High Voltage Circuit BreakerHV AC Switchgear

Typical difference between SF6 and vacuum switchgear in high voltage

  • Interruption of fault current with an associated very steep rate of rise of transient recovery voltage is superior with vacuum interrupters because of their very fast dielectric recovery, compared to SF6.
  • The breakdown statistics of vacuum and SF6 gaps differ. Although a vacuum gap in principle has a very high breakdown voltage, there remains a very small probability of breakdown at relatively moderate voltage.
  • Vacuum gaps are known to show spontaneous late breakdown, up to several hundreds of milliseconds after current interruption. However, the consequences of such an event are very limited because the vacuum gap
    immediately restores its insulation.
  • In inductive load switching, notably shunt reactor switching, the number of repeated re-ignitions (at one power frequency current zero) is signifcantly higher in vacuum than in SF6.
  • When switching capacitor banks with vacuum switchgear, very high inrush current must be avoided. This is because the contact system can be dielectrically deteriorated by the pre-strike arc.

Pic show contact system of SF6 CB (left) and vacuum CB (right).

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Source
Electra magazine 589

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