Modeling Operation of Liquid Metal Fuses When Breaking Overcurrents

Abstract

This paper shows that successful switching of extremely high short-circuit currents I> 50 kA can be achieved by joint operation of a liquid-metal self-resetting current limiter and a circuit breaker connected in series. The type NFU-225 device from Mitsubishi was taken as an example. The time-current characteristic of joint operation of a liquid-metal self-resetting current-limiting device and a circuit breaker was compiled. However, further in the article physical processes occurring in a liquid-metal self-resetting current limiter with a complete transformation of fusible unit are considered. The result of work is modelling of operation of liquid-metal fuses when overcurrents are switched off based on the pilot studies obtained by the Japanese scientists. It is proposed to simulate the break process not at every time moment, but at specific time moments (reference points). At other time moments, current and voltage should be considered as approximately linearly changing characteristics. The work of current limiter can be represented by three stages: the pre-arc, the main arc and the final arc. If the current density is less than 1000 A/mm2, then the pre-arc operation stage of the current limiter includes the following sections for heating the fusible unit: primary heating to the melting temperature; melting and its transition to liquid state; secondary heating to evaporation temperature; evaporation of fusible unit.