People in the power industry understand inertia and its importance to grid stability. As large thermal power plants and other inertia-providing units are replaced with renewable resources that provide no inertia, grid stability is at risk. Cost-effective solutions are available today, however, to maintain and even enhance grid operations.
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.
Historically speaking, inertia has been essential for grid stability for several reasons. Because inertia limits the rate of change of frequency (ROCOF) during disturbances, it can prevent protective relays from tripping unnecessarily. Additionally, the valuable seconds of stability it provides allows primary frequency response mechanisms, such as governor controls, to adjust.
Meanwhile, the short circuit power of synchronous machines, such as a turbine generator in a thermal power plant, has also been essential for power system protection and stability. When a short circuit occurs, the generator’s immediate high-current response activates protective relays and circuit breakers, allowing for quick isolation of the fault.
Synchronous generators are specifically designed with robust mechanical structures to withstand the electromagnetic forces generated during short circuits, along with thermal capability to handle the heat produced by fault currents. 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.
The transition to renewable energy creates challenges for grid inertia. Wind turbines and solar panels are typically connected through power electronic converters, providing no natural inertial response. 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.
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Full issueSynchronous Condensers: A Grid-Enhancing Solution
There are several ways to address declining inertia in power grid operations. One is with synchronous condensers. “Synchronous condensers are a sophisticated solution to the inertia problem in modern power grids,” said Ruediger Jansen, head of Siemens Energy’s Sales for Europe and Africa of FACTS (flexible alternating-current transmission systems) product lines. “They’re essentially synchronous motor-generators that operate without being connected to a prime mover, such as a turbine, or a mechanical load.”
As Jansen suggested, a synchronous condenser consists of a synchronous machine connected directly to the grid. It includes a large rotating mass, sometimes with additional flywheels (Figure 1), but it has no mechanical power generation. The inertial response is due to the conversion of stored kinetic energy in the rotating shaft, which injects or absorbs electrical power to or from the grid, effectively operating as a motor in the system.
When connected to the grid, the synchronous condenser spins at synchronous speed, matching grid frequency. Reactive power exchange can be controlled under steady state conditions. By over-exciting the machine, a synchronous condenser can provide reactive power, and by under-exciting, it can absorb reactive power. With this, the voltage level of the grid connection point can be controlled to a stable voltage level. Meanwhile, it provides inertia through its rotating mass and can absorb or deliver real power briefly during disturbances.
Synchronous condensers offer several advantages. They provide immediate inertial response during frequency disturbances, just like conventional generators. They can also supply or absorb reactive power, helping with voltage regulation (Figure 2). They contribute to system strength by increasing short-circuit current capability, improving the grid’s ability to ride through faults. And, perhaps most importantly, they work seamlessly with existing grid infrastructure and protection systems.
Synchronous condensers can be deployed in several ways. One is as a new installation. Purpose-built units installed at strategic grid locations can be planned appropriately to provide a great grid stabilization option. New build is attractive for long-term investment and can be optimized to meet customer needs.
Converting a retired thermal generator into a synchronous condenser by removing the turbine is also an option. Conversion means repurposing already-existing assets, which makes good use of previous investments and generally allows grid connection on short notice.
“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.”
Repurposing an existing asset can be very cost-effective. “By utilizing power plants that may otherwise become stranded assets, RGS conversions provide necessary system inertia, short circuit power, and reactive power to the grid for that balance,” Henkel explained. “It can be a great option in many cases.”
The basic conversion provides a cost-optimized solution that converts existing turbogenerators into synchronous condensers quickly. As mentioned previously, generators can also be extended with additional rotating mass from a flywheel to provide maximum inertia. Flywheels (Figure 3) are operated in a partial vacuum and equipped with a cooling system to minimize friction losses and reduce cooling efforts. Siemens Energy says its extensive rotor dynamics knowledge, and decades of experience with countless other engineering aspects, allows the company to guide and confirm customer benefits with detailed technical support.
“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.”
Successful Grid-Stabilizing Projects
Synchronous condensers have proven to be critical grid-strengthening assets across the global energy landscape, providing essential stability services as power systems transition toward renewable-dominated generation. As grids worldwide face the dual challenges of retiring conventional generation and accommodating intermittent resources, the examples that follow demonstrate how synchoronous condensers can be utilized to solve modern power system challenges without compromising reliability.
Moneypoint Power Station, Ireland. The Moneypoint Power Station has been a significant fixture in Ireland’s energy landscape for decades. Located near Kilrush on the Shannon Estuary, it was developed as a major component of Ireland’s strategy to diversify its energy resources following the oil crises of the 1970s. Construction of the station began in the late 1970s and was completed in 1987. The facility was designed with a deepwater port capable of receiving large coal shipments, making it strategically important to Ireland’s energy security. With an output capacity of 915 MW, Moneypoint became Ireland’s largest electricity generation station and its only coal-fired power plant.
The station has been operated by the Electricity Supply Board (ESB), Ireland’s state-owned electricity company. For more than three decades, Moneypoint served as a cornerstone of Ireland’s electricity system, typically generating about 25% of the country’s electricity needs.
In recent years, as Ireland committed to decarbonization targets and renewable energy goals, Moneypoint’s role began to shift. Coal-fired generation has been gradually reduced, and the ESB announced plans to cease coal burning at the site by the end of 2025 as part of Ireland’s climate action commitments. The “Green Atlantic @ Moneypoint” project represents the station’s transformation from coal-powered generation to a renewable energy hub, with a synchronous condenser installation marking the beginning of this new chapter. This repurposing leverages the site’s existing infrastructure, including its electrical connections to the grid and deepwater port facilities, to support Ireland’s renewable energy future, while balancing the energy trilemma of security, affordability, and sustainability.
As renewable energy sources, most notably wind, increase in Ireland, grid stability becomes more challenging. The synchronous condenser at Moneypoint (Figure 4) helps alleviate some of those challenges. It includes 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.
The ESB said the synchronous condenser represented an investment of €50 million. It has enabled the site’s transformation by stabilizing power grid frequency variations that could otherwise cause blackouts, essentially providing a low-carbon alternative to the grid-stabilizing functions of the old coal-fired plant. As part of the ESB’s vision for the Moneypoint site, it expects to tie in a 1,400-MW offshore wind farm, delivered in two phases; a wind turbine construction and service hub; and the development of a green hydrogen production, storage, and generation facility. The Moneypoint installation is likely just the first of as many as six synchronous condensers needed for Ireland to reach its climate goals.
Townsville Power Station, Australia. RATCH-Australia Corp.’s Townsville Power Station (TPS) is a 242-MW combined cycle power plant that utilizes a single Siemens Energy SGT5-2000E gas turbine and an associated steam turbine (Figure 5). The plant is located about 20 kilometers north of Townsville, Queensland, Australia, and was commissioned in 1999. It supplies electricity to the National Electricity Market (NEM). The output from TPS’s gas turbine is connected to the 132-kV Powerlink electrical network, and its steam turbine output is connected to the Ergon 66-kV network.
The station has undergone several upgrades and refurbishments over the years to improve efficiency and reduce emissions. Most recently, however, the owners decided to convert the gas turbine and generator at the site to a Hybrid Rotating Grid Stabilizer (RGS). This was spurred by concerns Powerlink had around grid stability as the share of renewables in Australia’s energy mix has risen.
The Australian Energy Market Operator (AEMO) requires grid operators, including Powerlink, to provide a minimum fault level and procure system strength. After reviewing eight different possible solutions, Powerlink concluded the addition of a clutch to the shaft between the gas turbine and the synchronous generator at TPS was the least-cost option to address the shortfall. Thus, it entered into an agreement with TPS for the provision of system strength services.
To carry out the project, Siemens Energy will provide integrated service solutions to convert the gas turbine to a Hybrid RGS during a scheduled major outage this year. Replacing the intermediate shaft of the gas turbine with an SSS clutch will provide an instantaneous switch from power generation to synchronous condenser mode. When in synchronous condenser mode, the Hybrid RGS unit can provide rotating inertia and short-circuit power without the need to produce power. It has been calculated that this will provide a short-circuit contribution of approximately 350 MVA to 400 MVA. 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 is Siemens Energy’s first project in Australia to convert an operating gas turbine unit to a Hybrid RGS. The project also marks the first Hybrid RGS conversion project on this size of gas turbine worldwide. The benefits to RATCH-Australia include a new revenue stream from providing grid services, and the capability to switch flexibly between power generation and grid stabilization modes. Moreover, the grid stabilization capability comes at up to 50% less cost than a new synchronous condenser, and it can be completed in 18 months rather than three years. The RGS can also be serviced in the same maintenance cycle as the gas turbine.
Intermountain Power Project, U.S. The Intermountain Power Project (IPP) is located in the Great Basin region of western Utah. It has generated an average of more than 13 TWh of energy each year from its two coal-fired units.
Operating continuously since 1986, the IPP is a story of unprecedented cooperation. The energy it produces is delivered over the project’s alternating-current and high-voltage direct-current (HVDC) transmission systems to 35 participants that have operations across parts of six states, although most of the energy is supplied to customers in Utah and California. The participants include 29 cities and towns that operate their own electric utilities. Those municipalities range in population from 254 in rural Utah to 3.9 million in Southern California.
As these entities’ current power purchase agreements near expiration, Intermountain Power Agency (IPA)—the owner of the plant—has expanded its role as a regional energy hub, including utilizing renewable energy resources to produce and store hydrogen that can be drawn upon to generate carbon-free electricity. As part of a project IPA calls “IPP Renewed,” new natural gas–fueled electricity generating units have been installed capable of utilizing hydrogen for 840 MW of net generation output. The project utilizes much of IPP’s existing infrastructure but also includes modernization of IPP’s Southern Transmission System.
Simultaneously, underground storage caverns in a unique salt dome located deep beneath the power station are being developed to store hydrogen, which will be used to also fuel the power station. IPP will use this clean hydrogen—made from renewable energy–powered electrolysis—as dispatchable fuel. The new power station turbines will be designed to utilize 30% hydrogen fuel at startup, transitioning to 100% hydrogen by 2045.
The project will also highlight the crucial role of Siemens Energy’s technology in integrating renewable energy resources and enhancing grid stability. Among the items supplied by the company are generator step-up transformers and synchronous condensers. The transformers are equipped with the Siemens Energy Sensformer solution, which uses proprietary models to assess data from physical and virtual sensors, allowing optimal efficiency for maintenance and operations. Three SGen-2000P series synchronous condensers, meanwhile, will greatly increase the amount of energy that can be delivered from IPP to project participants in Southern California. In fact, the HVDC line from Delta, Utah, to Adelanto, California, could not operate at 100% capacity without these condensers. Notably, this is the largest condenser project at one location in the U.S. and possibly the world.
Killingholme Power Station, UK. Commissioned in 1992, Killingholme has a rich history of innovation and is home to one of the biggest gas-fired power plants in the UK. As one of the leading power producers in the UK, Uniper has always delivered grid stabilizing services. When the country’s grid system operator, National Grid ESO, launched the Stability Pathfinder project, Killingholme was identified as a prime site for Uniper to provide dedicated grid stability services. For this, Uniper sought to repurpose steam turbine generators decommissioned after the power station was converted from a combined-cycle plant to an open-cycle plant in 2017.
Converting an existing turbine generator and retrofitting a flywheel was a major undertaking for the station (Figure 6), so a team of engineers from Uniper and Siemens Energy collaborated on the project. Teamwork was essential in developing a bespoke solution for the Killingholme facility.
The team removed the steam turbine and reused the generator as a grid stabilizer. Then, they added a flywheel in place of the turbine to provide maximum inertia. This conversion provides much needed services to the area’s power grid.
Siemens Energy was responsible for the supply, installation, and commissioning of major equipment including flywheels, auxiliary systems, electrical startup systems, excitation systems, and protection. The scope of supply also 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. Siemens Energy also provided civil engineering, pipework, and electrical system integration.
The partnership between Uniper and Siemens Energy yielded numerous benefits. The group was able to make use of retired generators, while seamlessly providing essential grid stabilizing services to National Grid ESO. This proactive approach supports the UK grid’s stability and enables the integration of renewable energy sources.
Effective Solutions for the Future
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. The examples featured above demonstrate that these seemingly simple rotating machines offer sophisticated solutions to some of the most pressing challenges facing modern grids.
By providing essential system services—inertia, short-circuit strength, and reactive power support—synchronous condensers enable higher renewable penetration while enhancing reliability. Their ability to retrofit existing infrastructure at conventional power plants offers both economic and environmental advantages, transforming potential stranded assets into valuable grid resources.
As the world looks toward a future with ever-increasing levels of inverter-based resources, the humble synchronous condenser—a technology with century-old roots—may well prove to be one of the most important enablers of a clean energy future. For grid operators and policymakers alike, these installations represent not merely a technical solution but a strategic investment in creating resilient, stable, and ultimately sustainable power systems capable of supporting the renewable revolution.
—Aaron Larson is POWER’s executive editor.
