WIND ENERGY SYSTEMS - Cd3wd

Chapter 5—Electrical Network 5–43
Figure 22: Load variation in model utility system.
R a =
∑365
R d (i) = 150(0.05) 5 + 120(0.05) 4 + 60(0.05) 3 + 35(0.05) 2
i=1
= 4.6875 × 10 −5 +7.5 × 10 −4 +7.5 × 10 −3 +87.5 × 10 −3
= 95.8 × 10 −3 =0.0958 day per year
This result shows that generation is inadequate to meet load about 0.1 days per year or about one
day in ten years. This level of reliability is a typical goal in the utility industry.
We see that a relatively small number of days with the highest peak load contributes the largest part
of the annual risk. If these peaks could somehow be reduced through conservation or load management,
system reliability would be improved.
It should be emphasized that even when load exceeds the rating of generators on a system,
the utility may still meet its obligations by purchasing power from neighboring utilities or by
dropping some less critical loads. Only when the generation of many utilities is inadequate
will load actually be lost.
The effective capability or effective capacity of a proposed generating plant is determined
in the following manner. The annual risk is determined for the original system for the year
under investigation. This requires a loss-of-load probability calculation based on (1) the rating
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001