Abstract
An important task that arises during the operation of electric steelmaking units is to ensure the reliability of their power supply. A simplified connection diagram for the transformer of a ladle furnace unit (AFK), the electrical equipment of which is installed in a closed 110 kV switchgear, is considered. It is noted that frequent causes of accidents are non-full-phase modes that occur during switching of vacuum circuit breakers (CBs). A brief description of the design of the explosive is given, and the reasons for its malfunctions are noted. Based on the time diagrams of currents and transformer phase powers, an analysis of currents in normal and open-phase modes was performed. The possibility of relatively long-term operation of the agro-industrial complex when the transformer is powered from two phases is shown. A literature review has shown that such currents cannot be identified by known protection systems for furnace transformers. The task has been set to develop a device for protecting a transformer from open-phase mode; a principle based on amplification and subsequent identification of no-load currents in the phases of the transformer has been substantiated. For this purpose, it is proposed to use fast-saturating current transformers that perform the functions of integrating measured signals. A diagram of a system for protecting an electric furnace transformer from non-phase operation is presented, and the use of commercially produced sensors designed to measure and amplify low currents is justified. The desired current-voltage characteristic of the current transformer used in the device is given. An algorithm has been developed to control the emergency shutdown of a gas-insulated circuit breaker connected in series with the explosive and providing protection for the electrical installation. The performance of the proposed protection system was tested. To do this, an imitation of the open-phase current mode of phase A was performed without connecting the transformer to the network. As a result, the reliability of the sequence of formation of logical signals for controlling the switching of explosives was confirmed. Next, the operation of the protection for no-load currents in each phase was checked. Identification of open-phase switching and reliable generation of a signal to turn off a gas-insulated circuit breaker have been confirmed. The device was implemented on the ladle furnace unit of an electric furnace melting shop; its long-term operation confirmed the effectiveness of preventing emergency shutdowns of the furnace.
Keywords
ladle furnace unit, transformer, vacuum circuit breaker, circuit breaker, single-phase mode, identification, protection, system, development, implementation, testing
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