WIND ENERGY SYSTEMS - Cd3wd

Chapter 5—Electrical Network 5–45
capacity factor of less than 10 percent because of the limited hours of operation, but have
an effective capacity of nearly 90 percent because of its high availability when the peak loads
occur. Wind electric plants will almost always be operated when the wind is available because
of the zero fuel cost. If the wind blows at the rated wind speed half the time and is calm the
other half of the time, then the capacity factor would be 0.5 except for the reduction due to
forced and planned outages. If the winds occurred at the times of the utility peaks, then the
effective capacity would be close to unity. However, if the wind is calm when the utility peaks
are occurring, the effective capacity will be near zero. The timing of the wind plant output
relative to the utility hourly demand profile is critical.
General Electric has performed a large study to determine the effective capacity of wind
turbines on actual utility systems[5]. They selected a site in Kansas, another in New York,
and two in Oregon. Detailed data for Kansas Gas and Electric, Niagara Mohawk, and the
Northwest Power Pool were analyzed using state-of-the-art computer programs. Actual load
data and actual wind data were used. Results are therefore rather specific and somewhat
difficult to extrapolate to other sets of circumstances. However, they represent the best
possible estimate of capacity factor and effective capacity that could be obtained at the time
of the study, and are therefore quite interesting.
Figure 24 shows the effective capacity and capacity factor for wind turbines on the assumed
1990 Kansas Gas and Electric System. Dodge City wind data for the years 1950, 1952, and
1953 were used in the study. These wind data were recorded at 17.7 m and extrapolated
to hub height of a model 1500 kW horizontal axis, constant speed wind turbine by the oneseventh
power law equation. A total wind generation capacity of 163 MW or 5 percent of
total capacity was assumed. This is often referred to as a penetration of 5 percent. A forced
outage rate of 5 percent was also assumed. There is no energy storage on the system.
Figure 24: Impact of weather year on capacity factor and effective capacity.
It may be seen that effective capacity varies from less than 30 percent in 1950 to almost
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001