IEEE Power & Energy Magazine - November/December 2015 - 24

Integrating wind power plants into grids is complicated by
a number of issues that are related mainly to wind variability
and the electrical characteristics of wind generators.

have a significant practical impact. However, wind generation is now reaching substantial penetration levels in many
regions, and grid integration has emerged as a potential limit
on further development of this environmentally friendly
resource. Consequently, interconnecting utilities and regulatory agencies are imposing grid codes that demand performance from wind plants similar to what is provided by
conventional power plants that use steam, gas, and hydro
turbines with synchronous generators.
Today's wind plants are quite capable of providing the
full range of ancillary services that conventional thermal and
hydro plants offer, often with greater speed and accuracy. To
demonstrate the capability of today's technology, this article
describes several examples of wind plants that use GE wind
technology. Other large mainstream wind equipment manufacturers provide functional capabilities similar to those
described here, although implemented using different strategies and methods.

Wind Plant Control Systems
Wind plant control systems regulate the net real and reactive power interchange between a wind plant and the grid.
This allows the wind plant to regulate the voltage magnitude of the grid, provide governor frequency response, and
minimize rates of power change. The reactive capability can
be achieved with or without sufficient wind velocity to operate the wind turbines. This wind plant control system is a
hierarchical scheme that controls individual wind turbines
in order to implement highly accurate, stable, closed-loop
regulation of grid parameters, such as voltage or power, and
grid-interface parameters, such as power factor or net power
output. In the following sections, we describe the functionality of each of these controls and examine its opportunities
for providing various ancillary services.

Wind Plant Control of Real Power
The advanced active power controls offered as part of
today's wind systems manage the electric power output just
as well as any conventional fossil-fired power plant. In some
regards, wind plants with these advanced controls offer even
better control than conventional units because the output
may be tuned to mitigate frequency excursions shortly after
a loss of a large amount of generation in the system. The
flexibility and performance of these modern controls offer
more tools to address the challenges regarding frequency
control that system planners and operators face today.
24

ieee power & energy magazine

Governor Frequency Response
One set of active power control functions offered in wind
plant control systems is closely akin to the governor controls
in thermal and hydro generation. They respond to significant
deviations in grid frequency, increasing or decreasing power
output according to higher or lower grid frequency events,
respectively. To accomplish this, the controls alter the active
power control reference targeted by the turbine controls.
Grid over-frequency events are stressful to power components. Furthermore, temporary high-frequency swings can
present a reliability concern. For example, during one recent
and well-publicized grid occurrence, the high-frequency
backswing from a major disturbance in the grid caused
power plant trips and aggravated an already severe event.
When enabled, the response of the plant control system will
rapidly reduce power output for the duration of the over-frequency event. This behavior is similar to that of the governor control on thermal generation, except that it is faster and
allows deeper runback of power than is typical of conventional thermal generation.
To demonstrate wind plants' ability to provide primary
governor response, we present the results of field tests
obtained at a 60-Hz North American site with 40 operating GE 1.5 MW turbines and a GE WindCONTROL plant
control system. These tests were performed during the
commissioning phase of the wind plant; for some portions
of the tests, only 38 turbines were in operation.
Figure 1 illustrates the power response of the wind plant
due to a grid over-frequency condition. For this test, the
controller settings correspond aggressively to frequencies
greater than the 0.02-Hz dead band. During the test, the site
was operating unconstrained at prevailing wind conditions.
It was producing slightly less than 23 MW (~40% of rated
for the turbines online) prior to the over-frequency condition. The system over-frequency condition was created using
special test software that injected a 2% controlled ramp
offset into the measured frequency signal. The resulting
simulated frequency increased at a rate of 0.25 Hz/s, from
60 Hz to 61.2 Hz. As can be observed in the figure, when
the frequency increases, the plant power drops at a rate of
2.4 MW/s. After 4.8 s, the frequency reaches 61.2 Hz, and
the power of the plant is reduced by approximately 50%. The
over-frequency condition is removed with a controlled ramp
down to 60 Hz at the same 0.25-Hz/s rate. In response, the
plant power increases to its unconstrained power level. This
is slightly higher than the unconstrained level prior to the
november/december 2015



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