Behavior of vacuum switching devices for short gaps.

摘要

The behavior of vacuum switching devices has been studied for short gap lengths. Reignition phenomena and events associated with high frequency arc quenching have been of particular interest.; When used for switching inductive loads, vacuum interrupters can generate excessive transients, frequently in the form of multiple reignitions leading to voltage escalation. The magnitude of the overvoltage depends on the circuit parameters, circuit configuration, and the properties of the switching device. Circuit related data, obtained from experimental tests, was included in the study. The main effort in this phase of the work concentrated on the properties of vacuum devices.; Two of these properties dominate the escalation process: the breakdown strength characteristic as a function of time, and the high frequency current clearing capability. While the breakdown strength characteristic is mainly a feature of the contact travel and physical properties of the contact material and the contact surface, the second property, high frequency current clearing capability, depends on complex vacuum arc physics. A computer program was developed which used this data to predict the amount of overvoltage to a reasonable level of accuracy. The experiments showed considerable differences among commercially available vacuum interrupters.; To gain the physical insight into what is happening during a high frequency reignition in vacuum, a theoretical model has been proposed in which the arc plasma is presented as a compressible fluid, obeying conservation laws of mass, momentum, and energy, and the principle of current continuity.; By solving a set of simultaneous equations a novel computer program provides data for the distributions of: ion density and velocity, electric field, and potential, along with the ion sheath parameters, in the interelectrode space.; A comparison between test and computer generated results indicates that ionization of residual metal vapor after a hf current zero can accompany the diffusive decay of arc plasma. The simulation study has also revealed that the electric field distribution exhibits a local maximum at the anode of the discharge some tens of nanoseconds after the ion sheath forms during the post arc period. This phenomenon may be responsible for the early breakdowns across the sheath. It depends on the plasma/circuit parameters and is not always present.