Streamer criterion : foreword
General
As said before, in Flux, the dielectric breakdown study is based on Streamers inception criterion. This criterion does not give precise information about arc path and breakdown voltage. The aim is rather to identify the Streamer starting area and a “Streamer inception voltage”.
Concerned dielectric medium
The “Streamer criterion” tool allows studying Streamers in gas medium. In fact, the gas has a lower breakdown voltage than liquids and solids.
Access in Flux
The tool is available in Flux electrostatic application (2D plane, 2D axisymmetric and 3D), in postprocessing context.
It is available through the menu “Computation”.
General principle
Each gas has a coefficient called αeff defined by a Streamer mathematical model depending on electric field and pressure. This model is determined by fitting an experimental curve. Consequently, a model will be optimal for some configurations (of electric field, geometry, pressure, temperature, etc.) but can not be universal.
Air formula used in Flux:
With :
Er: reduced electric field, E: electric field in kV/mm, P: pressure in bar
K is computed by integrating αeff on a path (Streamer path), and its value is compared to the threshold value. If the value is higher than the threshold, then the Streamer has a high probability to start.
As the law between the electric field and the electric potential is linear, a dichotomy is applied on the electric field until K is equal to the threshold. It allows finding the Streamer inception voltage on the considered path.
Streamer path
Streamers paths are not predictable by modelization.
The paths used in the modeling are:
- Previously identified with experiments
- Or approximated by physical quantities evolution laws. In Flux, the electric field lines are considered.