Lightning impulse conditioning of a combined field grading and shielding arrangement for vacuum double break

Advantages like environmentally friendly operation and relatively low maintenance costs of vacuum circuit breakers (VCB) in comparison to other interrupters have established the VCB in the medium voltage range. In order to enlarge the operational voltage level, a common approach is the usage of a double break. One possible solution for the symmetrisation of the voltage distribution over the series gaps is the use of external grading capacitors. In this paper a double breaking vacuum chamber with additional field grading and shielding electrodes, in a separate surrounding vacuum chamber to symmetrize the voltage distribution, is presented. In the first step, the shielding behaviour of the arrangement is analysed. Therefore, a grounded surrounding electrode is used to disturb the symmetric field distribution over the arrangement. In a second step the dielectric behaviour in dependence of the shield preparation before and during conditioning is investigated. Complete breakdowns of the arrangement are detected by using a common voltage divider. To detect partial breakdowns, the shields own capacitances are determined and used to generate a voltage divider. This approach also allows the estimation of the floating middle potential. All tests were performed with lightning impulse voltage stress (LIV). The paper presents a dielectric strength evaluation for vacuum double break arrangements and a test procedure to detect partial breakdowns inside.