Modelling and control of an electrode system for a three-phase Electric Arc Furnace

Abstract

This dissertation investigates the control of the electrical energy input to a three-phase electric arc furnace (EAF). Graphite electrodes are used to convert electrical energy into heat via three-phase electric arcs. Constant arc length is desirable as it implies steady energy transfer from the graphite electrodes to the metallic charge in the furnace bath. With the charge level constantly changing, the electrodes must be able to adjust for the arc length to remain constant. In this dissertation electric arc current is used as the control variable. This is the most often used control variable in the electric arc furnace industry and implies fast adjustments of short circuits between the electrode tips and the metallic charge. The motivation behind the modelling of the electrode system for a three-phase electric arc furnace is to extend an existing EAF model developed at the University of Pretoria. The existing model investigates the control of the electric arc furnace process itself and it is assumed that the applied electrical energy input is constant. Proportional-Integral-Derivative (PID) control as well as Model-Predictive-Control (MPC) is applied to the electrode system. Time delays on the outputs of the hydraulic actuators makes it necessary to include approximations of time delays on the outputs of the linear model, which is needed for controller design. A well known general control problem is followed in this dissertation. All models are derived from first principles, and complete controller design is carried out. Most available literature lack in at least one of these fields.