Frequently Asked Questions About Systems Integration

Wind's variability does increase the day-to-day and minute-to-minute operating costs of a utility system because the wind variations do affect the operation of other plants. But investigations by utility engineers show these costs to be relatively small — less than about 2 mills/kilowatt-hour (kWh) at penetrations under 5% and possibly rising to 5 mills at 20% penetration. In fact, when the Colorado Public Service Commission issued a ruling in 2001 on the 161-megawatt (MW) wind project in Lamar, Colorado, the commission determined that wind energy provided the lowest cost of any new generation resource submitted to an Xcel Energy solicitation bidding process (except for one small hydropower plant). The commission also noted that unlike the other generation resources considered, the Lamar project avoided the risk of future increased fuel prices.¹ And, in a recent landmark study of wind integration into the New York State electric power system, a 10% addition of wind generation (3,300 MW of wind in a 34,000-MW system) actually projected a reduction in payments by electricity customers of $305 million in one year.²

Back to Top

No power plant is 100% reliable. During a power plant outage — whether a conventional plant or a wind plant — backup is provided by the entire interconnected utility system. The system operating strategy strives to make best use of all elements of the overall system, taking into account the operating characteristics of each generating unit and planning for contingencies such as plant or transmission line outages. The utility system is also designed to accommodate load fluctuations, which occur continuously. This feature also facilitates accommodation of wind plant output fluctuations. In Denmark, northern Germany, and parts of Spain, wind supplies 20% to 40% of electric loads without sacrificing reliability. When wind is added to a utility system, no new backup is required to maintain system reliability.

Karl Pfirrmann of the PJM Interconnection, the entity charged with operating the largest electric operating system in the country, provides a detailed explanation of the role of wind on the electric grid (PDF 107 KB) Download Adobe Reader.

Back to Top

In situations with weak distribution grids (long lines with thin wires and few customers — maybe even single-phase), this can be true. However, in many cases wind generation can be connected to the distribution system in amounts up to about the rating of the nearest substation transformer. One study of a rural Midwestern county estimated that several tens of megawatts of turbines could be installed on the local distribution grid with a minimum of upgrade expense and minimal power-quality impacts. A number of single wind turbines and clusters of turbines are currently connected to the distribution system.³

Back to Top

Reliability of wind turbines is typically defined as the quantity of time they are on-line and ready to produce electricity divided by the total time in the year (also called "availability"). Whether or not they produce electricity is a function of the wind. Ignoring external influences on turbine availability such as grid outages or forced curtailments by the utilities, wind turbines have demonstrated very high reliability, meaning they are available to produce energy approximately 97% of the time. However, due to intermittency of the wind resource, the turbines do not always produce the maximum power output possible. When averaged over a year, wind projects typically operate at levels equivalent to 30% to 40% of their full capacity (also known as capacity factor).

Back to Top

Utilities can typically add wind generation to their power supply mix without major adjustments in the planning, operations, or reliability of their systems, according to several recent studies that included modeling wind integration scenarios, as well as experience in Europe.

Costs range from $5/MWh to zero beyond the integration costs that are the norm for conventional technologies. The level varies with size of region, type of tariff, transmission capacity, flexibility of other generation sources, and level of penetration.

Back to Top

While the wind doesn't blow all the time, reliable patterns (on a daily and seasonal basis) exist. These patterns are used to project long-term energy production from wind projects, as well as to forecast short-term energy delivery.

As part of project development, long-term wind measurements are compiled from nearby monitoring stations (such as airports and government environmental monitoring stations). These long-term data are correlated to the on-site wind measurements, which are then adjusted up or down depending on the correlation to reflect the long-term resource. Although it is expected that the wind resource will vary from this estimate in any given year, the intention is to predict the long-term average output from a wind project over its estimated design life.

Short-term forecasts serve a different purpose. Utilities use short-term forecasts (i.e., up to 48 hours in advance) to plan the dispatch of their generation resources. In the past few years, sophisticated wind prediction computer models have been developed to predict the hourly output from a wind project for several days into the future. Continued improvement in these models is expected to result in increasingly accurate forecasts.

Back to Top