Analysing of the Trip Current Influence in EAF’s Furnace Transformer Under Load on the Appearance of Switching Overvoltages

To date, operational switching on and off furnace transformers of arc steelmaking furnaces (ASF) and ladle-furnace units (LF) is realised by means of vacuum circuit-breakers (VСB). The main disadvantage of these vacuum circuit breakers is the characteristics of switching transients associated with repeated breakdowns of arc gaps and voltage escalation. They arise due to the phenomena of current shear and virtual current shear during switching the unloaded and loaded furnace transformer. These overvoltages can have a negative effect on the furnace transformer insulation and suitable cable lines. This paper reflects the dependencies of a number of factors that have the greatest influence on the possibility of emergency situations in 35 kV networks related to switching overvoltages. To determine these dependencies, the experimental studies of overvoltages during switching vacuum circuit breakers of electric arc furnaces at Cherepovets Iron and Steel Works of PJSC "Severstal" were carried out. Based on the evaluation results of experimental studies, a number of measures were developed to limit switching overvoltages, by reducing the current level when switching off the furnace transformer LF and special setting of emergency automation.