report to the Texas Legislature, the Texas PUC says that wind generation historically has supplied, on average, only 2.6 percent of its rated capacity during summer peaks. 11 ERCOT re-evaluates the wind-farm capacity percentage every year based on historical data. In 2005, ERCOT was using a 10-percent effective capacity for wind. In December 2006, Global Energy did a study for ERCOT that used a probabilistic model to estimate that the effective capacity of wind generation is roughly 8.7 percent of nameplate capacity.

Cal-ISO (California): Living with Mandates

Under California’s proposed Market Redesign Technology Upgrade (MRTU), the coordinators for load-serving entities (LSEs) will have to submit annual and monthly resource adequacy and supply plans to the ISO, which will validate and coordinate all plans. The types of resources that qualify for resource-adequacy consideration include: Generation within and outside of the Cal-ISO control area, resources under construction, demand-side resources, a portfolio of physical resources with a common bus-bar (e.g., multiple wind generators), and more. To qualify for consideration, a resource must pass a deliverability test (e.g., the output can reach load under peak conditions.) Each LSE must meet at least a 15-percent reserve requirement. The supply plans must include details about the resources the LSE has contracted for and each generating unit’s net qualifying capacity value. Intermittent resources, such as wind and solar, are considered limited-use, non-dispatchable resources and are expected to schedule into the day-ahead and hour-ahead markets, rather than the real-time market. Cal-ISO will not penalize these units for failing to meet their expected commitment. However, the ISO will track the performance of these resources and use that to understand the variances between planned and actual contributions. This tracking is vitally important because California has some of the nation’s most ambitious targets for the reduction of greenhouse gases through the employment of both interruptible (e.g., demand-side) and intermittent (e.g., wind) resources.



1. “Wind Integration Impact Studies,” Phase I, November 2005, and Phase II, July 2006.

2. “Wind Integration Stakeholder Workshop,” posted presentation, AESO Web site, July 19, 2006.

3. The PRISM model is a legacy FORTRAN program which has been used by PJM for adequacy studies since the mid-1960’s. PRISM stands for: Probabilistic Reliability Index Study Model.

4. MAAC is a NERC region called Mid-Atlantic Area Council. It recently has been incorporated into the new, larger NERC region called Reliability First Corporation.

5. 2006 Reserve Requirement Study, letter to the PJM Planning Committee from the Capacity Adequacy Planning Department, April 19, 2006.

6. PJM Manual 21, Rules and Procedures for Determination of Generating Capability, Appendix B-1: Determination of Wind Generator Capacity Values, Aug. 15, 2005.

7. The ECAR and MAIN regions recently have joined and are now called RFC, Reliability First; and the MAPP region is now called MRO, Midwest Reliability Organization.

8. “MISO Generation Deliverability Study Method,”