Affect of height of HV sphere above the ground in HV measuring sphere gap

Abstract

Sphere gaps are the standard gaps used for measurement of peak value of high voltages. Standards specify a minimum and maximum height of the spheres above the horizontal ground plane is specified as a function of its diameter 'D'. This height 'A' of the sparking point has to be within limits according to the guidelines of these standards. In the present work, both simulation and experimental results to see the affect of the height 'A' on breakdown voltage is presented. These results are presented for the worst case deviation from the guidelines by placing the ground sphere on the ground plane and the upper sphere above the ground sphere with gap separation S?D/2 units (considering commonly used vertical arrangement). The distance of the nearest grounded object is kept 'B' units away as specified in the standards. In continuation, in order to study the nearby grounded object a hemi-spherically tipped rod (needle) is brought near, radially, towards the gap axis. This needle is placed vertically on the ground plane maintained at ground potential. Experimental results are obtained with the radial distance of the needle to the gap axis as the parameter. The results are reported for both positive and negative polarity voltages. The simulation is carried out by developing charge simulation model (CSM). Some of the results (even with out the grounded needle in the vicinity) indicate that variation in 'A' modifies the surface electric fields of both the spheres. As the height 'A' is decreased the surface field intensity of upper sphere increases and that of the lower sphere decreases. The correlation between the experimental breakdown results and the simulation results is attempted. These results may help, the users, who have some constraints in actual practice in the laboratory to strictly follow the guidelines given in the standards. �2007 IEEE.