IEEE Power & Energy Magazine - March/April 2017 - 70

Power system stability can be divided into three
major categories: rotor angle stability, frequency
stability, and voltage stability.
voltage and frequency; however, some wind turbines use the
DFIG configuration with only a partial conversion of power
(up to 30%). But even for the DFIG topology, the grid frequency is fully decoupled form turbine rotational speed.
Wind, solar PV, and battery storage systems can provide
active and reactive power control in a similar fashion to conventional synchronous generators. The presence of inverters
allows for the control of active and reactive power independent from each other. With proper controller design, they can
also provide synthetic inertia-like response. Wind turbines
are capable of injecting additional active power into the grid
by extracting the energy stored in the rotating mass of blades
and generators. PV inverters can also provide inertia-like
response if curtailment is utilized. Energy storage can also
be programmed to modulate its active power to mimic the
inertial response of rotating machines.
In addition, inverter-based generators have superior fault
ride-through performance. With the proper converter design,
wind, PV, and storage inverters can ride through various
types of balanced and unbalanced under- and overvoltage
faults and frequency excursions, thus improving the overall
reliability of a power system. If desired, they can also inject
desired levels of reactive current during the fault to assist in
faster postfault voltage recovery.
Removing a significant number of synchronous generators from the system has several effects on power system
stability. The loss of synchronous generators will reduce
system inertia and effect transient and small-signal stability.
✔✔ Transient and small-signal stability: The loss of
system inertia could reduce the ability to respond to
disturbances. To enhance the responsiveness to faults,
VRE interfaces need ride-though capabilities. In a
100% VRE system, the angle stability of the remaining machinery, such as synchronous motors and
synchronous condensers, can be frequent and severe
because of the lack of inertia in the system.
✔✔ Frequency regulation: The electrical frequency of
an interconnection must be maintained very close to
its nominal level at all times. Significant frequency
deviations can lead to load shedding, instability, machine damage, and even blackouts. There is rising
concern in the power industry in recent years about
the declining amount of inertia and primary frequency response in many interconnections. This decline
may continue due to increasing penetrations of inverter-coupled generation and the planned retirements of
conventional thermal plants. VRE controllers, if care70

ieee power & energy magazine

fully designed, can provide primary, secondary, and
tertiary response that is superior to the response from
conventional generators because of the fast-response
speed from the power electronics interfaces.
✔✔ Volt/volt ampere reactive (VAR) regulation: Maintaining acceptable voltages at all buses in a power
system is essential to ensure that power is reliably
delivered across the transmission network. Voltage
regulation from conventional generators' excitation
systems keeps their terminal voltage stable. VRE
can provide voltage regulation using voltage controllers; however, it is likely to reduce their ability to
provide real power while providing voltage services.
Volt/VAR control and the optimization of VRE will
provide necessary voltage support and minimize the
impact on the ability of renewable generators to produce real power.
The inertial response is the immediate response to a
power disturbance based on a frequency change. This is a
key determining factor of both transient stability and smallsignal stability by slowing down the rate of change of frequency immediately following a disturbance. Synchronous
machines inherently provide the inertial response to power
systems. If correctly designed, active power controllers for
VRE can provide a synthetic inertia response to stabilize
frequency excursions.
Inertial control utilizes the kinetic energy of the rotating mass of wind turbine generators to provide an inertial
response capability for wind turbines, thus emulating the
inertial response from conventional synchronous generators. The response is provided by temporarily increasing
the power output of the wind turbines in the range from 5
to 10% of the rated turbine power by extracting the kinetic
energy stored in the rotating masses. This short, quick power
injection can benefit the grid by essentially limiting the rate
of change of frequency at the inception of the load/generation imbalance event.
The impact on system frequency with wind-based synthetic inertia control is shown in Figure 8. Here, the controls act quickly to minimize the drop in system frequency.
Although the severity of the frequency nadir is mitigated
with the utilization of synthetic inertia, the frequency takes
longer to stabilize.
Primary frequency response control from wind turbine
generators can be tuned to provide droop-like response
and significantly improve the frequency nadir as well as the
steady-state frequency. Figure 9 gives an example of the
march/april 2017



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