UWIG Power System Impact Study
A Proposal for
Characterizing the Impacts of
Significant Wind Generation Facilities on
Bulk Power System Operations Planning
January 2000
Prepared for
The Utility Wind Interest Group
By
2111 Wilson Boulevard
Suite 323
Arlington, Virginia 22201-3001
Tel: 703 351 4492, ext. 121
FAX: 703 351 4495
www.uwig.org UWIG Proposal Characterizing the Impacts of Wind Facilities on System Operations Planning
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Introduction
A recent survey of the UWIG membership identified "an improved understanding of the impact of wind
plants on utility system operating requirements" as the highest priority activity to be pursued with
available resources within the organization. For almost two decades, research and development efforts
have addressed the range of technical and economic questions that together make up this general issue.
Now, however, the "answers" to those questions, and perhaps more appropriately the approaches and
methodologies for resolving them in specific contexts, are taking on increased importance. Commercial
wind plant development has expanded beyond the traditional wind resource areas in California, bringing
a larger number of power system operators and interconnection issues into play.
Electric generating resources that are part of the interconnected power system must be controlled so that
their aggregate output matches the electric load at any given time. Frequency deviations and
unscheduled tie-line transfers between control areas are indicators of a mismatch between load and
generation. Reliable and economic operation of interconnected power systems relies on multiple layers
of automatic control systems to insure that generator output follows the changes in load. At the same
time, these systems must keep the operation of individual system components within safe limits and
maintain adequate security margins for likely contingencies.
On the smallest time scale, individual governors on the prime movers of individual generators act to
keep the mechanical system at a speed corresponding to the nominal 60 Hz power system frequency.
Automatic generation control (AGC) coordinates the individual actions of multiple generators in an area
to quickly respond to load changes and frequency deviations in the most economic (least cost) manner
given the profile of generating units in operation at the time. These actions occur on time scales from a
fraction of a second (for speed governor adjustments) to several minutes.
At the bulk power system (transmission) level, changes in aggregate system electrical demand on a
second-by-second or minute-by-minute basis reflect the diversity inherent in a very large number of
relatively independent and small electric loads; i.e. diversity has a filtering effect on the magnitude of
load changes on these time scales. Furthermore, many years of operating experience have contributed to
a very good understanding of these changes and to the development of generation control and dispatch
strategies for dealing with them.
The intermittent and mostly uncontrollable nature of wind generation introduces new variables into the
power system control problem. Since wind generation on a significant scale (relative to the bulk electric
power system) is relatively new, there is a general lack of historical operating experience. The effects
of fluctuating wind generation resources on generation control have been previously addressed in
general or hypothetical terms, with the objective of many of these evaluations being to determine the
wind generation penetration level below which no impacts would be expected. NREL has recently
begun an effort to monitor the long-term output of several wind plants at very high time resolution,
which should result in a valuable database for assessing system operating impacts. This work is to be
conducted by Electrotek, so its results will be readily available for this proposed project.
Wind generation development in the U.S. has progressed to a point where some individual wind plants
and projects have reached the size of a single medium-to-large conventional generating plant. At this
size, there is some anecdotal evidence that system operating and control strategies are being impacted.
The fluctuating output of the wind plant, along with the potential loss of that resource due to a UWIG Proposal Characterizing the Impacts of Wind Facilities on System Operations Planning
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transmission system event, must be taken into consideration in the overall equation for deploying and
controlling other generating plants in the control area.
The objective of the activity proposed here is to conduct a quantitative investigation into the impacts of
large wind generation resources on power system operations and scheduling functions. It is proposed
that the work be based on actual case study data, rather than a hypothetical situation or synthesized data.
The case study will utilize conventional utility analyses and software tools. It is likely, however, that
these conventional tools and approaches might not be suited for representation of wind plants or wind
generation characteristics. Hence alternative approaches and methodologies will be developed to
facilitate appropriate characterization and representation of wind plants in the various technical and
economic analyses.
A utility member of the UWIG has agreed to participate in and cosponsor the case study. The focus of
this project will be to answer questions regarding the specific impacts of wind generation on operations,
scheduling and planning for the case study system. Extrapolating and generalizing the results of this
project for application to a wider range of utility systems and wind generation technologies will be a
substantial undertaking in itself, and is recommended as a future, follow-on activity. However, a
preliminary attempt to generalize findings will be conducted within the scope of this project.
The UWIG considers this to be the most critical problem for the future integration of large amounts of
wind capacity into utility systems. Consequently, UWIG will cosponsor the work with a contribution
from its modest discretionary funds. Selected experts from within UWIG-member utilities will also
provide ongoing detailed technical review of the work to enhance its validity and credibility within the
electric sector. A successful outcome will require addressing a very wide range of technical and
economic issues. Tools and methodologies utilized in utility-system operations planning must be
modified to accommodate the technical characteristics of wind plants and wind generation. As
representatives of major utility organizations, the UWIG utility experts are well-versed in the relevant
technical disciplines, and can provide essential guidance for the work. UWIG Proposal Characterizing the Impacts of Wind Facilities on System Operations Planning
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Background - Power System Operations and Scheduling
The primary objective in power system operations and scheduling is to continuously provide sufficient
generation to match load at the lowest possible cost to the utility. This objective must be accomplished
while maintaining system voltage and frequency within specified tolerances, and providing for system
security. Time frames of interest range from seconds to weeks, months, and even years. As installed
wind plant capacity approaches and possibly surpasses that of other single units within a given power
system, integrating the performance characteristics of wind plants into the various algorithms and
programs used for planning and monitoring system operation will become more important.
AGC and Economic Dispatch
Stable operation of the interconnected power system is dependent on an instantaneous match between
load and generation. System frequency is a primary indicator of this balance. When electric load on a
synchronous generator exceeds mechanical input, the generator will begin to slow down as kinetic
energy is extracted from the machine's rotational inertia and is converted to electric power. The
decrease in shaft speed corresponds to a decrease in frequency in a synchronous generator. Conversely,
when power supplied by a generator prime mover exceeds electric demand, the generator mechanical
system will accelerate as the excess input is stored as rotational energy, with a corresponding frequency
increase.
Speed governors on individual generating units maintain constant generator speed by adjusting the
mechanical input from the prime mover (steam turbine, penstock, diesel engine, etc.) in response to a
speed error signal. These systems provide the fastest response to speed deviations caused by mismatch
between generator input and output.
Automatic generation control, or AGC, is the principle mechanism for coordinating the output of all
system generators to match aggregate electrical demand. In a given control area, governor setpoints of
generating units assigned responsibility for system regulation are coordinated by AGC to maintain
system frequency at 60 Hz. AGC involves two interrelated functions: 1) Load/frequency control; and
2) Economic dispatch. Response of load/frequency control is on the order of seconds; variations which
occur more quickly are handled by the individual units speed governors.
The economic dispatch function in AGC attempts to minimize the cost of meeting the load demand by
adjusting individua