Power-Electronic Transformer Tap-Changer for Increased AC Arc Furnace Productivity

The productivity of AC electric arc smelters widely used in the nonferrous metals industry is related to the arc voltage. Attempts to improve productivity with longer arcs and higher arc voltages give rise to power fluctuations which cause voltage flicker and frequent arc reignition failures. The furnace power is commonly regulated by moving the electrode rods to adjust the arc length. A 100 MW AC arc furnace power supply equipped with semiconductor switched regulation windings which greatly enhance furnace power control bandwidth is presented, along with suitable winding arrangements, semiconductor topologies, harmonic filters and commutation methods. This system was simulated together with a Cassie-Mayr dynamic arc model. The simulation model as well as the four-step commutation sequence has been experimentally verified by building and testing a single phase 230 V, 2 kW PWM modulated tap-changer. The simulations and experiments demonstrate that power-electronic transformer tap-changers with the ratings necessary for AC arc furnace power supplies are within reach of current semiconductor technology. They are significantly more effective at AC arc furnace power regulation in terms of device rated power than controlled series reactor approaches.