Synchronous Condenser

A synchronous condenser (also called a synchronous capacitor or synchronous compensator) is a DC-excited synchronous machine (large rotating generators) whose shaft is not attached to any driving equipment. This device provides improved voltage regulation and stability by continuously generating / absorbing adjustable reactive power as well as improved short-circuit strength and frequency stability by providing synchronous inertia. Its purpose is not to convert electric to mechanical power or vice versa, but to make use of the machine’s reactive power control capabilities and the synchronous inertia. It constitutes an interesting alternative solution to capacitor banks in the power system due to the ability to continuously adjust the reactive power amount. Synchronous condensers are perfectly suited to control-ling the voltage on long transmission lines or in networks with a high penetration of power electronic de-vices as well as in networks where there is a high risk of ‘islanding’ from the main network.

Technology Types

A synchronous condenser is a conventional solution that has been used for decades for regulating reactive power before there were any power electronics compensation systems.

A conventional synchronous condenser is an AC synchronous motor that is not attached to any driven equipment. The device can provide continuous reactive power control when used with the suitable automatic exciter. An increase of the device’s field excitation results in providing magnetizing power (kVAr) to the system. Synchronous condensers have traditionally been used at both distribution and transmission voltage levels to improve stability and to maintain voltages within desired limits under changing load conditions and contingency situations. To do so, they use a small amount of active power from the power system to supply losses.

The development of high-temperature superconductivity enabled the development of a second type: HTS based machines which are smaller, lighter, more efficient and less expensive to manufacture and operate than conventional machines. Advancements in the HTS wire technology have resulted in superconducting electromagnets that can operate at higher temperatures than those made of low-temperature superconductor materials. Consequently, they utilise simpler, less costly and more efficient cooling systems. This makes HTS wires technically feasible and economically viable for condenser applications at power ratings lower than this can be achieved with the low-temperature superconductor wire.

Components & enablers

Typical components of a (complete solution) synchronous condenser are:

  • Stator and rotor with solid integral pole tips
  • Cooling system (hydrogen, air or water)
  • Excitation system
  • Lubrication oil supply
  • Step-up transformer and auxiliary transformer.

High-inertia synchronous condensers are similar to traditional synchronous condensers but with additional inertia. A dedicated flywheel for better system inertia matching could complement a synchronous condenser if this is required by the inertia requirements of the TSO.

Advantages & field of application

A synchronous condenser is a long standing well-known technology that provides the following advantages:

  • System inertia: Inertia is an inherent feature of a synchronous condenser as it is a rotating machine. The benefit of inertia is improved voltage ‘stiffness’, which improves the overall behaviour of the system.
  • Increased short-term overload capability: Depending on the type, a synchronous condenser can provide more than two times its rating up to a few seconds, which enhances system support during emergency situations or contingencies.
  • Low-voltage ride through: Even under extreme low voltage contingencies, it remains connected and provides smooth, reliable operation.
  • Fast response: By using modern excitation and control systems, a synchronous condenser is fast enough to meet dynamic response requirements.
  • Additional short-circuit strength: Another feature of a synchronous condenser is that it provides real short short-circuit strength to the grid, which improves system stability with weak interconnections and enhances system protection.
  • No harmonics: A synchronous condenser is not a source of harmonics and can even absorb harmonic currents. This feature enables ease of integration into existing networks. Typical applications of a synchronous condenser include: HVDC (provides short-circuit strength and dynamic reactive power support); Wind / Solar (increases short-circuit ratio); Grid Support (improves weak AC grid performance, voltage support during faults and contingencies, limits ROCOF); and Regulation (can replace dynamic voltage regulation and inertia from retired units). Disadvantages include higher level of losses, mechanical wear and a slower response time compared to power electronic technologies. It should also be mentioned that over the last three decades, a preference for synchronous condensers was given to alternatives based on highly dynamic, low-loss and low-maintenance power electronics solutions. As of today, in a world with the massive penetration of renewable energies, they again constitute a robust solution to ensure system stability in a scenario of the high penetration of renewable generation, and thus they play a role in the planning of the future grid (see H2020 Migrate project results on the competition between grid forming and synchronous condensers).

Technology Readiness Level

Conventional systems are rated TRL 9 - System ready for full-scale deployment, whereas HTS-based devices are assumed to be at a lower TRL. However, publications claim that HTC-based devices have reached the market.

Maturity for high inertia synchronous condensers with flywheel reaches levels between TRL 7 – System prototype demonstration in operational environment – and TRL 8 – system complete and qualified (see application case in Italy announced in May 2020).

Research & Development

Current fields of research: The increasing penetration of renewables in the energy mix fostered interest in synchronous condenser technology. Utilities are rediscovering the benefits of conventional synchronous condenser technology and are looking for ways to utilise them in their networks from a technical and economical perspective. From the technology standpoint, the HTS synchronous condenser capitalises on the progress in HTS research and innovation.

Innovation Priority: Grid forming, synchronisation of converters, virtual synchronous machines. Research is now focusing on analysing the application of synchro-nous condensers at different locations of the grid, their role in frequency response markets, and ways in which synchronous condensers could play a role in future system planning.

Best practice performance

Power range: Typical range of reactive power rating for synchronous condensers connected to the grid are in the range of 20 to 200 MVAr, but manufacturers can tailor synchronous condensers for power grid stability up to 350 MVAr.

Best practice application

Bjæverskov, Fraugde and Herslev substations (Denmark)


Denmark is one of the few countries to include a large share of wind energy in its energy mix, which is why the country need synchronous condenser solutions to help stabilize her electricity transmission system and to support higher wind power generation.

The scope of delivery for the synchronous condenser solutions included a synchronous generator with brushless excitation, a generator step-up transformer and the electrical auxiliary systems, such as control and safety systems, voltage regulators and startup systems. Each synchronous condenser solution can deliver more than 900 MVA of short-circuit power and +215/-150 MVAr of reactive power. The startup time is designed so that the generators can reach up to 3,000 rpm within 10 minutes and be synchronized with the transmission grid. Minimum availability of 98%. They feature high efficiency, low noise emissions and low installation and commissioning costs.

- Bjæverskov substation 250 MVAr synchronous condenser solution started operation in 2013 - Fraugde and Herslev substations synchronous condenser solution is capable of delivering more than 900 MVA of short-circuit power and +150/-75 MVAr of reactive power, trial operation as of August 2014.

Granite Substation, Vermont, USA


As part of the Northwest Vermont Reliability Project, a number of upgrades were investigated to provide for the reactive power needs at the Granite substation. Simulations indicated that the power system is very near to a point of voltage instability. In the case of an outage of the Vermont Yankee – Coolidge 345 kV line, a continuous reactive power control device is critical to prevent voltage collapse.

Selection of synchronous condenser over static devices due to the low voltage ride through capability and the high short time overload characteristics.

• Four +25/-12.5 MVAr sync condensers; • Four 25 MVAr shunt banks (MSC); • Two Phase shifting transformers; • Integrated control system. The overload capability of the condensers provides enough time for the mechanically switched 115 kV shunt capacitors to be placed into service.

Scotland, United Kingdom


Partnership between the UK Utility, the System Operator and academic institutions to demonstrate a sustainable design and operational control of a synchronous condenser with innovative co-ordinated control system combined with a STATCOM flexible AC transmission system device. Project was awarded a budget of £17.64m through the UK’s Network Innovation Competition (NIC) in 2016.

Synchronous condenser with innovative co-ordinated control system combined with a STATCOM flexible AC transmission system device.

Provision of an efficient and composite solution that will enhance system stability and security while maintaining power quality, resulting in minimizing risks of power outages and delivering significant benefits to Great-Britain customers.

Oberottmarshausen, Bavaria, Germany


The nuclear power plant in Gundremmingen with 1.34 GW capacity was shut down in enforcing the Atomic Energy Act. Therefore, measures to ensure grid stability with respect to inertia and reactive power compensation was required.

Synchronous condenser offered a wide reactive power +340 to -170 MVAr with loss optimized and life-time optimized operation. The solution ensured grid stability during voltage fluctuations.

Synchronous condenser was incorporated to provide wide range of reactive power capability for operating under ambient temperature condition in Oberottmarshausen.

Blackwater station, New Mexico, USA


Transmission lines (216 miles) between 362 kV Station (north of Albuquerque) to Blackwater 362 kV Station (near Clovis) enables exchange of power generated from wind farms between New Mexico and Texas. Consequently, the transmission lines of this length resulted in low short-circuit conditions that were challenging the control and operation of power electronic systems for the high-voltage direct current convertor and wind farms located at or near the Blackwater station. Studies revealed that installation of synchronous condenser provides the required short circuit capability to facilitate transmission services in 362 kV Black water station.

The synchronous condenser was selected to provide 959 MVA short circuit power at the 362 kV Blackwater station.

Installation of Synchronous condenser maintains acceptable system performance by increasing the short-circuit ratio and providing voltage support during faults and contingencies thereby enabling transmission of power in New Mexico.

Hesse, Germany


After the governmental decision to shut down nuclear power plants in Germany following the Fukushima incident, one of the generating units of the 2.5 GW Biblis nuclear power plant was converted into a rotating synchronous condenser.

A 14 MW medium-voltage startup converter was set up for generator startup. This was connected to a new 18.3 MVA transformer, which subsequently transforms its output voltage to the generator terminal voltage of 27 kV via a further 17 MVA transformer. With a gas-insulated 30 kV medium voltage switchgear, the new system was connected to the generator via the generator terminal lead.

The newly converted condenser regulates the reactive power from -400 to+900 MVAr, which is made available to grid operator Amprion in situations of low or high voltage.

Codrongianos, Italy


2x250 MVAr synchronous condensers for stabilizing the Sardinian grid. Speed: 3000 rpm. Voltage: 19 kV.

SC aim to allow a safer and enhanced utilisation of the Sardinian HVDC links. The two SC present innovative differences with respect to traditional ones: round rotor design (better cost/Mvar ratio), two poles design to reduce weight and cost, Air-to-water cooling to simplify maintenance, fast static starting system (<15 mn.), adiabatic cooling system for the primary water circuit, 200% overload capability (10 s), completely unmanned operation from remote control centre. Total losses are estimated at 1.15%.


Brindisi, Italy

(announced in May 2020)

Two synchronous condensers and flywheel will be provided to Terna for the Brindisi substation in southern Italy. Each synchronous condenser unit will supply reactive power of up to +250/-125 MVAr and 1750 MWs inertia to support the stability of Italy’s grid.

These synchronous condensers will complement the park of synchronous condensers operated by the Italian TSO. The offer from the manufacturer includes the design, civil works, supply, installation and commissioning of the 2 electrical two-pole generators and related equipment (step-up transformers, circuit breakers, auxiliaries and balance of plant, protection and controls systems) as well as the monitoring and diagnostic systems and 20 years of planned maintenance. Each of the generators will be equipped with a flywheel to respond to the inertia requirements from Terna.

Both equipment are expected to supply a combined 500 MVAr reactive power and 3500 MWs inertia to help stabilize grid and support the integration of more renewable energy.


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[12] H2020 MIGRATE. The MIGRATE project website downloads [Link] and brochure ‘The Massive InteGRATion of power Electronic devices’. [Link]