A synchronous condenser (also called a synchronous capacitor or synchronous compensator) is a conventional solution that has been used for decades for regulating reactive power before there were any power electronics compensation systems.

A synchronous condenser is a Direct Current (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 and improving grid system strength through its inherent short-circuit current contribution. In addition, a flywheel can be added to enhance grid frequency stability by providing additional 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, short-circuit current contribution and the synchronous inertia. It constitutes an alternative solution to mechanically switched capacitors with damping network (MSCDN) or compensation choke in the power system due to the ability to continuously adjust the reactive power amount in both directions. Its overload capability is also superior to static var compensators (SVCs) or static synchronous compensators (STATCOM) of the same rating.

Synchronous condensers are perfectly suited for controlling the voltage on long transmission lines or in networks with a high penetration of power electronic devices as well as in systems where there is a high risk of “islanding”.

Technology Types

A conventional synchronous condenser is an Alternating Current (AC) synchronous motor that is not attached to any driven equipment. The device can provide continuous reactive power control through field excitation control. Both a static and brushless system may be used. By increasing or decreasing the field current, the system can make the machine generate or absorb reactive power, thereby maintaining voltage stability and regulating reactive power flow in the power system. With proper design, the device can supply multiple times (typically 2–2.5 times) the rated field current and voltage (field forcing capability) for up to 10 seconds, offering dynamic voltage recovery support during depressed grid voltage conditions. 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. It is worth noting that there are also electrical losses associated with the synchronous condenser, which is the active power the device will consume at a different operating point. These electrical losses directly impact the operating cost of the synchronous condenser plant.

The development of high-temperature superconductivity (HTS) 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 those that can be achieved with the low-temperature superconductor wire.

The benefits of synchronous condensers are listed below:

• System inertia: inertia is an inherent feature of a synchronous condenser as it is a rotating machine. The benefit of inertia improves 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-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.

Given the higher levels of losses, mechanical wear and a slower response time compared to power electronic technologies, a preference over synchronous condensers was given to alternatives based on highly dynamic, low-loss and low-maintenance power electronics solutions during the last three decades. As of today, in a world with the massive penetration of renewable energies, synchronous condensers again constitute a robust solution to ensuring system stability in a scenario of the high penetration of renewable generation, and thus play a role in the planning of the future grid (see H2020 Migrate project results on the competition between grid forming and synchronous condensers [1]).

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; and
• Step-up transformer and auxiliary transformer.

Typical applications of a synchronous condenser include:

• HVDC: provides short-circuit strength and dynamic reactive power support;
• Wind and solar: increases short-circuit ratio;
• Grid support: improves weak Alternating Current (AC) grid performance, voltage support during faults and contingencies, and limits rate of change of frequency (ROCOF); and
• Regulation: can replace dynamic voltage regulation and inertia from retired units.

Most of the ongoing developments regard the:

• Development and implementation of high-temperature superconducting synchronous condensers;
• Increase in the inertia by coupling with larger flywheels;
• Minor general improvements in the coupling between synchronous condenser and flywheel, and improvements in bearing e.g., magnetic bearings;
• Development of a market for use of thermal losses for other customers; and
• Alignment and standardisation of general requirements for faster deployment of projects.

The technology is in line with milestone “New and optimised control concepts of reactive power” under Mission 3 of the ENTSO-E RDI Roadmap 2024-2034.

Examples

TRL 9 for conventional synchronous condensers.
TRL 3 to 5 for a high-temperature superconductive synchronous condenser in extra high voltage grid.

1. Synchronous Condenser: Design, Working, Phasor Diagram & Its Applications

2. H2020 MIGRATE, “The MIGRATE project website downloads and brochure ‘The Massive InteGRATion of power Electronic devices.’”, Cordis.europa.eu.

3. ABB, “Synchronous condensers for reactive power compensation”.

4. GE, “Synchronous condenser systems”, gevernova.com.

5. WEG, “Synchronous condensers”, Weg.net.

6. SP Energy Networks, “Phoenix – System Security and Synchronous Condenser”, Spenergynetworks.co.uk.

7. Siemens, “The stable way. Synchronous condenser solutions”, Siemens-energy.com.

8. T. Abdulahovic and S. TelekChalmers, “University of Technology. Modeling and Comparison of SC and SVC”, Chalmers University of Technology.

9. Siemens, “Renaissance of a classic – the synchronous condenser”. Ansaldo Energia, “Brochure on Synchronous Condenser”.

10. S. Kalsi, D. Madura and M. Ross, “Performance of Superconductor Dynamic Synchronous Condenser on an Electric Grid”, presented at Transmission and Distribution Conference and Exhibition: Asia and Pacific, 2005 IEEE/PES, Feb. 2005.

11. F. O. Igbinovia, G. Fhandi, Z. Muller and J. Tlusty, “IEEE Reputation of the Synchronous Condenser Technology in Modern Power Grid”, presented at IEEE International Conference on Power System Technology, Nov. 2018.

12. F. Palone and M. Rebolini, “Synchronous condensers in multi-infeed HVDC systems”, presented at AEIT/CIGRE HVDC Transmission Workshop at Milan, May 2014.