Accommodating High Levels of Variable Generation - NERC

Characteristics of Power Systems & Variable Generation
2. Characteristics of Power Systems & Variable Generation
This chapter provides an overview of the inherent characteristics of variable generation, along
with the power system modeling and analysis needed to accommodate large-scale integration of
variable generation resources. Although there are many varieties of variable generation, this
chapter focuses on wind and solar generation technologies, which currently have the largest
growth potential in North America over the next 10 years.
2.1. Power systems
Reliable power system operation requires ongoing balancing of supply and demand in
accordance with the prevailing operating criteria and standards, such as those established by
NERC. Operating power grids are almost always in a changing state due to fluctuations in
demand, generation, and power flow over transmission lines, maintenance schedules, unexpected
outages and changing interconnection schedules. The characteristics of the installed power
system equipment and its controls and the actions of system operators play a critical role in
ensuring that the bulk power system performs acceptably after disturbances and can be restored
to a balanced state of power flow, frequency and voltage.
The impacts of large-scale penetration of variable generation should be considered in terms of
timeframes: seconds-to-minutes, minutes-to-hours, hours-to-days, days-to-one week and beyond.
Planners also must address longer time frames, sometimes up to 30 years, for both transmission
and resource adequacy assessments.
In the seconds-to-minutes timeframe, bulk power system reliability is almost entirely controlled
by automatic equipment and control systems such as Automatic Generation Control (AGC)
systems, generator governor and excitation systems, power system stabilizers, automatic voltage
regulators (AVRs), protective relaying and special protection and remedial action schemes, and
fault ride-through capability of the generation resources. From the minutes through one week
timeframe, system operators and operational planners must be able to commit and/or dispatch
needed facilities to re-balance, restore and position the bulk power system to maintain reliability
through normal load variations as well as contingencies and disturbances. For longer
timeframes, power system planners must ensure that adequate transmission and generation
facilities with proper characteristics are built and maintained so that operation of the system
remains reliable throughout a range of operating conditions.
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