Recent Developments of Field Grading for High Voltage Vacuum Circuit Breakers

In the medium-voltage range, circuit-breakers with vacuum as interruption medium have advantages such as environmental friendliness and lower maintenance costs compared to sulphur hexafluoride (SF6). Current political and industrial developments suggest an effort to expand the field of application of vacuum circuit breakers (VCB) up to high voltage levels. With a rising operating voltage level, the influence of parasitic capacitances on floating components, such as metal vapour shields, result in an increased absolute voltage drift of these components. This reduces the dielectric strength of the VCB, especially in case of lightning impulse stress. To ensure the dielectric strength of the high voltage VCBs, larger insulation distances are required. An approach with pure (linear) upscaling of all components and the gap distance is not sufficient from the technical as well as economical point of view. This paper presents a novel approach to stabilize floating potentials in VCBs based on a combination of shielding from external influences and grading floating potentials by internal capacitive coupling mechanisms. This is achieved through an additional shield arrangement in an outer surrounding vacuum chamber. In this chamber, shields on defined potentials are coupled with the dielectric displacement to floating shields and thus stabilize those potentials. Following this approach, the coupling between the shields is simulated with COMSOL as well as proven over empirical studies. To determine the dielectric strength of vacuum and thus to derive the required distances between the shields, empirical investigations are considered. A prototype of the shield arrangement for a double breaking VCB is developed and tested. Finally, comparative measurements of the double breaking VCB arrangement with and without external disturbances are presented and show the functionality of the combined shielding and grading. The main goal of this work is an approach to stabilize floating potentials in high voltage VCB and thus ensure dielectric insulation under lightning impulse stress.