IEEE Power & Energy Magazine - November/December 2015 - 26

15
10
Power (MW)
Wind Speed

90
10
0
11
0
12
0

Wind Speed (m/s)

Power MW

-0
8

35
30
25
20
15
10
5
0
Time (min)

figure 3. A demonstration of power ramp-rate limitation.

wind resource to produce rated output of 30 MW throughout the two-hour period, so, of course, the final output is a
steady 30 MW. It is of interest to note that the scheduled
limit is held tightly; the power variations around the limit
are barely visible in the plots, even though the wind speed is
moderately variable. The figure also shows the power ramp
limiter maintaining a specified rate of change in power output between each successive step. In this case, the plant is
not allowed to increase its power output at a rate faster than
5% per minute. However, a wide range of ramping speed
(both faster and slower) may be set in the controls if needed.
This ability to tightly hold and ramp the output enables
wind generation to provide the same benefits to the grid as
any other conventional technology, and because the speed
of ramping may be set faster or slower for wind plants, it is
arguably more flexible than what conventional units provide.
A unique limiting technique is used to maximize energy
capture of the plant while at the same time enforcing an overall power ramp limit for the system. The ramp limiter does
not impose a rate of change on any single power-producing
turbine until the plant power rate of change approaches the
limit. This technique allows each turbine to respond to local
changes in wind conditions and each turbine to ramp its
power independently of the other turbines. Only when the
entire response of the collective plant approaches the ramp
limit will the control enforce a ramp limit for the plant.

Power Scheduling as an Ancillary Service
As with the governor response discussed earlier, this functionality is most likely to be valuable and economic at
times of high wind and light load. Ramp-rate limits can be
set to meet the requirements for specific grids and applications. Ramp-rate limits can be imposed for grid operating
conditions that warrant their use and need not be continuously enabled.

Controlled Inertial Response
(Fast-Frequency Response)
The response of bulk power systems to system disturbances
is of great concern to those responsible for grid planning
and operations. System events that include loss of generation
normally result in transient depressions of system frequency.
26

ieee power & energy magazine

The rate of frequency decline, the depth of the frequency
excursion, and the time required for system frequency to
return to normal are all critical bulk power system performance metrics that are affected by the dynamic characteristics of generation connected to the grid. Typically, in the first
few seconds following a loss of a large generating plant, the
system's frequency dynamics are dominated by the inertial
response of the operating generation. The behavior of conventional synchronous generation is well understood, and it
is relied upon by the grid for secure operation. These synchronous turbine-generators inherently contribute some of
their stored inertial energy to the grid due to the unit's physical properties, reducing the initial rate of frequency decline
and allowing slower governor actions to stabilize grid frequency. However, most modern MW-class wind generation
does not exhibit this inertial response. This raises concerns
that systems with a high penetration of wind generation will
exhibit unacceptable frequency response.
Now an inertial response capability for wind turbines,
similar to that available with conventional synchronous generators for large under-frequency grid events, is available.
The response from wind generation is neither inherent nor
based on physics alone; it is a controlled response in the
first ten seconds of a large event. Therefore, this response in
this time frame can also be characterized as fast-frequency
response. Most large mainstream wind equipment manufacturers offer inertial fast-frequency response functionality
today, albeit by means of various different control methodologies and implementation strategies. This offers the industry
the advantage of diversity regarding how this function generally mitigates the reliability risk of frequency excursions
due to the loss of large generation pockets.
The GE WindINERTIA feature demonstrates the effectiveness of fast-frequency inertial control for wind plants.
For large under-frequency events, the feature temporarily increases the power output of the wind turbine in the
range of 5-10% of the rated turbine power. The duration
of the power increase is on the order of several seconds.
Below rated wind, stored kinetic energy from the turbinegenerator rotors is temporarily donated to the grid but is
recovered later. This inertial response is essentially energy
neutral. At higher wind speeds, it is possible to increase
the captured wind power, using pitch control, to temporarily exceed the steady-state rating of the turbine. Under
these conditions, the decline in rotor speed is lessened, and
the energy recovery is minimal. This feature utilizes the
energy stored in the rotor to provide an increase in power
only when needed. Hence, this feature does not adversely
impact annual energy production.
Unlike the inherent inertial response of synchronous
machines, inertial wind turbine generator (WTG) response
depends on active controls. Further, the response is shared
with controlled variations in the active power necessary to
manage the turbine speed and mechanical stresses. These
stress-management controls take priority over inertial
november/december 2015

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