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Appendix C
Comparison of Different Power-Generation Methods 263

Different locations will have different wind-speed distributions. The
Rayleigh model closely mirrors the actual distribution of hourly wind
speeds at many locations.
Because so much power is generated by higher wind speed, much of
the energy comes in short bursts. The 2002 Lee Ranch sample is telling;
half of the energy available arrived in just 15 percent of the operating
time. The consequence is that wind energy does not have as consistent
an output as fuel-fired power plants; utilities that use wind power must
provide backup generation for times that the wind is weak. Making
wind power more consistent requires that storage technologies must be
used to retain the large amount of power generated in the bursts for later
use.

Grid Management
Induction generators often used for wind power projects require reac-
tive power for excitation, so substations used in wind-power collection
systems include substantial capacitor banks for power factor correction.
Different types of wind turbine generators behave differently during
ptg
transmission grid disturbances, so extensive modeling of the dynamic
electromechanical characteristics of a new wind farm is required by
transmission system operators to ensure predictable stable behavior dur-
ing system faults. In particular, induction generators cannot support the
system voltage during faults, unlike steam or hydro turbine-driven syn-
chronous generators. (However, properly matched power factor correc-
tion capacitors along with electronic control of resonance can support
induction generation without grid.) Doubly fed machines, or wind tur-
bines with solid-state converters between the turbine generator and the
collector system, have generally more desirable properties for grid inter-
connection. Transmission systems operators will supply a wind farm
developer with a grid code to specify the requirements for interconnec-
tion to the transmission grid. This will include power factor, constancy
of frequency, and dynamic behavior of the wind farm turbines during a
system fault.
Because wind speed is not constant, a wind farm’s annual energy pro-
duction is never as much as the sum of the generator nameplate ratings
multiplied by the total hours in a year. The ratio of actual productivity
in a year to this theoretical maximum is called the capacity factor.
Typical capacity factors are 20 percent to 40 percent, with values at the