Abstract-High-voltage design optimization requires a fundamental understanding of electrical breakdown mechanisms under different stress situations. The impulse withstand voltage is normally used as dimensioning criterion for medium voltage air-insulated systems as flashover mechanisms in air are rapid. Prediction of withstand voltage relies on streamer inception and propagation models that are not always sufficiently accurate. Positive impulse voltage experiments were performed on a rodplane gap with a dielectric barrier at different positions parallel to the ground plane. Streamers initiate from the rod tip and propagate in the field direction. Charge deposited on the dielectric surface changes the field situation and can result in a higher inception voltage. The streamer propagation was recorded with a fast ICCD camera. Finite element method field simulations of the background field were used to evaluate the effect of a barrier surface potential on the streamer inception voltage. Streamers reach the ground electrode without initiating electrical breakdown. The discharge activity from the rod was reduced by deposited charge on the barrier.

Prediction of withstand voltages in air-insulated systems are made on the basis of empirical models that are not sufficiently accurate for complex geometries. Better understanding of the spatiotemporal development of electrical discharges is necessary to improve the present models. Discharges in lightning impulse stressed 20-100 mm rod-plane gaps are examined using a highspeed camera, photo-multiplier tubes (PMTs) and a highbandwidth current measurement system. The images and measurements of gaps larger than 20 mm show a fast initial streamer discharge with a current rise time of some tens of ns, followed by a dark period of a few µs and a propagation of a slower leader-type channel leading to breakdown. The breakdown mechanisms in the shortest gaps are faster and geometry dependent, probably occuring by heating of initial streamer channels. Different light filters used with the PMTs indicate that all parts of the leader-type discharge development emit light over a spectrum from UV to IR. The initial discharges emit low amounts of warm light and IR compared to the leader-type channel. Finally, it is suggested that empirical breakdown voltage prediction models should be interpreted in light of the leader-type breakdown mechanism.