IEEE Power & Energy Magazine - November/December 2015 - 25

test, due to an increase in the wind speed. The droop and
deadband settings for this test are typical values.
This control can also respond to under-frequency events,
as shown in Figure 2. To allow for an increase of wind plant
active power output in response to an under-frequency condition, some active power production must be kept in reserve.
Therefore, the maximum power production of the wind plant
is constrained to a value less than that available from the wind.
These settings, unlike those of typical governors, are
deliberately asymmetrical between high- and low-frequency
response and can be adjusted to meet specific grid and application requirements.

Governor Response as an Ancillary Service
Provision of governor response has not historically been
regarded as an ancillary service but rather as an expectation for generation on interconnected systems. By traditional
practice, it is common for thermal and hydro plants to have
governor functions enabled, although exceptions are widely
found. For example, nuclear power plants rarely have governor response.
In the case of providing governor response from wind
plants, it is critically important to recognize the inherent
asymmetry of wind power production: the available wind
power at any given moment sets the upper bound on available power. Wind turbines are designed to maximize the
electrical energy output based on the wind energy input with
the highest possible efficiency. No control action on the part
of the wind turbines can increase this upper bound. Thus,
it is normally possible to make less power than is available
from the wind, but never more on a sustained basis because
operation becomes less efficient and economical.
As shown in the field tests described previously, in order
to increase power over a sustained period, the power must be
limited a priori by "spilling" wind. Thus, providing this "up"
governor response amounts to a continuous loss of energy
production and an intentional underutilization of the wind
plant. This is fundamentally different from plants that are

inherently able to temporarily generate power beyond their
rating or predisturbance operating point, in that the use of
this function has a substantial opportunity cost. Thus, just as
with other types of generation that cannot provide governor
response without high opportunity (or other) costs, a market
mechanism is required for this function.
Provision of governor response could be enabled by an
ancillary services agreement. It might be scheduled to operate only at defined times of the day or year. Alternatively, it
could be selectively enabled according to overall grid operating conditions. The function is most likely to be valuable
and economically feasible under conditions of high wind and
light power system load.
By comparison, the "down" response has essentially no
opportunity cost, and can be provided very effectively as an
ancillary service. Because of its controllability and speed,
the down response of wind plants can be markedly superior to that available from conventional thermal or hydro
plants and, thus, has significant value for system reliability.
It makes sense for this function to be enabled under all operating conditions.

Power Scheduling and Ramp-Rate Control
As noted in the discussion on governor functions, wind generation can usually produce less power than is available from
the wind, but not more. Limiting the output of a wind plant,
called "curtailment," may be necessary for several reasons.
Two major reasons are congestion (the grid's inability to
accept or deliver power because of transmission infrastructure limitations) and reserve limitations (inability to accept
wind power because of constraints on other resources providing various reserve ancillary services).
Figure 3 demonstrates the ability to curtail to a tightly
controlled power schedule. In this field measurement, a
30-MW plant with GE 1.5 MW wind turbines is shown curtailed at various levels. Over the course of two hours, the
scheduled curtailment is lifted from 10 MW at the start to
no limit by the end. The wind trace shows there is sufficient

Power
2% Frequency
Increase
(1.2 Hz ∆f)
50% Power
Reduction

Frequency

10 s/div

figure 1. A demonstration of over-frequency governor
response.
november/december 2015

10 s/div
Frequency
10%
Power
Increase

4% Frequency
Reduction
(2.4 Hz ∆f )
Power

figure 2. A demonstration of under-frequency governor
response.
ieee power & energy magazine

25



Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - November/December 2015

IEEE Power & Energy Magazine - November/December 2015 - Cover1
IEEE Power & Energy Magazine - November/December 2015 - Cover2
IEEE Power & Energy Magazine - November/December 2015 - 1
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IEEE Power & Energy Magazine - November/December 2015 - Cover3
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