Accommodating High Levels of Variable Generation - NERC

Characteristics of Power Systems & Variable Generation
In each of the operational planning and real-time operations domains, the characteristics of the
bulk power system must be understood to ensure reliable operation. For example, regulating
reserves and ramping capabilities are critical attributes necessary to deal with the short-term
uncertainty of demand and generation, as well as with the uncertainty in the demand forecasts
and generation availability.
At higher levels of variable generation, the operation and characteristics of the bulk power
system can be significantly altered. These changes need to be considered and accommodated into
the planning and operational processes. For example, as shown in Figure 2.2, wind generation
can increase the gap between net demand at peak and off-peak periods, increasing the need for
more dispatchable ramping capability from the resources on the system that provide this ramping
capability. 14 Figure 2.2: Variable Generation can Increase System Flexibility Needs
14 If we assume that conventional generation resources provide all the ramping capability for the system, Figure 2.2
shows that in the absence of wind generation, these conventional resources must be able to ramp from 9,600 MW
to 14,100 MW (4,500 MW of ramping capability) in order to meet the variation in load demand during the day
shown in the figure by the red curve. With additional wind generation, the variation in net demand, defined as
load demand minus wind generation, must be met using the ramping capability from the same conventional
generators on the system. As shown in Figure 2.2, wind generation is significantly higher during the off
peak load period than during the peak load period. Hence, the net demand during the day, shown in blue, varies
from about 7,000 MW to 13,600 MW requiring the conventional generators to ramp from 7,000 MW to 13,600
MW (6,600 MW of ramping capability) which is approximately 45% greater than the ramping capability needed
without wind generation.
Accommodating High Levels of Variable Generation 8