Open-Phase Mode Protection System for Vacuum Circuit Breaker

One of the reasons behind the reduction of operational reliability of steel-making factory power grids is the open-phase mode that occurs during the switching of vacuum circuit breakers (VCB). This mode results in overvoltage in furnace transformer windings, insulation breakage, and short circuits causing accidents and facility downtimes. Using Joslin VCS in a ladle-furnace unit, we analyzed the causes of this mode occurrences. We analyzed the VCB pole switching control system installed in the high-voltage grid of the furnace transformer. The VCB-ready-for-switching signal is generated depending on the charge in just one phase. When the condenser voltage is insufficient (or absent), the respective pole fails to close or open. Another cause is the lack of control and indication for the voltage values in high-voltage line phases. To reduce the risk of accidents, we developed an improved control system that prevents breaker switching in the open-phase mode. It features condenser battery charge control and indication for each phase, as well as voltage monitoring for the 110 kV grid. The article presents the system diagram and description and reviews its operating principle. The system was integrated into the closed switchgear VCB of an operating ladle-furnace unit. During its operation, we confirmed the reduction of accidents due to the open-phase switching of VCB and grid disconnection.