IEEE Power & Energy Magazine - November/December 2015 - 27

Inertial Response as an Ancillary Service
Inertial response has not traditionally been considered an
ancillary service, but rather a natural and uncontrollable
characteristic of the power system. Consequently, introduction of technology that makes inertial response controllable
and that requires specific changes to generation is a substantial change. Ultimately, grid codes may be (and, in a few
cases, have already been) modified to include some type of
inertial response requirement. This demonstration shows
that such functionality is, indeed, possible. However, it also
shows that inertial response identical to that of synchronous generation is neither possible nor necessary. Inertial
response of wind generation is limited to large under-frequency events that represent reliability and continuity-ofservice risks to the grid. The crafting of new grid codes and,
ultimately, consideration of inertial response as an ancillary
service should, therefore, proceed cautiously and focus on
functional, systemic needs.
In the case of wind plants using GE technology, the
WindCONTROL active power control, if enabled, will also
november/december 2015

respond to significant under-frequency events as a speed governor would on a conventional unit. The command for this
response emanates from the wind plant level and is delivered
to each individual WTG. In order to increase active power,
the plant must be partially curtailed so that additional power
can be extracted from the available wind. This incremental
power order signal will add to that from WindINERTIA,
which is local to the individual WTG. The total response of
the WTG to these two signals is coordinated to respect the
physical capabilities of the WTG. Other wind manufacturers
provide similar functionality.

Wind Plant Control of Reactive Power
For many wind plants, especially large remote projects,
traditional approaches to managing reactive power are no
longer acceptable. A large wind plant may consist of a hundred or more individual wind turbines, separated by tens or
even hundreds of kilometers of electrical collector system.
However, the power system needs are dictated at the point
of interconnection with the host grid. Wind plant controllers by some manufacturers achieve improved voltage/voltampere reactive (VAR) control by using the inherent VAR
capabilities integrated into each wind turbine and precisely
controlling the turbine's VAR output to maintain voltage at
the point of interconnection. This coordinated control system senses ac system conditions and instructs the individual
turbines within a plant to adjust their local control objectives
to meet system needs. The control system provides tight
closed-loop regulation of utility system voltages. This hierarchical control minimizes voltage flicker, improves system
stability, reduces the risk of voltage collapse, and minimizes
the impact of system disruptions.
This provides two major benefits. First, the impact of
active power fluctuations from wind variation on the grid
voltages is minimized. Second, the fast and precise voltage control effectively strengthens the grid, improving
the overall power system's resilience to large disruptions.
While some manufacturers use wind turbines' dynamic
capability alone to meet voltage regulation requirements,

1,800
1,500
Power (kW)

control. Turbulence may mask the response for individual
turbines at any instant in time, but overall plant response
will be additive.
Overall, the control is designed to provide functional
response similar to that of a synchronous machine. However,
unlike the inherent response of a synchronous machine, the
response is not exactly the same under all operating conditions. The design has sufficient margin over the turbine
operating range to meet the equivalent energy (kW-s) contribution of a synchronous machine with 3.5-s p.u. inertia for
the initial seconds.
Field test results of the control for various wind speeds
on a single wind turbine are shown in Figure 4. The field
data was generated by repeated application of a frequency
test signal to the control. The results, at various wind
speeds, were then averaged and plotted. Below rated wind
speed (<14 m/s), the results clearly demonstrate the inertial
response and recovery. Above rated wind speed, the inertial
response is sustained by extracting additional power from
the available wind.
The control has a number of differences from the inherent
inertial response of a synchronous machine. First, and most
important, the control is asymmetric: it only responds to low
frequencies. High frequency controls are handled separately
by a different controller, as discussed previously. Second, the
deadband ensures that the controller only responds to large
events-those for which inertial response is important to
maintain grid stability and for which seriously disruptive consequences, like under-frequency load shedding, may result.
Finally, a controlled inertial response means the speed is a
function of the control parameters. The standard settings are
tuned to provide good coordination not only with the inertial
response of other generation on the system but also with the
governor response of conventional generation as well.

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900
600
8 m/s Avg Meas
14 m/s Avg Meas

300
0

10 m/s Avg Meas

40 50
Time (s)

figure 4. A field demonstration of the GE WindINERTIA
response.
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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - November/December 2015