DSO TSO Technopedia

An alternating current (AC) circuit breaker refers to a mechanical switching device that is used to close or open an electrical circuit. Circuit breakers are capable of carrying and breaking currents under normal and abnormal (e.g. short circuit) grid conditions.

To break the flow of current, a circuit breaker uses the natural property of AC current passing through the value of zero for 50 Hz and in normal conditions. At this point, current stops naturally flowing and the circuit breaker needs to avoid restrike between contacts and withstand the extinction recovery voltage.

A circuit breaker is a necessary safety device to operate and control the AC electrical power grid. Hence, very high reliability of device is demanded.

There are three basic designs of the circuit breakers:

• Life tank: The interrupter is inside an insulated enclosure that is air-insulated from ground potential.
• Dead tank: The interrupter is situated inside a grounded metallic enclosure filled with insulating gas. It is connected via bushings to the air-insulated substation (AIS) busbar. Typically, the current transformer is also attached to this module.
• Gas-insulated high-voltage switchgear (GIS): The interrupter is located in a grounded enclosure filled with insulating gas like dead tank with an extension that the circuit breaker is connected with other GIS components such as disconnectors, voltage transformers, and earthing switches, busbars in GIS technology.

In the recent decades many improvements of the breaking technology have been observed:

• Due to the increase in short circuit currents, arc quenching (the process of extinguishing the highly ionised path that allows current flow between open contacts) technology has evolved through several stages. It started with air-blast, then developed to minimum oil, further to SF6 blast, then to SF6 self-blast, before today's solution with SF6 double motion self-blast.
• The number of arc quenching chambers has been reduced over time.
• The reliability has significantly been improved by adapting the interrupters, so that it can be operated using spring drive (the mechanism through which the switch is opened or closed). There have been several different solutions over the years, compressed air, then hydraulic with N2 energy storage, then hydraulic with spring energy storage, and now only with spring charged by an electric motor.

At present, the F-Gas-Regulation introduced in 2024 requests application of alternative technologies to SF6 with significantly lower greenhouse gases emission. New technologies rely on insulating gases such as CO2, O2, N2, Fluoronitrile C4FN, pure air). The development of these alternative technologies progresses stepwise from lower voltages to higher and from lower short circuit current breaking capability to higher currents. The following technology trends for circuit breaker can be observed:

• Arc quenching using a vacuum chamber (single or two connected in series) which is insulated with natural-origin gases.
• Arc quenching and insulation using alterative gas (Mixture of C4FN with natural-origin gases or mix of natural-origin gases, like N2, CO2, O2).

AC circuit breakers are unavoidable devices for appropriate operation of AC grids.

The change of the insulating gases from SF6 towards low GWP gases options will significantly reduce greenhouse gases emissions.

• Safely extinguishing arcs—especially at high voltages and currents—is complex. AC arcs are particularly difficult due to the current's zero-crossing behaviour.
• Breakers (alongside protection relays) must coordinate to isolate faults without disconnecting healthy sections of the grid.
• Traditional breakers are designed for stable conditions and may not respond optimally to dynamic scenarios.
• SF₆ is a potent greenhouse gas, and its use is increasingly regulated. Alternatives are being developed but are not yet developed for all voltage levels.
• Predictive maintenance is difficult without embedded sensors and analytics, leading to either over-maintenance or unexpected failures.

The enablers of AC circuit breakers are listed below:

• Pilot projects of first SF6-free technologies and verification of their technical performance as well as CAPEX and OPEX costs.
• Rising short-circuit current levels in meshed AC grids.

Several R&D activities can contribute to further improve the technology:

• The development of SF6-free equipment for further standard and special (niche) applications.
• Monitoring of SF6-free circuit breakers.
• Standardisation of the circular economy, sustainability (life cycle analysis), and resilience approaches for traceable comparison, which can be used in the purchasing process.
• Further development of the circuit breaker digital interface integrated with IEC 61850 [2].
• Development of controlled switching algorithms for SF6-free circuit breakers.
• Implementation of digital twins for improving and speeding up planning processes.

The technology is in line with milestones “SF6-free solutions operating in high voltage and extra high voltage grids” and “Circular economy and environmentally friendly components included in planning and asset management” under Mission 1 of the ENTSO-E RDI Roadmap 2024-2034.

The technology readiness level (TRL) is as follows considering live tank, dead tank, and GIS circuit breakers:

TRL 9 for medium-, high-, and extra-high-voltage SF6 -circuit breakers.

TRL 7-9 for medium- and high-voltage (<= 145 kV) SF6 -free circuit breakers.

TRL 5-7 for high-voltage (> 145 kV) and extra-high-voltage SF6 -free circuit breakers.

What Are the Types of High Voltage Circuit Breakers in a Substation? - PEAK Substation Services

ICE, “Find out more abouve IEC 61850,” IEC [Online]