VCB as GCB and limits versus SF6

Precautions when a VCB is used as a generator CB

A generator breaker sees a very high fault current with a large DC component (because of the high X/R ratio), delayed current zeros, and a very fast TRV rate of rise. A VCB interrupts so well that it can chop the current before natural zero. On the inductive generator and transformer windings this gives $V = L\,di/dt$ overvoltages and multiple re-ignitions. So:

1. Install metal-oxide surge arresters close to the breaker on both sides.
2. Install surge capacitors or RC snubbers at the generator terminals to slow the voltage rise and damp chopping overvoltages.
3. Choose a breaker type-tested for generator duty (IEEE C37.013 / IEC 62271-37-013), including asymmetrical breaking with high DC content and out-of-phase switching.
4. Check the TRV/RRRV capability; add TRV capacitors if needed.
5. Make sure the continuous current rating and temperature rise are adequate (forced cooling if needed).
6. Use low-chopping contact material (e.g. Cu–Cr) and monitor vacuum integrity.

Limitations of VCB compared with SF6 in HV application

Summary of VCB limitations at HV: the dielectric strength of a vacuum gap does not rise linearly with gap length, so a single bottle cannot be built economically above about 40 kV. Several bottles in series need voltage grading. Chopping overvoltages are higher and the continuous current rating is lower. For these reasons SF6 dominates above 72.5 kV, although SF6 is a greenhouse gas.