Rationale for Thyristor Voltage Control of Electric Arc Furnace Transformers

We hereby review the inter winding circuit of the transformer of EAF 180, a 180 ton electric arc furnace used by Magnitogorsk Iron and Steels Works (OJSC "MMK"). It is demonstrated that the furnace transformer is a sophisticated unit consisting of three electromagnetic units, i.e. the reactor with an iron core, the main transformer, and the boost transformer. In its turn, the main transformer is a three-winding transformer which can adjust the load voltage in the tertiary winding. We briefly describe the ArCOS electric process control system. The system controls the hydraulic electrode positioning system and switches the transformer and reactor load voltage controllers (LVC) stage by stage. It is demonstrated that EAF might become less reliable and controllable due to low system performance. We present oscillograms of the arc current as affected by switching the transformer voltage stages; these oscillograms prove the long pauses occurring in each switching. This causes a loss of control over the melting parameters. Another drawback of the ArCOS actuators is its poor performance due to the inertia of the hydraulic electrode positioning system. We present oscillograms of transients occurring at different impedance controller configurations, which prove this finding. A promising area of research that we note consists in designing EAF transformer circuits with contactless voltage control. We analyze the proposed transformer circuit with thyristor voltage control in the tertiary winding, which uses the known concept of high dynamics EAF voltage control. The paper presents the results of mathematical modeling, which prove that voltage is smoothly adjustable in the specified range. We assess the capabilities of this circuit with respect to adjusting the primary parametric coordinates for the specified power and voltage ratios of the main and the boost transformers. We propose further research into improving performance of EAF electrical parameter control systems.