IEEE Power & Energy Magazine - November/December 2015 - 80

Wind Generation's Evolving Dynamic Response Capability
The evolving dynamic response capability of wind offers an

power oscillations as generators swing against each other.

example of technical, regulatory, and reliability progress for

Studies show that wind turbine "synthetic inertia" provides

both wind turbines and the electric power industry. Lack of

benefits similar to those of synchronous inertia to help the

dynamic grid support from any generation technology in the

power system ride through a disturbance while also damp-

event of a power system disturbance is problematic both

ening undesirable oscillations. Figure S1 shows the superior

because the generator is not helping to stabilize the power

stability response of doubly fed asynchronous wind tur-

system and because it may be displacing generation that

bines compared to conventional synchronous generators

could. Both the power industry's understanding and wind

following a grid disturbance.

turbine capabilities have evolved so that reliability is now
increased with the addition of wind.
age ride-through requirements, and voltage support, as
previously discussed. More recently, the ability of wind turbines to actively control the energy injected into the power
system has been exploited. With electronics coupling the

Voltage (%)

These reliability improvements include frequency, volt-

generator to the power system, modern wind turbines are
able to control their output much faster and more accurately
control is possible in cycles (milliseconds, which is the inertial time frame) rather than in seconds (the response time
of conventional generator governors). The rotating mass of
the wind turbine blades themselves coupled to the shortterm overload capability of the power electronics provides
an additional source of completely controllable stabilizing
energy. Unlike conventional synchronous generators, which
provide uncontrolled inertia response, the response from
wind turbines is completely controllable. Power system
transient stability response to a major disturbance involves

that can provide many ancillary services that were not envisioned several years ago. When properly incentivized, VG
can contribute to system balance and control.

VG Can Contribute to
System Balancing Needs
Two of the primary balancing functions are automatic generation
control (AGC) or AGC (regulation) and economic dispatch.

System Balance
The operational planning cycles in bulk system operations, whether under wholesale energy market structures
or in vertically integrated utility settings, have the objective to position the system to meet the net load and to provide the range of services needed to guarantee operational
security on multiple timescales. As shown in Figure 1,
this involves scheduling generators the day before via a
process called unit commitment based on the forecasted
net load. In real time, available generating capacity is dispatched to meet the continually changing net load. Load
80

ieee power & energy magazine

Voltage Revovery of
Wind Farm Is Superior
Ten-Cycle Grid Fault
Gas Turbine
Wind Farm

250
Power (MW)

than conventional synchronous generators. Wind turbine

1.2
1.0
0.8
0.6
0.4
0.2
0.0
200
150
100

Synchronous Generator
Swings Dramatically

50
0
0

5
Time (s)

figure S1. Wind power plants with doubly fed
asynchronous generators are more stable than those
that have conventional synchronous generators.

following capacity is adjusted manually, on a frequent
economic dispatch cycle, or through the operation of subhourly energy markets to track changes in net load on a
timescale between 5 min and 1 h. Computer-controlled
"regulation" from AGC redispatches capacity on a secondto-minute basis to support interconnection frequency control (60 Hz in North America).
Traditionally, demand for electricity tends to peak in the
afternoon or evening and is low in the middle of the night.
System operators use intermediate units, such as combinedcycle gas units, to follow the load, whereas less-expensive
base-load units run at high output 24 h per day.
Large amounts of VG change the way the system is operated. The variations in net load and short-term uncertainty
are increased, thereby increasing the amounts of required
regulating capability and increasing load following or ramping to meet the increased variability of the net load. In some
regions, wind power output can be higher at night when
demand is low, which could make system operation more
difficult. When the base-load units are generating at their
november/december 2015

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