Environmental Engineering Reference
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output. Weather and wind forecasting becomes more important as the level of penetration
by wind power plants increases. Although weather has always been an important generation
planning consideration since it drives power demand for space heating and cooling, and fore-
casting models have been improved to the point that this is not considered problematic.
Generation Dynamic Response
Variability within the grid is not new to a utility, as illustrated by the display of time
scales in Figure 13-6. The longest of these is the daily load following scale. All utilities
need to cope with the continual fluctuations in demand caused by variations in industrial,
commercial, and residential consumption. The left end of this bar, at a time scale of ap-
proximately 250 seconds, represents the local level where there is a continual load “ripple”
caused by the many and varied demands on the utility system. The next-shorter time scale
is that for tie-line regulation . Fluctuations in transmission line demand are at the right-hand
end, with perturbations that tend to occur on time scales approaching 1,000 seconds. At the
left-hand end of this bar, voltage fluctuations on distribution lines are minimized by ensuring
that consumer loads are connected at a voltage level which ensures they can be absorbed by
the power-carrying capacity of the local network. Long-term dynamic behavior occurs on a
similar time scale, but extends down to about 0.3 second.
The next time scale is the one of greatest concern to utilities dealing with current or
future wind power generation, that of transient and dynamic stability phenomena. The inter-
action between wind plants and the grid are important design issues that are well understood.
We see that if significant fluctuations in the output of a wind power plant have periods of
about 15 seconds or more -- by virtue of their multiple units, extended land area, active con-
trols, or a combination of all these factors -- utilities need have no concern about its impact
on system dynamic stability.
Figure 13-6. Time scales of utility system dynamic phenomena.
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