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AWEA Publications on
Utility Integration
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How Difficult is it to Integrate Wind
Turbines With Utilities?UTILITY
WIND INTEGRATION ISSUES NOT
SERIOUS, SAYS ELECTROTEK'S PUTNAM
[from Wind Energy Weekly #680,
15 January 1996]
Integration of wind power plants into
utility operating systems "has not been a problem," according to a new paper by
Robert Putnam of the consulting firm Electrotek Concepts, "and any issues that have
developed, such as intermittency and voltage regulation, can be addressed by accepted
power system procedures and practices."
Putnam's paper, which has been
submitted to the annual conference of the Institute of Electrical and Electronic Engineers
(IEEE), was funded by the U.S. Department of Energy and is based on interviews with system
operators and dispatchers from Pacific Gas & Electric Co. (PG&E) and Southern
California Edison Co. (SCE). Both utilities have had extensive experience with the
integration of wind energy on their systems since the early 1980s.
Putnam divides integration issues that
have arisen with wind into "interface (or engineering) issues, operational issues,
[and] planning issues." He describes how the interface and operational issues which
were the focus of his investigation have been handled as follows:
Interface
Issues
Interface issues include harmonics,
reactive power supply, voltage regulation, and frequency control.
Harmonics:
"Harmonics are undesirable distortions of the utility AC sinusoidal voltage and
current waveforms. . . . Harmonics are of concern due to potential damage to both utility
distribution and customer load equipment. Some first-generation wind power plants
installed in the early 1980s employed older, alternative conversion systems such as those
using 6-pulse thyristor bridge configurations without external harmonic correction or
filtering, resulting in the production of lower order harmonics . . . Advanced converter
systems available today produce output with very little harmonic distortion, well below
that specified in the IEEE Recommended Practice for Monitoring Power Quality. With the
addition of harmonic correction devices and the current trend towards the use of advanced
power electronics in variable-speed wind turbines, harmonics are no longer a significant
utility concern."
Reactive
Power Supply: "Early wind plants using induction generators were
installed with inadequate hardware for reactive power compensation. As a result, utilities
experienced increased line losses and difficulty controlling system voltage. . . . Wind
plant operators were economically incented to improve the quality of power injected into
the PG&E system when PG&E began to charge for excessive VAR [reactive power]
support. SCE and PG&E now require small power producers using induction generators to
provide near unity power factor at the point of interconnection. Power electronics
technology used with modern, variable-speed wind turbines have demonstrated a full range
of power factor control under all operating conditions, even with the wind turbine shut
down."
Voltage
Regulation: "Difficulty in controlling voltage regulation is accentuated
when the wind plant is located in a remote area and connected to the utility through
transmission lines originally designed to service only the load in the area. SCE
experiences periodic voltage limitations on its 66 kV system in Tehachapi due to the weak
system interconnection. Solutions considered by SCE include new transmission lines,
alternative line arrangements, the addition of static or adaptive VAR controllers, and
wind plant curtailment. Based on an economic analysis of each of these alternatives, SCE
has determined that the least cost option is to curtail wind plant production and to
compensate wind plant operators accordingly . . . "
Frequency
Control: "Utilities operating wind power plants connected to weak,
isolated grids can have difficulty maintaining normal system frequency. System frequency
varies when gusting winds cause the power output of wind plants to change rapidly. While
maintaining normal system frequency has not been a problem in the windfarm areas of
California, it has been well documented on the Hawaii Electric Light Company (HELCO)
system. [An] EPRI [Electric Power Research Institute] study showed that a reduction in
capacity or an increase in demand of 10 MW per minute, caused by a combination of wind
power output changes and/or unscheduled load changes, would cause HELCO's load-following
generation plant, Hill 6, to trip, resulting in a loss of ability to regulate system
frequency within acceptable limits. The report concludes that in order to accommodate more
wind energy, the HELCO system would require
- the use of modern, variable-speed wind
turbines with power electronic control and interface to the grid (the power electronic
system can be controlled to limit wind turbine output during gusty or strong wind periods)
and/or
- automatic generation control with
additional spinning reserve.
In the case of SCE and PG&E, the
short-term variations in wind plant output are small relative to normal load fluctuations
and therefore to not significantly impact the ramping and cycling duties of available
system regulating capacity." |
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Operational Issues Operational issues include operating reserve,
unit commitment and economic dispatch, system stability, and transmission and distribution
system impacts.
Operating
Reserve: "Utilities carry operating reserve to assure adequate system
performance and to guard against sudden loss of generation, off-system purchases,
unexpected load fluctuations, and/or unexpected transmission line outages. Operating
reserve is further defined to be spinning or non- spinning reserve. Typically, one-half of
system operating reserves are spinning, so that a sudden loss of generation will not
result in a loss of load, with the balance available to serve load within 10 minutes. Any
probable load or generation variations that cannot be forecast have to be considered when
determining the amount of operating reserve to carry. . . . At current wind plant
penetration levels in California, the variability of wind plant output has not required
any change in operating reserve requirements. The exact point at which the integration of
intermittent generation such as wind begins to degrade system economics is unclear, but
the technical literature suggests that it is at penetration levels in excess of five
percent. Intermittency is becoming an increasing concern to utility operators in
California, particularly during low demand periods, since wind plant penetration is
beginning to reach this level. . . . As markets for electricity become more competitive,
the ability to forecast and control the wind resource will increase the value of wind
energy to utilities.
Unit Commitment and Economic
Dispatch: Unit commitment is the scheduling of specific power plants on the
utility system to meet expected demand. Units are committed to the schedule based on
"generation maintenance schedules, generator startup and shutdown costs, minimum fuel
burn requirements, and seasonal availability of intermittent resources such as hydro and
wind. This schedule is usually made at least 24 hours in advance. . . . The most
conservative approach to unit commitment and economic dispatch, and the one adopted by
PG&E and SCE, is to discount any contribution from interconnected wind resources . . .
In fact, wind plant output may be fairly predictable as in the case of the Altamont Pass
region of California, due to seasonal and diurnal wind resource characteristics observed
over many years of wind farm operation or as a result of wind resource monitoring
programs. Further research is needed to develop the capability to accurately forecast wind
plant output on an hourly basis over time periods ranging from one day ahead to one week .
. . "
System
Stability: " . . . Large wind turbines typically have low-speed,
large-diameter blades coupled to an electric generator by a high-ratio gear box. This
feature results in a large turbine inertia and low mechanical stiffness between turbine
and generator [which] gives large wind turbines excellent transient stability properties.
Operating experience with wind power plants in California confirms that wind turbine
transients due to speed fluctuations or network disturbances have not resulted in system
stability problems."
Transmission and Distribution
System Impacts: Wind systems can affect transmission and distribution systems
by "[altering] the design power flow or [causing] large voltage fluctuations . . .
" Also, "islanding," in which a wind plant might energize a line that
otherwise would be dead, has been a concern. "Operating experience with wind power
plants in California has not shown system protection or safety to be an issue.
Circumstances that may have led to islanding in the past have been identified, and
hardware and detection schemes have been tested and approved. In PG&E's case, for
example, the installation of direct transfer trip equipment is designed to trip the wind
farms to prevent them from islanding."
Concludes Putnam, "The positive
integration experience with wind energy in California . . . can provide valuable insights
to utilities planning new projects and needs to become more widely understood." |
