Protection of arc furnace supply systems from switching surges

An arc furnace supply system consists of one or several furnace transformers connected to a larger supply transformer through cables and overhead lines. In a conventional arc furnace, controlled arc between three phase electrodes heats a scrap material to the melting temperature. Due to the nature of the metallurgical processes involved, arc furnace transformers are very frequently energized and de-energized at their higher voltage side. Several transformer failures related to switching transients have been experienced in different arc furnace supply systems. The paper presents a practical approach to study switching transients in actual arc furnace systems. Both field tests and computer simulations have shown that current chopping and repetitive reignitions in circuit-breakers result in severe overvoltages at transformer terminals. An RC filter can reduce the risk of failure due to switching surges and improve the transformer protection. A practical approach is presented for selection of the optimum filter parameters.