Inverter Based Power Generating Resources - Synchronous Inertia of Grid - Synchronous Condensers

Background

Replacement of conventional synchronous power generation with inverter based generation results in overall deficiency of "Reactive Power Support for Voltage Stability", "System Inertia helpful for Frequency Stability" and "System Short Circuit Strength" in Grid.

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, 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 machines reactive power control capabilities and the synchronous inertia. They constitute an interesting alternative solution to capacitor banks in power system due to the ability to continuous adjustment of the reactive power amount. They are perfectly suited to control the voltage on long transmission lines or in networks with high penetration of power electronic devices and in networks of high risk of ‘islanding’ from the main network.

Technology Types

Synchronous condenser is a conventional solution that is used for decades for regulating reactive power before there were any power electronics compensation system.

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 that 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: high temperature superconductor (HTS) based machines being 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. As a result, they utilize simpler, less costly, and more efficient cooling systems. This made HTS wires technically feasible and economically viable for condenser applications at power ratings lower than that could be done with the low-temperature superconductor wire.

Components & enablers

Typical components of a (complete solution for a) 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 condenser are similar to traditional synchronous condenser but with additional inertia. A dedicated flywheel for better system inertia matching could complement 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, since it is a rotating machine. The benefit of inertia is improved voltage “stiffness”, which improves the overall behavior on 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 to 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 massive penetration of renewable energies, they constitute again a robust solution to feed the debate to ensure system stability in a scenario of high penetration of renewable generation and thus play a role in the planning of future grid (see H2020 Migrate project results on the competition between grid forming and Grid forming and synchronous condensers).

Technology Readiness Level

Conventional systems are rated TRL 9-System ready for full scale deployment while HTS based device are assumed to be at lower TRL. However, publications claim that HTC based device 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 the interest to synchronous condenser technology. In a way, utilities are rediscovering the benefits of conventional synchronous condenser technology and are looking for ways to make use of them in their networks from technical and economical point of view. On the technology standpoint the HTS Synchronous Condenser capitalizes the progress on HTS research and innovation.

Innovation Priority: Grid forming, synchronization of converters, Virtual Synchronous machines. Research is now focusing on analyzing the application of synchronous 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.

References

[1] ABB. Synchronous condensers for reactive power compensation.

[2] GE. Synchronous condenser systems.

[3] WEG. Synchronous condensers.

[4] SP Energy Networks. Phoenix-System Security and Synchronous Condenser.

[5] Siemens. The stable way. Synchronous condenser solutions.

[6] Chalmers University of Technology. Modeling and Comparison of SC and SVC.

[7] Siemens. Renaissance of a classic-the synchronous condenser.

[8] Ansaldo Energia. Brochure on Synchronous Condenser.

[9] Porto University. Swarn Kalsi, David Madura and Mike Ross Performance of Superconductor Dynamic Synchronous Condenser on an Electric Grid.

[10] Famous O. Igbinovia, Member, IEEE, Ghaeth Fandi, Member, IEEE, Zdenek Muller, Senior Member, IEEE and Josef Tlusty, Fellow, IEEE Reputation of the Synchronous Condenser Technology in Modern Power Grid Conference Paper Nov. 2018 DOI: 10.1109/POWERCON.2018.8601540.

[11] Terna. F. Palone, M. Rebolini. Synchronous condensers in multi-infeed HVDC systems.

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

P.S. Above contents are a direct extract from an entsoe article. https://www.entsoe.eu/Technopedia/techsheets/synchronous-condenser