Accommodating High Levels of Variable Generation - NERC

Introduction
• Variability: The output of variable generation changes according to the availability of
the primary fuel (wind, sunlight and moving water) resulting in fluctuations in the plant 9
output on all time scales.
• Uncertainty: The magnitude and timing of variable generation output is less predictable
than for conventional generation.
It is important to distinguish between variability and uncertainty when discussing planning and
operations of the bulk power system. The effects of variability are different than the effects of
uncertainty and the mitigation measures that can be used to address each of these are different.
When accommodating large amounts of variable generation, these two attributes can have
significant impact, requiring changes to the practices and tools used for both bulk power system
planning and operations.
Power system planners and operators are already familiar with designing a system which can be
operated reliably while containing a certain amount of variability and uncertainty, particularly as
it relates to system demand and, to a lesser extent, to conventional generation. However, largescale
integration of variable generation can significantly alter familiar system conditions due to
unfamiliar and increased supply variability and uncertainty.
1.2 NERC’s Planning and Operating Committees Create a Task Force
To date, North American experience with variable generation has been limited to integration of a
relatively small amount of the total generation within a Balancing Area (i.e. typically less than
5% of annual energy). Integration of this level of variable generation typically has not
appreciably impacted the reliability of the bulk power system. Future projections, however,
forecast a substantial increase in variable generation additions across North America, particularly
wind resources (i.e. up to 145 GW of wind generation over the next 10 years). 10 Bulk power
systems can accommodate the large-scale integration of variable generation energy in a variety
of ways; therefore a complete understanding of reliability considerations is vital.
In addition to forecasts for significant wind resource additions, it is also worth noting that during
the time period during which this report was prepared, activity (as measured by interconnection
requests) for large solar plants increased dramatically. For example, in the California ISO
generation connection queue, requests for solar (all types) increased from 51 applications
representing 17,600 MW in January 2008 to 91 applications representing nearly 30,000 MW. 11
9 Plant is a term used to describe a collection of variable generators as they typically occurs in groups, for example
multiple wind turbines constitute a wind plant.
10 http://www.nerc.com/files/LTRA2008%20v1.1.pdf
11 Source: California ISO website
Accommodating High Levels of Variable Generation 4