Sparkover in sphere gaps with alternating voltages and perturbed electric fields

Quasi uniform field gaps, namely sphere gaps are quite often used in high voltage laboratories. In the present work, both simulation and experimental results to see the electric stress distribution on breakdown voltage are presented. Considering commonly used vertical arrangement of sphere gaps, disruptive discharge voltages are measured experimentally. These experiments are with one sphere grounded. In order to study the redistribution of electric stresses, experiments have been conducted by placing a hemi-spherically tipped rod (needle), near the gap axis. This rod is placed vertically on the ground plane being at ground potential. Experimental results are reported with the rod diameter of 3 mm and its height 10 cm as a function of sphere-gap separation. Experimental results reported are with power frequency alternating voltages. The simulations corresponding to these experimental conditions are reported to correlate the change in electric field distribution obtained by charge simulation models (CSM). Interpreting the experimental breakdown results in relation with simulation results of geometric electric field is attempted. The results with rod in the vicinity of sphere gap results in to a non axi-symmetric field. These are simulated to obtain the maximum stress on the electrodes.