Several detailed technical investigations of grid ancillary service impacts of wind power plants in the United States have recently been performed.These studies were applied to Xcel Energy (in Minnesota) and PacifiCorp and the Bonneville Power Administration (both in the northwestern United States). Although the approaches vary, three utility time frames appear to be most at issue: regulation, load following and unit commitment. This article describes and compares the analytic frameworks from recent analysis and discusses the implications and cost estimates of wind integration. The findings of these studies indicate that relatively large-scale wind generation will have an impact on power system operation and costs, but these impacts and costs are relatively low at penetration rates that are expected over the next several years. PrefaceWith the continuing decline in the as-delivered cost of wind energy and the continuing spread of favourable renewable policies at the state and federal level, many US utilities are taking a serious look at wind power. Foremost in these examinations are concerns associated with accommodating the variable nature of power production in the interconnected grid system. At first blush the uncontrollability of output presents a formidable obstacle, often resulting in high estimates of ancillary service costs or assumptions that wind capacity must be 'backed up' with large amounts of dispatchable conventional technology such as natural gas-fired combustion turbines. However, these cursory examinations often overlook key factors such as:• the stochastic nature of grid systems, which must routinely contend with varying and uncertain demand and unexpected transmission and generation outages; • the ability to forecast wind power output in both hourly and day-ahead time frames; • actual wind farm power output characteristics, including multiple-generator smoothing (intra-and inter-site) and new generator and wind farm interface abilities; • large-scale geographic diversity resulting in smoothing of aggregate power output;• the evolution of US competitive wholesale markets, including near-real-time operations and unscheduled deviation practices.Recently, several more detailed technical investigations of grid ancillary service impacts have been performed. This article will summarize the issues of grid integration, approaches and results of recent studies, and implications for future work.Recent studies relevant to costs of grid integration of wind energy have been performed for the following utilities: Xcel Energy (in Minnesota) and Pacificorp and the Bonneville Power Administration (both in the northwestern United States). In addition, market rules from the Mid-Atlantic region (Pennsylvania, New Jersey and Maryland, or PJM, power pool) have been examined for cost impacts. Although the approaches vary, three utility time frames appear to be most at issue: regulation, load following and unit commitment. Market-based and integrated provider, cost-based approaches to evaluation have been examined....

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