From 126kV To 252kV: The Voltage Leap in Vacuum Interruption Technology - News

Why Must the Vacuum Interrupter Be Installed in a Unified Manner in Deep-Fusion Circuit Breakers with Embedded Poles?

For decades, vacuum circuit breakers dominated medium voltage applications up to 52kV. Extending vacuum interruption into transmission voltage levels, first 126kV and then 252kV, represents one of the most significant engineering achievements in modern switchgear technology.

Breaking the 126kV Barrier

The first challenge was 126kV. Early designs used two vacuum interrupters in series, with external insulation provided by SF₆ gas and grading capacitors ensuring even voltage distribution across the breaks. A two-break 126kV prototype demonstrated the feasibility, but complexity and cost drove research toward a single-break solution. By the 2000s, single-break 126kV vacuum interrupters achieved ratings of 2000A and 40kA, passing dielectric and short-circuit tests. The key enabler was the axial magnetic field electrode, which kept the vacuum arc diffuse during high-current interruption.

Why 252kV Is Different

Doubling the voltage is not simply scaling up. The critical obstacle is a phenomenon called "saturation" in long vacuum gaps. Beyond a certain contact gap, increasing distance yields diminishing insulation gains. This makes the conventional design approach insufficient for 252kV. Vacuum interrupters also face a heat conduction problem: the long internal path from contact to terminal limits current-carrying capability. And with larger contact strokes and higher closing speeds, contact bouncing becomes severe, requiring specialized damping techniques.

The Path to Commercial 252kV

In 2025, the world's first 252kV single-break vacuum circuit breaker completed all type tests and entered live-grid operation in China. The achievement required advances in insulation grading within the interrupter, manufacturing processes for large ceramic envelopes, and new conditioning techniques for the internal vacuum environment. The 252kV unit uses pure nitrogen as external insulation, eliminating SF₆ entirely. With this voltage class now proven, attention turns to 550kV, where multi-break configurations and voltage grading will be essential.