Accommodating High Levels of Variable Generation - NERC

Characteristics of Power Systems & Variable Generation
distribution system is functioning properly. However, a comprehensive approach is needed for
planning from the distribution system through to the bulk power system particularly with the
increased penetration of variable generation on distribution systems. Local area issues severely
stressing a distribution system can also impact bulk power system reliability. Therefore, these
impacts need to be understood and resolved in the bulk power system planning and operation.
Planners and operators would benefit from one or more reference manuals which describe the
evolving changes required to plan and operate a bulk power and distribution systems
accommodating large amounts of variable generation. Therefore, the following recommendation
is made for NERC’s Planning and Operating Committees:
NERC Action: NERC should prepare a reference manual 21 to educate bulk power and
distribution system planners and operators on reliable integration of large amounts of
variable generation. The reference manual should outline concepts, processes and best
practices to be used by bulk power and distribution system planners and operators to reliably
integrate large amounts of variable generation.
The following sections will describe the technical characteristics of variable generation and
highlight their inherent characteristics including capabilities and limitations. Understanding these
technical characteristics is vital to comprehend how to reliably integrate them into the bulk
power system.
2.3. Variable Generation Technologies
As described previously, variable generation technologies generally refer to generating
technologies whose primary energy source varies over time and cannot reasonably be stored to
address such variation. Variable generation sources which include wind, solar, ocean and some
hydro generation resources are all renewable based. 22 There are two major attributes of a
variable generator that distinguish it from conventional forms of generation and may impact the
bulk power system planning and operations: variability and uncertainty.
Steady advances in equipment and operating experience spurred by policy incentives and
economic drivers have led to the maturation of many variable generation technologies. The
21 Note that a reference manual is not a NERC Standard. If acceptable, it may become a NERC Planning Committee
Guideline. “Reliability guidelines are documents that suggest approaches or behavior in a given technical area for
the purpose of improving reliability. Reliability guidelines are not binding norms or mandatory requirements.
Reliability guidelines may be adopted by a responsible entity in accordance with its own facts and circumstances.”
See Appendix 4, of the Planning Committee’s Charter, entitled “Reliability Guidelines Approval Process,” at
http://www.nerc.com/docs/pc/Charter_PC_Approved_29Oct2008.pdf.
22 Note the reverse is not necessarily true i.e. renewable does not imply variable as there can be a storage element.
For example biomass is renewable and can be stored and used to fuel a thermal power plant and is therefore not
variable. Another example is hydroelectric power with a large storage reservoir.
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