In three-phase arc furnaces, scrap metal is melted by means of electrical energy, with the conversion of the electrical energy into thermal energy that is required for the melting process taking place in the three arcs which burn between the electrode tips and the material to be melted. For process control, the operating point of the arc furnace can be varied by step-by-step adjustment at the voltage supplied to the arc furnace via a furnace transformer, as well as by step-by-step, separate adjustment, of the distances between the electrode tips and the material to be melted. The chosen operating point is maintained by controlling the distances between the electrodes and the material to be melted. This is generally done by means of impedance regulation b forming an actual impedance value for each electrode from continuously measured electrical variables such as the phase voltage and the phase current, and using the error between this actual impedance value and a predetermined set impedance value to define a manipulated variable for adjusting the height of the respective electrode.
In order to match the operating point of the arc furnace to the variable requirements for furnace operation during the melting process, those operating points which are respectively assigned to a voltage level on the furnace transformer from predetermined characteristics for furnace operation are stored, in the case of a method that is described in European Patent No. 0 036 122, as set impedance values in a digital memory. The furnace operator then manually selects suitable operating points from this digital memory during the melting process.
The presetting of the operating points and set impedance values for setting the height of the electrodes has until now been carried out as a function of previously defined process phases (first bucket, second bucket, refining) and sections within these process phases (placing the electrodes against the cold scrap, melting down, final melting on a flat melt bath), with the set impedance values in each case being predetermined and fixed during these sections. This previous procedure is based on the assumption that the conditions within a section remain sufficiently constant and are reproducible from charge to charge. Steady-state balancing of the three-phase arc furnace is also carried out on the basis of similar principles.
On the basis of relatively detailed knowledge about the behavior of alternating-current arcs in three-phase arc furnaces, one may, however, assume that the preconditions for steady-state setting of the arc furnace do not occur. In fact, it should be assumed that the conditions in the individual arcs vary even during the sections of the process phases and, in particular, unbalanced operating conditions can occur with respect to the arrangement of the electrodes in the three-phase arc furnace. If the set impedance values are then set in a balanced and steady-state manner on the assumption of balanced conditions in the arc furnace, then, if the furnace conditions are actually asymmetric, this leads to different manipulated variables which then produce arcs of different length via the adjustment of the heights of the electrodes. Optimum utilization of the power supply to the arc furnace is not possible in this case.