Study And Application Of Electric Arc Model For AC Electric Arc Furnace

In the operation of electric arc furnace, the electric arc resistance being typically nonlinear in its nature is responsible for the load current distortion and the production of harmonics which are detrimental to the power supply system. In the melting stage, the voltage fluctuations and flicker as a result of quick changes in the reactive power consumption are connected with random variations of the load current which are mainly caused by arc length acute fluctuations. Especially in recent years, as the further increasing of electric arc furnace power input ratings and capacity, so do these power quality problems as a result of this progress. Therefore, it is necessary to develop an accurate and easy-to-use electric arc model for the purpose of minimizing these large power quality problems on the electrical network. Meantime, this accurate and easy-to-use electric arc model can lay a foundation for the development of electrode system model for the electric arc furnace. The electrode sysem that is a key link of electric arc furnace steelmaking is a highly nonlinear, time-varying, serious-coupling and strong-random-disturbance system and its unstable operation can absolutely lead to the increase of power consumption per ton steel, the reduction of steelmaking efficiency and the impact of power quality. Furthermore, the more advanced control algorithm can be investigated by utilizing the electrode system model. These studies on the electric arc model of alternating current electric arc furnace, parameter determination and model applications are carried out by taking the 40t alternating current electric arc furnace as the research background in this dissertation.Aiming at these problems about the lack of universal electric arc model and the indefinite physical meaning, the impedance characteristic of electric arc is analyzed initially and the main cause that the operating reactance is more than the short circuit reactance in the electric arc furnace operation is obtained. Afterwards, a new time domain electric arc model of electric arc furnace in the form of a nonlinear differential equation is developed on the basis of the energy conservation and the electric arc physical knowledge, which takes the arc conductance as state variable and the arc instantaneous current plus arc length as input.According to the question that parameters in the electric arc model are difficult to determine, the variation range of model parameters is derived from mechanism analysis and then the adaptive mutation differential evolution algorithm is applied to estimate the value of model parameters by using the actual data on the basis of the above mentioned variation range. The proposed determination method, compared with the measurements taken from field, is validated.In the light of these problems on power quality aspects introduced by the electric arc furnace, the electrical system model that is composed of the power supply system model and three electric arc models is put forth. Then, these power quality problems of electrical network can be simulated and analyzed by means of the electrical system model. Based on the comparison of simulation results with actual recorded data, the conclusion can be easily drawn that the new electric arc model may be used effectively to study harmonics, the voltage fluctuations and flicker. In order to take precaution against these power pollution problems, the compensation scheme is put forward for the load compensation using thyristor controlled reactor (TCR) and fixed capacitor (FC) type static var compensator (SVC) device. The simulation results demonstrate the effectiveness of this presented scheme.In order to investigate the electrode system model, the hydraulic system model is built firstly. By using the relation between the piston displacement and arc length, the electrode system model can be established, which consists of three hydraulic system models and the electrical system model. The complete electrode system model can be considered as controlled object and the fuzzy-integral compound controller based on variable universe is designed because of the various control performance requirements of electrode regulation in the different metallurgical stages, the controlled object’s own characteristics and the steady state error caused by the variable universe fuzzy control and deadzone. Based on the simulation results and practical applications, the developed controller may be proved to be reasonable. It also indicates the new electric arc model can be useful for the electrode system model and control.