Optimisation of high voltage AC electric arc furnace control

The aim of this project was to improve the performance of AC electric arc furnaces with high transformer voltages of 1000 V and above. With stable long arc operation, advantages of high electrical power input for high productivity under lower production costs shall be utilised. Disadvantages like loss of energy efficiency and damage to water cooled panels from long arcs as well as mechanical stresses to electrodes from arc instability shall be avoided as far as possible. Based on results from earlier extensive investigations in electrical variables of AC arc furnaces, Betriebsforschungsinstitut (BFI) extended the analysis of fluctuation effects to furnaces with reactor and higher voltages. For this purpose, further measurements were made at the 145 t Bochum furnace of Krupp Thyssen Nirosta (KTN) which is equipped with a 20 MVA reactor and a 135 MVA / 1200 V transformer. It was found that transfer of arc voltage fluctuation to fluctuation of magnetic forces in the high-current system and to fluctuation of the power supply voltage can be described by the same empirical equations which had previously been derived for furnaces without reactor and with voltages below 1000 V. This indicates how to design and to operate AC arc furnaces electrically stable also at higher voltages; the ratio of arc resistance to short circuit reactance should be kept below 1.5. On this basis, a dynamic fluctuation control system was developed and implemented at a 135 t furnace with 75 MVA / 800 V transformer. Despite this moderate voltage, electrical instability had often been observed at this furnace, especially during melting of stainless steel scrap. The furnace is not equipped with a reactor to adapt the short circuit reactance. Therefore the arc resistances are adapted by modifying the current setpoints for the existing electrode controllers, depending on the actually observed intensity of current fluctuation. BFI developed graphical tools to simplify and to visualise the optimisation of furnace operation. The voltage-reactance chart serves to assign reactor taps to transformer taps with respect to electrical stability. Currents are assigned to selected tap combinations in the current-power diagram under the aspects of active power, arc voltage and radiation. In the furnace operating diagram, selected combinations of transformer taps, reactor taps and currents are assigned to the subsequent steps of the baskets according to the progress of melting. Several operating diagrams were developed, tested and evaluated at the Bochum furnace of KTN. Altogether, the steel plant reported remarkable improvements from the installation of the transformer with higher reactance, from the operation of the additional reactor and of voltages up to 1200 V, and from subsequent optimisation of furnace operation by means of the above tools: Stable operation at 1200 V and low currents with sufficient short circuit reactance. More stable operation reduced the frequency of electrode breakages by 75%. With this and with lower currents, electrode consumption was reduced by 25%. With improved operating practice, frequency of wall defects was reduced by 50%. In the first part of its work, IRSID improved the performance of the electrode regulator for arc resistance control. The aim was not to define a new controller, but to improve its adjustment.