Vacuum switch-disconnectors : 1. Dielectric behaviour

This study on vacuum switch-disconnectors addresses two major questions: dielectric behaviour of the VI and the reliability in a practical situation. In the experimental study, the ageing of a specially designed vacuum switch-disconnector is studied. Several internal design parameters (shape of the shields, contacts and contact gap) are varied. The ageing of VIpsilas is simulated by no-load switching, by load current switching as well as short-circuit closing; the dielectric behaviour is analysed by applying a lightning impulse voltage after each operation. A total of 200 operations are performed on each single VI out of a series of 10 interrupters. It is demonstrated that : (1) no-load switching and nominal current switching donpsilat change the dielectric behaviour; the breakdown probability can be described by a Gaussian distribution with as main characteristic that the standard deviation is about 16% of the mean breakdown value. (2) Closing in on short circuit leads to a temporary reduction of the breakdown voltage; a single interruption of the load current permits to restore the original breakdown strength.