Synthesis and Evaluation of Fast Onload Multitap Changers for Flicker Compensation in AC Arc Furnaces

This paper evaluates the abilities of fast tap-changer topologies by using insulated-gate bipolar transistors to mitigate flicker generated by the ac electric arc furnaces, based on technical and economic criteria. The design problems derived from the very high powers and currents involved have been solved to a preliminary industrial level. The operation of the best resulting topology is verified on a real 80-MVA arc furnace with a dedicated graphical-mathematical multilevel simulator developed to perform short-and long-term tests. Selected pattern windows of the arc voltage, taken from exhaustive logs of electric variables of a real 80-MVA furnace during a complete meltdown process, have been loaded into the simulator. This novel simulation strategy enables the testing of the proposed compensator under reproduced "real working conditions" with several degrees of flicker severity. The simulator includes an IEC flicker meter capable of checking the flicker level online at any point in the installation. The simple open-loop and closed-loop control tests confirm the new tap changer as a fast and economical competitor to other arc furnace compensators. It manages 16% of the furnace system power at a subcycle or super-cycle tap changing rate (between 0 and 10 changes per half cycle), as required. This fast switching capacity reduces the compensating time to the very limited values imposed by the L/R time constant of the furnace installation.