Wind power, now recognized as a utility-scale energy source, represents unique challenges for the industry in terms of system planning and operation. A significant challenge is how to address the variable nature of wind – it does not blow at the same speed all of the time. This drives concerns about changes in the operating costs of the nonwind portion of the generation mix, as well as overall electric system reliability. For a number of years, the Utility Wind Integration Group (UWIG) has conducted research and coordinated with electric utilities, research organizations, and government and industry groups to quantify these impacts and examine practices, technologies, and mechanisms to manage or mitigate them.
There have been a number of studies that have examined the implications of various wind penetration scenarios. A 2006 study conducted for the Minnesota Public Utilities Commission evaluated and quantified reliability, operating and market impacts of three levels of wind generation: 15, 20 and 25 percent of Minnesota retail sales in 2020. The study found that the total integration cost for up to 25 percent wind energy delivered to Minnesota customers is less than $4.50 per MWh of wind generation. It should be noted that the 25 percent penetration level of wind is predicated on operating in the Midwest Independent System Operator (MISO) market, control area consolidation in Minnesota, geographic diversity of the wind power and adequate transmission.
Penetration levels of this extent are already being seen in Europe, where wind provides more than 20 percent of the electric supply in Denmark as well as portions of Germany and Spain. Although penetration levels in the United States are nowhere near those levels – less than 1 percent of total energy generated and less than 10 percent for some individual utilities – a number of states – California, New Mexico, and most recently Minnesota, Oregon, and Colorado – have or are implementing Renewable Portfolio Standards of 20 percent or more where wind will play a significant part in the electricity supply scenario.
Responding to Industry Requests
UWIG was asked by the Power Engineering Society of the Institute of Electrical and Electronics Engineers (IEEE) to assist in putting together an issue of its Power & Energy Magazine focusing on integrating wind into electric utility systems. That issue (November/December 2005) became widely accepted as a reference document on the subject. UWIG is now working with the IEEE Power Engineering Society on an update of the issue, which will be released this November.
In early 2006, drawing from the articles in the November/December 2005 issue of Power & Energy Magazine, UWIG developed a summary of the findings published in the 2005 issue and repackaged it into a document available on the UWIG web site (www.uwig.org). The document was prepared by UWIG in coordination with the Edison Electric Institute, American Public Power Association, and National Rural Electric Cooperative Association. The summary document noted that wind generation can have impacts on the power system, but can be managed through proper plant interconnection, integration measures, transmission planning, and system and market operations. The document noted that in regards to the integration cost associated with up to 20 percent wind capacity penetration, system operating cost increases arising from wind variability and uncertainty amounted to less than 10 percent of the wholesale cost of the energy produced by wind. The costs can be managed through a variety of means – most notably accurate wind forecasting. And it should be noted that the costs should be viewed in light of wind replacing reliance on fossil generation, where fuel price risk and carbon risk are of increasing concern.
The compatibility of wind plant operation with the electric power system has always been a significant question. The UWIG document noted that wind plant terminal behavior is different than that of conventional power plants, but can be compatible with existing power plants through the use of sound engineering practices and technology. The document states that “with current technology, wind power plants can be designed to meet industry expectations such as riding through a three-phase fault, supplying reactive power to the system, controlling terminal voltage, and participating in SCADA system operation.” It should be noted that the Federal Energy Regulatory Commission (FERC) codified these parameters into Order 661-A, Interconnection for Wind Energy. This rule constitutes a "Grid Code" for large generators with a capacity of 20 MW and above.
Wind is not a Capacity Resource
In terms of integrating wind, a common assumption made about wind power is that it requires an equivalent amount of backup generation to mitigate its variability. Wind is an energy resource – as opposed to capacity resource – and does not require backup generation provided that wind capacity is properly discounted in the determination of overall generation capacity adequacy. Significant levels of wind penetration will impact overall generation mix and dispatch as nonwind generation is needed to maintain system reliability when winds are low or not blowing. There is a real need to understand and properly treat the capacity value of wind plants within the overall system. The capacity value of wind generation is typically 10 to 40 percent of its nameplate rating and depends heavily on the correlation between the system load profile and wind plant output. Effective integration of wind generation can be ensured through good wind forecasting capability, implementing plant output forecasting in both power market operation and operations planning, adequate system flexibility, adequate transmission, and well-functioning markets.
In terms of transmission planning and market operation, a lot of work has been conducted to examine and evaluate wind development scenarios and how to accommodate more wind generation. In terms of transmission, significant efforts have been underway in the Western United States with the Western Governors’ Clean and Diversified Energy Action Plan and in Texas with the creation of Competitive Renewable Energy Zones (CREZ). These are primarily focusing on addressing one corollary about wind – it tends to blow best where transmission is not readily available. Work has also been underway to better understand and manage the interplay between transmission system tariffs and wind. There have been generator imbalance penalties in place for generator output schedule deviations through FERC Order 888, which led to a drive to remove the penalty-based imbalance tariff and replace it with a cost-based imbalance tariff. This will remove the barriers to wind plant development in large regions of the country. This was recently accomplished through FERC Order 890, which moves most wind generation imbalance to a cost-based payment, and addresses several other issues of importance to wind, including a flexible-firm transmission tariff, redispatch, and regional transmission planning.
In addition to building more transmission and properly treating wind power in terms of system operations, the development and expansion of well-functioning day-ahead and real time markets will provide an effective means of dealing with the variability of wind generation. There is also recognition that consolidation of balancing, or control, areas can assist in the management of variability, as can the aggregation of broadly geographically dispersed wind plants.
The Utility Wind Integration Group, as well as followers of the wind and electric power industries, has noted that there is a continuing need to expand the understanding and quantification of the impacts of wind generation on utility power systems. A critical facet of this is effective modeling of wind plant output and behavior and improvement in the quality and utilization of wind forecasting. Advances in wind turbine technology, as well as plant design and operation procedures, will also facilitate effective integration of wind. On top of all this is the need to expand and enhance the nation’s transmission infrastructure and optimize market design and operation to address wind’s variability. Since its beginning, UWIG has recognized that the key to making this all work is through an improved understanding of how increasing amounts of wind capacity affect the system, increased dialogue and information exchange. UWIG remains committed, through its mission of facilitating the responsible integration of wind energy into electric power systems, to keeping the electric power industry informed about the state of the art in wind integration. Stay tuned.
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