A Utility Perspective of Wind Energy - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 734

An excellent review of the integration issues faced by wind power producers and utili-
ties [Milborrow 1993] is the source of much of the following information. From the utility
viewpoint, integration issues can be categorized as follows:
-- macro-scale issues involving controls, interconnection, and operation with high
penetration ( i.e. , fraction of the system demand filled by wind power)
-- micro-scale issues relating to connection and control of individual wind turbines
or small clusters of wind turbines
It is important to note that the cost of resolving either macro- or micro-scale operating prob-
lems must be weighed against the value of the wind-generated electricity to the utility.
Macro-Scale Integration Issues
Macro-scale concerns include problems with the control of a wind power plant that ex-
tends over a very large area and with grid electrical stability, which may result from high
penetration. An early concern was that arrays of wind turbines might lack sufficient controls
to prevent them from injecting power into the utility system that could cause operational
problems, such as significant deviations in frequency and voltage. When the wind-generated
electricity is purchased by the utility from an independent power producer, the burden of
correcting any frequency, voltage, and line loading deviations caused by the wind plant will
fall on the utility's conventional generating system. As a result, the operating procedures of
a utility need to be examined to determine the best means to manage the penetration of a grid
by wind power.
For moderate penetrations by wind power sources, operating performance of the utility
can be managed by modifying the utility's sequence and rate of committing generating units
as demand increases, through the scheduling of additional conventional regulating units.
This increases the ramp rate of control capability ( i.e. , the control authority), but regulat-
ing units are more expensive to operate than the intermediate units they would displace. If
operating problems become more acute, the development of more sophisticated grid control
systems could be required, but this has not been the case, even in Denmark where wind pen-
etration has exceeded 50 percent in some regions.
Micro-Scale Integration Issues
Utility technical concerns at the micro-scale level involve determining any impact of
wind turbines as an intermittent generating source. When penetration is low, such impacts
have been inconsequential. However, any potential impact on a substation or other local
equipment will increase with penetration level. Portions of the sub-transmission and distri-
bution networks may be subject to overloading and counter flows , which can give the utility
problems with reliability and voltage regulation. Reactive power or VAR support for the
utility line could be an additional requirement, although newer electronically controlled vari-
able-speed asynchronous wind turbine generators allow full control of power factor and can
provide VAR support to the grid and have virtually eliminated this concern.
Transmission and Distribution System Effects
At the local level, there are three factors that need to be considered in regard to this issue.
First, power swings caused by wind speed variations might cause voltage fluctuations that are
large enough to be detectable as flicker in fluorescent lights. Utilities can avoid this potential