ANEW LINE of steel-clad impulse oil circuit breakers' has recently been developed and put into production by the conmpany with which the authors are associated. These breakers are enclosed in lenticular tanks, Figs. 1 and 2, instead of the conventional cylindrical tanks. The new tanks reduce the size and weight of the breakers and reduce the quantity of oil to be handled during maintenance procedures. Because of significant changes in the electric field between the highvoltage interrupters and ground metal, an investigation was initiated early in the development program to apply recent advances in field mapping techniques2 to the problem of determining the optimum shapes and dimensions for the tank and for the electrostatic shields on the interrupters. The purpose of the investigation was to obtain optimum over-all arrangements with adequate dielectric strength.The results of the investigation have had definite practical application. The electrostatic shield curvatures and shapes were determined initially in the electrolytic tank, and required no subsequent modifications which were significant from an electrostatic field standpoint. It was found that electric stresses were so low on inside surfaces of the tank that breakdown does not initiate from these surfaces. Consistent with a previous paper,3 it was found that breakdowns would not initiate from certain surfaces on which electric stresses were the highest in the breaker. Instead the breakdowns would initiate from other surfaces on which the electric stresses were as much as 40 per cent (%) less but which exerted these lower stresses through a much greater volume of oil. good engineering accuracy while the designs were still on the drafting board. Because the electrolytic models could be made and modified so inexpensively, many variations in design were investigated, with the best being selected and constructed for full-scale test and subsequent use in production.
Summary of Over-all InvestigationTo establish how the investigation depended upon prior work, and why certain additional concepts and information were needed, a chronological synopsis of the over-all program seems necessary. The first step was to construct 3-dimensional models of various shapes under study; see Fig. 2. These were filled with filtered water, and by application of electrolytic field analysis techniques, flux plots such as Fig. 3 were obtained showing the distribution of voltage throughout the oil circuit breaker. However, because such flux plots are difficult of interpretation except in a plane of symmetry, a 2-pointed probe was developed so that the field gradients could be determined at any point in the breaker regardless of the direction of the field or whether it happened to occur at the surface plane of symmetry of the electrolyte. Use of this probe required development of a special electronic amplifier which would measure the small voltage difference between the points of the probe even though both points were at relatively high voltage. In this manner, it was found practical to ...
