An alternating current (AC) circuit breaker refers to a mechanical switching device that is used to close or open a circuit. Circuit breakers are capable of carrying and breaking currents under normal circuit conditions. In case of specific abnormal conditions such as a short circuit, circuit breakers are capable of carrying the fault current for a specified time, as well as breaking currents.

To break the flow of current, a circuit breaker uses the natural property of AC passing through the value of zero every 10 ms. At this point, current extinction takes place via the use of an interrupter.

A circuit breaker is a necessary safety device to operate and control the AC electrical power grid. Given that a device malfunction can trigger instability of the system, there are very high reliability demands.

There are three basic designs of the circuit breakers:

• Life tank: The interrupter is inside an enclosure that is insulated from ground potential.
• Dead tank: The interrupter is situated inside a grounded metallic enclosure filled with insulation 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 pipe filled with insulating gas. The circuit breaker is connected with other GIS components such as disconnectors, voltage transformers, and earthing switches in GIS technology.

Moreover, recent decades many improvements of the technology have been observed:

• Due to the increase in short circuits, 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.
• Increased reliability has been achieved by changing the drive (the mechanism through which the switch is opened or closed). There have been several different solutions over the years, first using air, then hydraulic with N2 storage, then hydraulic with spring storage, and now the spring.

At present, new technologies are appearing on the market offering devices with gases (dielectrics such as CO2, O2, pure air) with a much better CO2 footprint than SF6. Here, the development is not yet completed, although the following technology trends can be observed:

• Arc quenching using a vacuum chamber (single or two connected in series) isolated with natural-origin gases.
• Direct arc quenching in insulating gas, similar to SF6 technology.

The benefits of AC circuit breakers are listed below:

• The SF6 circuit breaker technology has been developed over the last 50 years for numerous applications. Hence, the SF6-gas emissions, maintenance efforts, number of special applications, and reliability of the technology have been significantly improved over time. Nowadays, the technology is well-known and extremely reliable.
• The new SF6-free technology will be introduced in the coming years, although it currently requires some further development steps.

The enablers of AC circuit breakers are listed below:

• SF6 equipment will be available for all kinds of applications (as well as niche applications) until alternatives are developed.
• Pilot projects of first technologies and their consequent improvements.

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

• The development of SF6-free equipment for standard applications.
• Improvements and development of SF6-free equipment for niche applications with improvements in switching capabilities.
• 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.

Examples

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 circuit breakers.
TRL 7-8 for medium and high voltage SF6-free circuit breakers.
TRL 5-7 for extra high voltage SF6-free circuit breakers.

1. What Are the Types of High Voltage Circuit Breakers in a Substation?

2. ICE. “Find out more abouve IEC 61850,”

3. Hitachi. “Hitachi Energy to provide world’s first SF₆-free 420 kV gas-insulated switchgear technology at TenneT’s grid connection in Germany,”

4. Schneider Electric. “SM AirSeT™”

5. GE Vernova. “GE’s Grid Solutions’ GL312g g³ Live Tank circuit breakers help Groupe E to get rid of SF₆,”

6. GE Grid Solutions. “g3: The SF6-Free Solution in Practice White Paper.”