WIND ENERGY SYSTEMS - Cd3wd

Chapter 5—Electrical Network 5–42
The utility bases its plans for expansion on the need to maintain a reliable system. Utilities
try to maintain a total installed capacity at least 15 percent greater than the expected yearly
peak load. This allows them to continue to meet the required load even if a large generating
plant has a forced outage. When the load is not at its peak, several generating plants may
have forced outages without affecting the ability of the utility to meet its load with its own
generation.
There is a certain probability of a forced outage occurring during a daily operation cycle.
This probability varies with the type, age, and general condition of the generating plant. A
typical forced outage rate for a hydro plant may be 1.5 percent, while that of a coal fired plant
may be 5 percent. A 5 percent forced outage rate means that, on the average, a given plant
will be out of service at least a part of the day for one day out of twenty. Forced outages
typically take the plant out of service for at least 24 hours before repairs are made and the
plant is put back on the line, so the daily peak load would normally occur while the forced
outage is present. This means that the daily peak is used in determining reliability of a system
rather than hourly loads.
The probability of two generating plants being on forced outage at the same time is just
the product of the probabilities that either one will be out. If each has a forced outage rate of
0.05, the probability of both being forced out at the same time is (0.05) 2 = 0.0025 or about
0.91 days per year. The probability of additional generation being out at this same time is
still smaller, of course.
Suppose for the sake of illustration that we have a utility system with ten 700 MW generators,
each with a forced outage rate of 0.05. Suppose that the load for several days is as
shown in Fig. 22. The peak load for the first day is between 4900 and 5600 MW, so three
generating plants have to be out of service before the utility is unable to meet its load. Two
plants being out will cause a loss of load on the third day while four plants would have to
be out on the fifth day to cause a loss of load. If the load ever exceeds 7000 MW then the
probability of generation being inadequate that day is 1.0
Each day has a certain probability R d (daily risk) that generation will be inadequate to
meet the load. If we add these daily risks for an entire year, we get an annual risk R a ,
expressed in days per year that generation will be inadequate[3].
Example
365
∑
R a = R d (i) (71)
i=1
The daily peak load on the model utility system of Fig. 22 is between 3500 and 4200 MW for 150
days of the year, between 4200 and 4900 MW for 120 days, between 4900 and 5600 MW for 60 days,
and between 5600 and 6300 for 35 days. What is the annual risk R a ?
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001