Accommodating High Levels of Variable Generation - NERC

Characteristics of Power Systems & Variable Generation
Consequently, additional flexibility may be required from conventional dispatchable generators,
storage, and demand resources so the system operator can continue to balance supply and
demand on the bulk power system. In this respect, the inherent flexibility of the incumbent
generating fleet may be assessed by the:
• Range between its minimum and maximum output levels;
• Ability to operate at any MW level from minimum and maximum output levels;
• Start time; and
• Ramping capability between the minimum and maximum output levels. 16
To maintain reliable and efficient operation of the power system, operators must use forecasts of
demand and generator availability. Today the majority of supply-demand balancing in a power
system is achieved by controlling the output of dispatchable generation resources to follow the
changes in demand. Typically, a smaller portion of the generation capacity in a Balancing Area
is capable of and is designated to provide Automatic Generation Control (or AGC) service in
order to deal with the more rapid and uncertain demand variations often within the seconds-tominutes
timeframe. AGC is expected to play a major role in managing short-term uncertainty of
variable generation and to mitigate some of the short-term impacts (i.e., intra-hour) associated
with variable generation forecast error. Hence, it may be necessary for planners and operators to
review and potentially modify the AGC performance criteria, capabilities 17 and technologies to
ensure that these systems perform properly.
AGC typically includes both load frequency and interchange control algorithms that work
together to optimally move generating units on AGC to maintain system frequency. The AGC
system resides in the system control center and monitors the imbalance between generation and
demand within a Balancing Area. At higher levels of variable generation, the AGC algorithms
and parameters may need to be modified for better performance. 18 Within a Balancing Area,
AGC adjusts supply automatically between dispatch intervals to ensure that the Balancing Area
is contributing to maintaining system frequency and keeps its interchange(s) with neighboring
Balancing Area(s) at scheduled value(s).
16 Ramping capability may require different characteristics for ramping up than for ramping down
17 Including an assessment of available AGC as a percent of total generation by individual Balancing Area and
interconnection
18 EPRI TR-1018715, “EPRI Evaluation of the Effectiveness of AGC Alterations for Improved Control with
Significant Wind Generation,” Hawaiian Electric Light Company Study Report, Oct 2006.
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