The peculiarities of interruption of the medium voltage motors by VCB with CuCr contacts [vacuum circuit breakers]

The Monte-Carlo simulation technique and appropriate software has been developed to analyze the switching overvoltages occurring at the interruption of 6 and 10 kV motors by vacuum circuit breakers (VCB) with CuCr contacts. VCB parameters have been determined experimentally. A comparison has been provided between VCB switching behaviour at the interruption of 6 and 10 kV motors. Voltage escalation has been shown to be the most important process for 6 kV motors with respect to switching overvoltages generation. In contrast to 6 kV motors, three-phase virtual current chopping predominates for 10 kV motors. It has been deducted that for the majority of the real field applications, the VCB with the most popular contact material (CuCr base) can create dangerous overvoltages. Switching surge analysis conducted with the aid of the Alternative Transients Program (ATP) proved a metal oxide surge arrester connected in parallel to VCB contacts to be the most effective way to suppress phase to ground overvoltages. Turn-to-turn overvoltages induced at interruption of medium voltage motors by VCB with the described protection have also been studied. These overvoltages have been compared with the turn-to-turn overvoltages induced by an "ideal" breaker that does not interrupt high frequency current at all. The authors' analysis revealed that the maximum turn-to-turn overvoltages induced by VCB with adequate protection do not exceed overvoltages induced by an "ideal" circuit breaker for more then 15% and are lower then similar overvoltages induced by conventional breakers under the same conditions.