The reliable operation of an electric power system depends on careful management of the balance between generation and load to ensure that system frequency is maintained within narrow bounds around a scheduled value. Yet, maintaining frequency at the scheduled value is challenging because the load served is continuously changing, and occasionally, events such as the sudden loss of a large generation plant or large amount of load, cause frequency to deviate abruptly. This report reviews the recent history of frequency performance for all three U.S. interconnections: Eastern, Western, and the Electric Reliability Council of Texas (ERCOT). The review is based on data collected by the North American Electric Reliability Corporation (NERC). The review focuses on frequency response, which measures the performance of the interconnections immediately following sudden, large imbalances between load and generation. Trends in frequency response are presented and preliminary efforts are made to relate frequency response to other aspects of frequency performance and to examine aspects of the methods used to calculate frequency response.

This report is a part of an investigation of the ability of the U.S. power system to accommodate large scale additions of wind generation. The objectives of this report are to describe principles by which large multi-area power systems are controlled and to anticipate how the introduction of large amounts of wind power production might require control protocols to be changed. The operation of a power system is described in terms of primary and secondary control actions. Primary control is fast, autonomous, and provides the first-line corrective action in disturbances; secondary control takes place on a follow-up time scale and manages the deployment of resources to ensure reliable and economic operation. This report anticipates that the present fundamental primary and secondary control protocols will be satisfactory as wind power provides an increasing fraction of the total production, provided that appropriate attention is paid to the timing of primary control response, to short term wind forecasting, and to management of reserves for control action.