RC filters employment for the protection of industrial arc furnace transformers during switching-off operations

摘要

The transformers employed in steel plants to supply industrial electric arc furnaces have to suffer switching on and switching off operations many times per day, in order to ensure the required steel production. These operation can occur up to one hundred times per day and can generate dangerous over-voltages. It is quite obvious that a failure of the transformer would end up in a relevant economic damage, first for the machine itself, and moreover, due to the production that would be lost. That's clear then the necessity to design exactly the devices used to protect the transformer from over-voltages and voltage oscillations that can occur at the arc furnace transformer terminals. These devices are RC filters and surge arresters, and are employed to smooth the dangerous over-voltages that could damage harshly the insulation materials of the transformer. Furthermore, the RC filter protection device have to prevent current re-striking when interrupting the circuit and the consequent voltage escalation phenomenon that could occur in the vacuum circuit breaker employed in this kind of plants. The following study deals with the problem of switching off transients of electric arc furnace transformers, in particular taking care of the possible occurrence of breaker re-striking during the switching operation. This study shows the potential simulating the plant circuit switching off operation to correctly design RC filters for furnace transformers.