INFLUENCES OF AXIAL MAGNETIC FIELDS AND DIFFERENT CONTACT MATERIALS IN VACUUM INTERRUPTERS ON THE CHOPPING BEHAVIOUR OF SWITCHING ARCS

Abstract

In state of the art vacuum interrupters the current chopping level is in the range of 0.4 … 1.5 A for WCAg40-wt% (weight percent) contact material and 3 … 6 A for CuCr25-wt% in case of interruption up to rated current. This event occurs abruptly, resulting in a high current gradient within nanosecond time scale. At inductive loads, these impulses can cause high overvoltages during interruption operations, which can lead to irreversible damages on the primary equipment. The arc's chopping behaviour can be influenced by different methods: One of these is to form the arc's plasma inside the vacuum chamber by applying a constant external axial magnetic field via the contact gap. The result is a diffuse arc, which forms a homogenous plasma evenly distributed over the entire contact surface. Diffuse arcs tend to burn more stable close to the current zero crossing. This minimizes high current gradients. Additionally, innovative alloys of contact materials can be used to optimize the chopping current behaviour under axial magnetic fields. The main focus of this paper is to investigate the influence of axial magnetic fields on different common contact material compositions for interrupters. In contrast to synthetic test circuits used in other publications with high feeding voltages a simple resistive load circuit is introduced in this contribution. Measurements show that low magnetic flux densities are able to reduce chopping currents by 20 to 30%. This improvement strongly depends on the used electrode alloys and interrupter geometries: In some configurations also an undesirable increase of chopping currents can be observed if an external magnetic field is applied.