Novel Field Grading Shield Design for Double Breaking Vacuum Chambers under Lightning Impulse Stress

In the medium range of high voltage levels, switches based on vacuum have advantages such as being environmentally friendly and featuring lower maintenance costs compared to switches with sulfur hexafluoride (SF6). In order to expand the operation voltage levels of vacuum circuit breakers (VCB) up to high voltages, series arrangements are promising. Using these arrangements a symmetric voltage distribution over the switching distance is one of the main challenges. External electrical fields can cause asymmetric voltage distributions, which in turn, could lead to failure of one of the series arrangements and thus trigger a complete breakdown. The most common approach, to ensure a uniform voltage distribution, is the usage of external grading capacitors. The presented research proposes a new approach for a combined external electrical field-shielding and grading, based on an additional surrounding shield arrangement in a separate vacuum chamber to compensate additional grading capacitors. As a first step, the coupling mechanisms between the shields are simulated in COMSOL Multiphysics software for a one-pole series model with open contacts under a transient lightning impulse voltage. Here the potential drift of the floating middle part of the double breaking VCB (without plasma) is most important. In a second step the arrangement is build up in a test field for lightning impulse voltage (LIV) tests. To avoid flashovers between the shields, vacuum is chosen as insulation medium. To measure the potential of the floating middle shield, different techniques are used. In additional tests the new shield arrangement is complemented with a series arrangement of vacuum interrupters to measure the dielectric strength of the series gaps with and without external disturbances. The current research gives first results for designing shields in an additional vacuum chamber surrounding the double breaking VCB.