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 VI's 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 don't 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.
Compactness of switchgear can be reached using a shielded solid insulation technology. Thus the size and costs can be reduced and the substation installation becomes easier and more attractive for customers. Constraints are then reported on the design of Vacuum Circuit Breaker in order to fulfill the whole standard requirements. A new embedded Vacuum Interrupter was developed that complies with requirements related to partial discharges, thermal cycles, mechanical endurance, breaking performance and disconnecting dielectric withstand. Multi-physics simulations were widely used to match industrial and technical requirements of such a complex system. For instance, overmolding simulation is mandatory to ensure the material homogeneity, the suitability of stresses with thermal constraints and that there are no holes. Radial Magnetic Field contacts were used to get compactness, cost effectiveness and breaking capacity. Contact material was also selected in order to obtain the best compromise between breaking performance, anti-welding property and low erosion. Thus the E2 electrical endurance class and 50 full interruptions at 25kA/15kV were achieved with less than 0.8mm erosion. No degradation of power frequency dielectric performance across isolating distance was found after breaking tests. Finally, this vacuum interrupter complies with the circuit-breaker and the disconnector standards.
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