WIND ENERGY SYSTEMS - Cd3wd

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Chapter 5—Electrical Network 5–44 of each generating plant and its forced outage rate, (2) the daily hourly-integrated peak loads (the greatest energy sales in any one hour of the day), (3) maintenance requirements for each unit, and (4) other special features such as seasonal deratings or energy interchange contracts. The single point resulting from this calculation is spread out into a curve by varying the assumed annual peak load for that year by ±20 percent and each daily peak by the same fraction. As the assumed peak load increases for the same generation, the annual risk increases. A curve such as the original system curve of Fig. 23 is the result. Figure 23: Annual risk before and after adding a new unit. The proposed new generating plant is then added to the system while keeping all other data fixed. We again vary the annual peak load with the daily peaks considered as a fixed percentage of the annual peak. Adding this unit reduces the risk at a given load, so we consider a somewhat larger range of loads, perhaps a zero to 40 percent increase over the previous midpoint load. The result can be plotted into the second curve of Fig. 23. The distance in megawatts between these curves at the desired risk level is the amount of load growth the system can accept and still retain the same reliability. This distance is the effective capability or effective capacity of the new unit. The effective capacity will typically be between 60 and 85 percent of rated capacity for new fossil or nuclear power plants. If it is at 75 percent, this means that a 1000-MW generating plant will be able to support 750 MW of increased load. The remaining 250 MW will be considered reserve capacity. The effective capacity is not identical to the capacity factor or plant factor, which was defined in Chapter 4 as the ratio of average power production to the rated power. Capacity factor is calculated independently of the timing of the load cycle, while effective capacity includes the effect of the utility hourly demand profile. Effective capacity may be either larger or smaller than the capacity factor. An oil fired gas turbine may have an actual Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001
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
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