IEEE Power & Energy Magazine - March/April 2017 - 68

Wind and solar power are different from most thermal generators
because they have variable and uncertain power output that is
determined by local weather conditions.
known as the system inertia and consists of summed generator masses. Induction motors/generators do not have coupling as tight, and therefore they are not usually considered
in the system inertia. The inertia of a power system is also
a measure of how well the system can "ride through" disturbances and still maintain stable frequency and voltage.
Future PV and wind systems will need to help maintain the
grid stability at high penetration levels.
Power system stability can be divided into three major
categories: rotor angle stability, frequency stability, and
voltage stability. Each of these categories can be further
divided into two subcategories: small-signal stability and
transient stability.
✔✔ Rotor angle stability is the ability of synchronous generators in an interconnected power system to remain
in synchronism after disturbances.
✔✔ Frequency stability is the ability of a power system
to maintain steady frequency during normal operation
and restore frequency to its scheduled level during
system contingencies when large imbalances between
load and generation may be present.
✔✔ Voltage stability is the ability of a power system to
maintain voltages within safe limits at all buses after
disturbances to prevent outages and blackouts.

Generators and loads are providing various types of ancillary services to ensure the stable operation of a power system
during steady-state and transient conditions. Ancillary services can be either cost or market based. Independent system
operators set the required amount of ancillary services
based on the least-cost option. In the current power system,
ancillary services are mainly provided by conventional generators and loads. If controlled properly, VRE is capable of
providing the full set of ancillary services, and it can successfully contribute to maintaining stable and reliable operation of the power system.
Many conditions of the power system, including generation
and load levels as well as transmission availability, are both variable and hard to predict; therefore, additional capacity called
operating reserves is made available during the dispatch process
to ensure the system's frequency stability during sudden imbalances. The variable nature of VRE increases the importance of
the reserves due to the uncertain nature of these renewable resources. The classification of the operating reserve categories
is shown in Figure 7. Both the normal and event-responsive reserve categories can be subdivided by their required response
speeds. For instance, instantaneous events need a generatorlevel response to arrest the rate of change of frequency (inertial response) and stabilize the frequency at some steady-state

Operating
Reserve

Eve

ent
nev

Regulating
Reserve

Following
Reserve

Ramping
Reserve

Automatic Within
Optimal Dispatch

Manual Part of
Optimal Dispatch

Instantaneous

Noninstantaneous

Correct the
Current ACE

Correct the
Anticipated
ACE

Primary

Stabilize
Frequency

Secondary

Tertiary

Return
Frequency
to Nominal

Replace
Primary and
Secondary

Secondary

Return
Frequency
to Nominal

Tertiary

Replace
Secondary

figure 7. Operating reserve categorization.
68

ieee power & energy magazine

march/april 2017

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - March/April 2017