We’ve heard the wind-integration success stories from Sweden and Denmark, but we haven’t heard as much about how wind is contributing right now to U.S. electricity supplies. A recent, well-timed report shows that wind power is supporting up to 20 percent of average demand in some regional transmission systems and up to 50 percent at some times during the year. Despite fears that such high penetration rates by an intermittent generation source could threaten grid reliability, those transmission systems are thriving and looking to add even more wind to their portfolios.


The report, “Integrating Renewable Energy into the Electricity Grid,” was developed by the Cambridge, Mass.-based Brattle Group, a leading utility­-industry consulting firm. Prepared for the Advanced Energy Economy Institute (AEEI), it investigates the success Xcel Energy Colorado and the Electric Reliability Council of Texas (ERCOT) have had integrating large amounts of wind energy into their generation portfolios.


The report comes as the U.S. Environmental Protection Agency (EPA) is set to finalize its Clean Power Plan (CPP) regulations, creating state-by-state caps on the amount of carbon dioxide power plants can release. Critics suggest the caps could result in the rapid retirement of coal-fired plants before the grid is ready to transition to a generation portfolio more heavily dependent on variable renewable resources. As the experiences of ERCOT and Xcel Energy Colorado illustrate, technology advances and some ingenuity offer system operators a range of options for integrating renewables with coal and natural gas generation.


“There are a lot of tools. It’s not just one technology. It’s many,” said Matt Stanberry, vice president of market development for AEEI’s parent organization, Advanced Energy Economy. “We’re excited by the diversity of the operational and technological techniques that people are putting together into packages.”


Texas stands alone


ERCOT is a unique entity. It is largely isolated from the U.S. transmission system, so any balancing to address wind’s inherent intermittency must be accomplished within Texas borders. However, it also gives system operators more flexibility in how they run their pricing market. By implementing location-based pricing and dispatch—along with evaluating system capacity and demand every five minutes instead of every 15—operators can be much more nimble in addressing variability concerns.


In West Texas, ERCOT has benefited from the state legislature’s work to connect areas of high wind supply, called Competitive Renewable Energy Zones (CREZ), to load centers in other parts of the state. New transmission lines have the capacity to deliver 18,500 megawatts (MW) around the Lone Star State.


However, observed experience is an advantage available to system operators anywhere. ERCOT has used it to upgrade estimates of how much of a turbine’s nameplate capacity can be counted on during peak-load conditions. Until late 2014, this estimate, called the “effective load carrying capability” (ELCC), was set at 8.7 percent, regardless of the turbine’s location or operation time. By evaluating performance, planners determined this location-neutral assessment oversimplified and understated wind’s contributions. Now ELCC calculations are based on location in coastal or noncoastal locations and time of year. ELCCs for turbines in all areas have been significantly upgraded.


Pushing boundaries in Colorado


Unlike ERCOT, which operates a completely deregulated electricity market, Xcel Energy Colorado is a vertically integrated utility, meaning it owns generation plants and controls transmission and distribution within its service territory. The company has tripled its renewable-generation capacity in the last 10 years, and wind supports close to 20 percent—at times exceeding 50 percent—of its customers’ demand.


While Xcel’s summer peak demand reaches 6,700 MW, off-peak load can drop to 2,700 MW—only slightly above its current 2,600 MW of installed wind. As it explores adding even more renewable generation, the utility has recognized the potential liability if wind levels dropped, so it has reserve generation that will kick in within 30 minutes. It has implemented an advanced wind-forecasting system developed by the National Center for Atmospheric Research to inform operations. The utility also has upgraded existing natural gas generators for faster start times and has added the capability for wind turbines to provide short-term frequency support for the larger grid, boosting overall system reliability.


Just in time


Even without the CPP’s anticipated caps, this report’s findings are timely. In just the first quarter of 2015, wind-farm developers had 13,600 MW of new generating capacity under construction in 100 projects across the United States, according to the American Wind Energy Association. Tax incentives supporting wind development remain a political hot potato, but they could become less important over time. Stanberry, for one, is certain wind’s role in U.S. electricity production will continue to grow.


“There’s no end in sight,” he said, referring to wind’s continued penetration. “We’ve got great examples of how this can be done and every indication that this can keep going.”