March 25, 2011

Electricity systems adjust operations to growing wind power output

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Electric power system operators face a challenge as they seek to integrate rising quantities of intermittent generation from wind plants into the system mix. Operators must continuously match electricity generation to electricity demand, a process that becomes more difficult with additional intermittency.

Today in Energy on March 22, 2011 described intermittent generation from wind turbines. In today's story, we describe how electric power system operators adjust their procedures to deal with increasing wind capacity as the demand for electric power changes over the course of the day. The unpredictability and sudden changes in production from wind generation (see chart above for a real-life example) create real engineering issues for operators.

Each afternoon, operators lay out their unit commitment for the next day, comparing the day-ahead demand forecast to their available generators (in market regions, those that have bid in their capacity). Operators need to commit enough generation to avoid shortages, yet not so much that they unnecessarily raise costs. Many regions are developing or have developed highly detailed near-term wind forecasting tools to give dispatchers advance notice of the projected wind resource.

Wind generators are subject to abrupt changes in wind speed, and their power output is characterized by steep ramps up or down. Power systems handle such moment-to-moment changes in intermittent generation just as they handle small fluctuations in demand: certain generators automatically raise or lower their output in response to imbalances between supply and demand.

As the percentage of intermittent generation on the system increases, it becomes more important to smooth out the fluctuations in wind generation. Spreading out wind generators across a wide geographical area reduces variability. Also, building a more robust transmission network not only connects wind resources to load centers but provides a wider set of resources for combating the effects of intermittent generation.

Late-model wind turbines are better able to control their output by changing the pitch of their blades and "spilling" wind (i.e., letting the wind blow past without extracting its energy, like water spilling over a dam). This allows them to respond to orders to reduce output.

Electric power systems with a large share of intermittent resources may rely more on flexible resources such as gas turbines or hydropower to "firm up" the output of intermittent generators. Demand response and energy storage are also potential approaches, but the deployment of storage—other than pumped hydro—is essentially zero at this time.

For more information on integrating wind generation, please see:

• Accommodating High Levels of Variable Generation (North American Electric Reliability Corporation)
• The Effects of Integrating Wind Power on Transmission System Planning, Reliability, and Operations (New York State Energy Research and Development Authority)
• Use of Frequency Response Metrics to Assess the Planning and Operating Requirements for Reliable Integration of Variable Renewable Generation (Lawrence Berkeley National Laboratory)