Disruptive discharge voltages in sphere gaps with perturbed electric fields

Abstract

Quasi uniform field gaps, namely sphere gaps are quite often used in high voltage laboratories. They exhibit very little polarity effect when subjected to direct high voltages. But if their electric field is disturbed by a near by earthed object they tend to show polarity effect. In the present work, both simulation and experimental results to see the electric stress distribution on breakdown voltage are studied. 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 polarity effect on redistribution of electric stresses, experiments have been conducted by bringing a hemi-spherically tipped rod (needle), radially, towards the gap axis (of varying rod dimensions). This rod is placed vertically on the ground plane being at ground potential. Experimental results with both positive and negative polarity dc voltages are reported. The simulations corresponding to these experimental conditions have been reported to correlate the change in electric field distribution using charge simulation models (CSM). The video of spark channel formation under positive and negative polarity with field perturbing rod will be shown during presentation. With rod in the vicinity, electric field gets redistributed and affects the disruptive discharge process. Under negative polarity voltage applied to the upper sphere with rod at the ground potential (along with the ground sphere), the spark is drawn towards the rod. Depending on the vicinity of the rod the spark channel is between the upper sphere and the rod. On the contrary, with the positive potential applied to the upper sphere, the spark channel is drawn towards the rod; (having got disturbed and not being along the sphere gap axis) but strikes the lower sphere and not the needle. Authors feel this research effort is likely to help better understand many aspects related to lightning rods and discharges.