IEEE Power & Energy Magazine - November/December 2015 - 29

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 specifications of this particular system required
regulation of the voltage at a remote point of grid interconnection. To avoid dependence on telecommunications, the
line-drop compensating feature was used to synthesize the
voltage at the point of interconnection, 42 mi from the measurement points at the wind plant substation.
Despite the challenges of a very weak grid and the
requirement of regulation of a remote voltage, performance
of this system has been excellent. Figure 5(a) shows the wind
plant voltage and the voltage at the point of grid interconnection. The wind velocity is also shown, but without a scale.
In Figure 5(b), the same wind velocity is shown, along with
the wind plant power output. Despite larger than 20% variations in generated power, the voltage at the interconnection
bus is quite invariant. The voltage flicker index, Pst, is less
than 0.02 for this high stress condition-well within industry expectations. Most of the voltage variations are within a
few hundred volts on the 230-kV system.
As the density of wind plant projects increases, especially in geographic areas best suited to wind power production, new challenges arise in coordinating multiple wind
power plants operated in voltage-regulation mode. One
such challenge is balancing reactive power contributions
from two or more plants so as to avoid the "fighting" that
tends to occur, with one plant sourcing reactive power and
another nearby plant consuming reactive power-resulting
in little net reactive power contribution to the grid. GE is
developing a new feature known as multiplant coordination to coordinate two or more wind power plants such that
multiple wind plants work together to regulate the voltage
of one upstream bus while simultaneously balancing the
reactive power contributions of each plant and allowing the
grid to better utilize the reactive capability of the collective
wind power plants.

Extended Reactive Capability as
an Ancillary Service
In North America, it is an interconnection requirement to
provide voltage regulation. In addition, North American
Electric Reliability Corporation regulations mandate that
voltage regulators be activated and follow a prescribed voltage schedule. Historically, the reactive capability requirement was placed on individual generators (e.g., +0.90/−0.95
at the machine terminals). With wind plants, these types of
requirements have morphed into minimum reactive power
ranges specified at the point of interconnection (e.g., ±0.95
november/december 2015

power factor). The intent is to achieve technology neutrality
and to allow a combination of reactive power from turbinegenerators and other equipment within a plant to meet the
requirement.
The control system that produced the performance demonstrated in Figure 5 uses shunt reactive devices to augment
the reactive capability of the turbines. This raises an interesting ancillary service opportunity: wind plants with these
types of controls could be designed to offer reactive capability ranges significantly greater than the minimum required
by the grid code. The locationally dependent value of this
extra capability could be incentivized with an appropriate
ancillary services agreement. This concept extends to the
technology introduced in the next section.

Reactive Power Control Without Wind
The latest advancement in WTG technology provides control of reactive power output even when the wind turbine
is stopped. Currently, all MW-class wind turbines stop in
response to sustained wind speeds below a minimum threshold or when wind exceeds a high speed cut-out. They may
also be disconnected from the grid in response to severe system disturbances. Under such conditions, both real power
to serve load and reactive power to support system voltage
are lost.
WTGs that are equipped with this reactive power control
provide smooth, fast voltage regulation by delivering controlled reactive power even when the wind turbines are not
generating active power. Such a function cannot normally be
provided by conventional (e.g., thermal or hydro) generation,
because production of reactive power from these generators
requires that the generator (and, therefore, the turbine) continue to spin at synchronous speed.
From a systemic perspective, the reactive power capability is similar to that provided by various dynamic reactive
devices (e.g., synchronous condensers, static VAR condensers, STATCOMs) that are used for grid reinforcement where
dynamic voltage support is required.
GE's WTGs use large power converters. This decouples
the generator speed from the power system frequency and
allows for a wide operating speed range. The power converters rely on two major components: the generator-side
converter and the line-side converter, which connects to the
grid. It is important to recognize that the line-side inverter
is self-commutating. This provides it the ability to independently deliver active and reactive power. When there is no
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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - November/December 2015