Grid inertia is a growing risk on power grids where thermal
power plants are shrinking and inverter-based resources (IBRs) are growing.
Aaron Larson of Power Magazine explains grid inertia as follows:
“Concerning power grid operation, inertia refers to the
energy stored in the rotating masses of synchronous generators, typically found
in conventional power plants such as coal, gas, nuclear, and hydropower
facilities. This stored kinetic energy provides an automatic and instantaneous
response to fast frequency changes in the grid. For example, when a sudden
imbalance occurs between power generation and load demand, such as when a large
generator trips offline or a factory suddenly engages massive equipment causing
an increase in demand, the rotating masses of generators naturally resist
changes in rotational speed. This is due to a release of energy as the rotating
mass speed is reduced, or conversely, energy is absorbed by the rotating mass
through a speed increase. This process slows down the rate of frequency change,
providing crucial time for control systems to respond, which helps maintain
grid stability during transient events.”
Synchronous Generators
According to
Electricity-Magnetism, how synchronous generators, also known as alternators,
work is explained as follows:
“Synchronous generators work on the principle of
electromagnetic induction, a phenomenon discovered by Michael Faraday in 1831.
This principle states that an electromotive force (EMF) is induced in a
conductor when it is subjected to a changing magnetic field. Synchronous
generators exploit this principle by rotating a conductor (the rotor) within a
static magnetic field (the stator).”
Synchronous generators
essentially convert mechanical energy into electrical energy.
For synchronous generators,
the frequency is independent of the load. It is instead determined by the speed
of the turbine or other spinning device. Synchronous generators can be
over-excited or under-excited, depending on loads, and they can adjust to those
imbalances according to their independent frequency control. For those
generators, automatic voltage regulators (AVRs) and governor control systems
are commonly used to maintain the voltage and frequency of the power.
Inertia limits the rate of
change of frequency (ROCOF) during disturbances. It can prevent relay switches
from tripping and allow frequency response mechanisms, such as governor
controls, time to adjust. Synchronous machines also protect and stabilize the
power system with an immediate high-current response, which activates
protective relays and circuit breakers, quickly isolating the fault.
Synchronous generators are designed to handle the electromagnetic forces and the
heat that can come with short circuits. Larson elaborates on the functional
features of synchronous machines:
“Their damper windings and field excitation systems help
maintain stability during transient conditions, while the generator’s physical
inertia prevents rapid frequency collapse. This combination of features enables
the power system to ride through faults without widespread instability or
equipment damage, making short circuit power a fundamental design parameter for
these machines.”
He also notes the increasing
power vulnerabilities that occur due to retiring thermal generation, which also
means retiring grid inertia. Wind, solar, and batteries are IBRs and have no
inertia since there is no spinning involved.
“As conventional generators are retired, system inertia,
short circuit response, and reactive power capabilities are all adversely
affected, which makes power grids more vulnerable to frequency excursions and
instability.”
Synchronous Condensers
Synchronous condensers are a
very good way to ensure that grid inertia, also known as spinning reserve, is
retained. According to Siemens:
“They’re essentially synchronous motor-generators that
operate without being connected to a prime mover, such as a turbine, or a
mechanical load.”
These do not generate power
but regulate power. They are essentially a motor that stores kinetic energy in
the rotating shaft, which can give or take power to or from the grid. This
allows for frequency and voltage adjustments.
Synchronous condensers can be
deployed in various ways. They can be planned, designed, and incorporated into
new builds, or they can be retrofits. A retired thermal generator can be
converted to a synchronous condenser. In this way, they are repurposing an
existing asset, which can be cost-effective.
“Siemens Energy offers rotating grid stabilizer [RGS]
conversion solutions leveraging our engineering expertise and service
capabilities,” said Dr. Norbert Henkel, Siemens Energy’s global head of sales
for Steam Plant Modernization and Transformation. “The RGS conversion comprises
complete solutions for converting existing power plant equipment into
synchronous condensers, from engineering to installation and commissioning.”
The addition of flywheels
provides an additional rotating mass to optimize inertia. These flywheels are
operated in a partial vacuum and equipped with a cooling system.
“We also offer a hybrid conversion package,” said
Henkel. “This option provides maximum flexibility. It basically couples a gas
or steam turbine with an additional SSS [synchro-self-shifting] clutch between
the generator and the turbine. The option also exists to add additional inertia
with a flywheel coupled to the existing shaft line. Our engineers can help
evaluate the best design for any given situation.”
Synchronous condensers have
proven their value in providing grid stability, especially where the share of
energy produced by IBRs is high.
The synchronous condenser at
the Moneypoint project in Ireland features “the world’s largest flywheel
(130 tons) paired with a 66-ton rotor spinning at 3,000 rpm, providing 4,000
megawatt-seconds (MW-sec) of inertia.”
There, they plan to
incorporate growing amounts of offshore wind power, which the projects will
accommodate.
Another solution, at a
combined cycle gas turbine plant in solar-heavy Australia, decided to add a
clutch to the shaft between the gas turbine and the synchronous generator, as
shown below. The clutch allows the plant to shift instantaneously from power
generation mode to synchronous condenser, or grid stabilization mode. This is
the largest project of its kind in the world.
“The electrical inertia while operating in the grid
stabilization mode is calculated to be about 250 MW-sec and approximately 1,000
MW-sec while operating in power generation mode.”
This solution is about half
the cost of building a new synchronous condenser and can be built in about half
the time, 18 months vs. 36 months.
Siemens is also providing
synchronous generators to the Intermountain Power Project’s Advanced Clean Energy Storage Delta Project in
Utah, which will make and store hydrogen and burn it along with natural gas and
renewable energy generation. The addition of three synchronous condensers will
allow the project to provide power at 100% capacity to Southern California via
the HVDC line from Delta, Utah, to Adelanto, California.
At the Killingholme Power
Station in the UK, a steam turbine was removed and reused as a grid stabilizer.
A flywheel was in place of the turbine to maximize inertia, providing
much-needed services to the area’s power grid. Siemens supplied the engineering
solution, which
“…included the fully redundant Omnivise T3000 control
system, including vibration monitoring for the synchronous condenser and the
system’s implementation into the existing control room.”
Larson recaps the benefits of
synchronous condenser solutions to providing grid inertia for grid stability:
“As power systems worldwide navigate the complex
transition toward renewable energy dominance, synchronous condensers are a
crucial bridge technology that marries traditional grid stability principles
with modern decarbonization goals.”
References:
How
Decreasing Inertia Is Affecting Power Grids and What to Do About It. Aaron
Larson. Power Magazine. June 11, 2025. How
Decreasing Inertia Is Affecting Power Grids and What to Do About It
Synchronous
Generators. Matan. October 26, 2023. Electricity-Magnetism. Synchronous
Generators – Electricity – Magnetism
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