Furnace Transformer Protection Device Mitigating Open-Phase Mode During Switching of Vacuum Circuit Breaker

The authors review the connection of a ladle-furnace unit (LFU) transformer to the 110 kV power line. Transformer protection is implemented as an SF6 Circuit Breaker series connected with a Vacuum Circuit Breaker (VCB) that performs switching in operating modes. The authors stress the importance of providing the reliable operation of high-voltage equipment, particularly through the mitigation of the open-phase mode that occurs during the VCB switching. They list the drawbacks of the existing protection devices associated with controlling low zero-sequence idling currents (hundredths of amperes). The authors present a device that can identify open-phase modes by using transformers with a fast-saturated magnetic core that act as integrators. This device is part of the protection system that amplifies signals and sends them to the controller that generates the disconnection signal for the SF6 Circuit Breaker. The authors tested the operability of the suggested device in two stages. During the first stage, emergency modes were simulated without network connections. This helped confirm the reliability of the signal generation sequence for power unit disconnection. During the second stage, the authors tested the protection using the idling currents during the switching of each phase. Thus, we confirm the operability of the suggested control system and the open-phase mode protection system using the transformer phase currents. The system was deployed on the VCB of a closed switchgear installed on a metal plant LFU.