Gas insulated high voltage devices usually contain solid insulation for mechanical support or as an enclosure. Although the dielectric strength of the gaseous and solid insulation may be sufficient individually, detrimental breakdown can still can occur because the gas-solid interface may be weaker. An insulator surface can for instance modify the local electric field, or emit electrons, and facilitate discharges along its surface flashover. The underlying fundamental physics of these creeping sparks however is poorly understood. To be able to use design rules based on knowledge rather than experience we need to gain a deeper understanding of the associated physics. In this research work, the streamer-like phase that precedes sparking will be investigated. We are currently setting up an experiment to measure charges present on a dielectric surface while a discharge is propagating on this surface. For this experiment, we use a BSO crystal, which exhibits the Pockels effect, with a thin coating of sample material. The polarization of light passing through the BSO crystal is changed under the application of an external electric field. This enables us to visualize the charge of a streamer on the sample surface. The measured electric field however is a summation of the electric field resulting from charges present in the electrode and charges present in the streamer and on the surface. To eliminate the electric field due to electrode charges, we need to calculate this field, for which we use COMSOL. We measure the voltage pulse used to generate the discharge and use this as input for the simulation. We then calculate the time-dependent electric field in 3D.
|Publication status||Published - 2014|