Kaveh Niayesh

Switching of capacitive currents

The new IEC 62271-100 requires an extensive proof of the capability of capacitive switching for a breaker under test. For vacuum circuit breakers. dielectric properties are mainly determined by the condition of the contact surfaces, which are modified by in-rush currents as well as load-breaking currents. A synthetic single-phase test device has been erected in order to investigate these dependencies in detail and to get knowledge of the statistical properties of the investigated interrupters. The results are evaluated with respect to pre-ignition field strengths and contact strokes. Application to commercial solutions are presented coping with the required standard class C2.

On the arcless commutation of currents higher than 1 kA

A new current limiting principle based on the application of liquid metals is proposed. This principle takes advantage of the arcless commutation of currents to a gradually increasing parallel resistance and makes it possible to realize a current-controlled resistor for very high currents, which can be used to limit the fault currents in power networks. The conditions for the arcless current commutation are investigated theoretically and the optimum profile for the parallel resistor is derived. An experimental set-up is constructed to investigate the behaviour of the separating contacts under different current amplitudes, different current steepnesses and different resistance profiles. The experimental results relating to the commutation phase are compared to the theoretical limits.