Module 2 · Arc interruption

How an SF6 puffer breaker puts out the arc

Send a trip and watch one interrupter pole open in slow motion: the main contacts part, the arc strikes between the arcing contacts, the puffer compresses gas, and the arc dies at a current zero. Scrub the timeline to stop at any millisecond.

Interrupter pole · 220 kV · puffer type

t = −5.0 ms

Slow motion

Readouts

Contact travel: 0 mm · 0 %
Puffer pressure: 6.00 bar
Current: 3.15 kA rms
Across gap: 0 kV
Arcing time: —
Break time: —

Test conditions

Current being interrupted

Rated load current, 3150 A.

SF6 density in the breaker (at 20 °C)

6.00 bar(g) Normal

Refill alarm at 5.40 bar, lockout at 5.00 bar — below lockout the drive is blocked. This is the same breaker as bay E-05.

Interruption stages

Current flows through the silver-plated main contacts: the fixed finger cluster grips the moving contact tube. The tungsten-copper arcing contacts are in parallel but carry almost nothing.
Protection energises the trip coil. The spring drive unlatches and the operating rod starts to accelerate the moving assembly — the opening time has begun.
The tube slides out of the fingers. Because the arcing contacts are still touching, the current simply commutates to them, so the main contacts part without an arc and never erode.
The tulip leaves the arcing pin and a plasma column of around 20 000 K forms in SF6 inside the PTFE nozzle. The arc keeps conducting the current until a natural current zero.
The moving cylinder compresses SF6 against the fixed piston, raising the pressure. Once the pin clears the nozzle throat, the compressed gas blasts axially through the arc, cooling it and sweeping away charge carriers.
As the current passes through zero the arc's energy input vanishes. SF6 is strongly electronegative: its molecules capture free electrons within microseconds and the gap turns from conductor to insulator.
The network drives a transient recovery voltage across the open gap. If the gap's dielectric strength rises faster than the TRV, the interruption holds and the pole is open.

Puffer versus self-blast interrupters

A puffer interrupter makes its own blast pressure mechanically, so the drive must supply the compression energy — it needs a stronger spring or hydraulic drive. Self-blast (thermal-assisted) designs use the arc's own heat to pressurise a heating volume at high currents and keep only a small puffer for low currents, so a lighter spring drive is enough. Most modern GIS breakers up to 245 kV are self-blast; the physics of current-zero interruption is the same.

Minimum arcing time and re-ignition

At the first current zero after the arcing contacts part, the gap may still be too short and the gas flow too weak. The arc then re-ignites and burns for another half-cycle. Set the fault to 100 % Isc and watch it happen at 30 ms: the gap is cleared only at the next zero, at 40 ms. Break time is the opening time plus the arcing time, and it must stay within the rated value — 60 ms at this voltage.

TRV, RRRV and why SF6 wins the race

After a fault current zero the network rings across the open contacts: the transient recovery voltage. Its peak (about 364 kVp here, from a first-pole-to-clear factor of 1.3 and an amplitude factor of 1.4) and its rate of rise are what the gap must survive. Interrupting load current is gentler, because the voltage is near its own zero when the current is. Hot SF6 recovers its dielectric strength about a hundred times faster than air, which is why one SF6 break can do the work of several air-blast breaks.

What the arc leaves behind

Most SF6 recombines as the arc cools, but some decomposes into products such as SF4, SOF2 and HF, and PTFE ablates from the nozzle. Molecular-sieve adsorbers in each breaker compartment trap them. Treat the gas and the powder in an opened breaker compartment as hazardous: use the gas cart, a respirator and gloves, and follow the OEM's handling procedure.