Fault Analysis of a Ladle Furnace Transformer caused by Switching Transient and Ferroresonance

Electric Arc Furnace and Ladle Furnace transformers are very important in steel production industry. The overvoltages caused by vacuum circuit breaker during operation are very repetitive and can lead to failures and, consequently, large financial losses. The use of protection devices, as RC filters and surge arresters, can attenuate theses transient overvoltages. However, the clamping of one of the circuit breaker phases, which is a common event to occur in a system like this, can cause failure to one of the phases of the transformer protection system, due to ferroresonance effect. Operation of this transformer without one of the phases of the RC filter and surge arrest will likely lead it to failure after repetitive overvoltages. This paper presents the study and analysis of the cause of insulation failure in a no-load Ladle Furnace (LF) transformer caused by overvoltages during vacuum circuit breaker switching transients and by ferroresonance effects. The system of a real steel mill is modelled and will be used to perform simulations in Simulink/Matlab.