Process Control and Simulation of Ferromagnetic Strip in the Power Transformers and Electrical Machines Applications : Electric power systems

Abstract: This thesis investigates optimization of the control of electrical and thermal equipment by using FEM and CFD modeling in combination with dynamic simulation models. The thesis focuses on the production of electrical strips and the control system with the aim of reducing losses and improving magnetic properties. Several parameters and factors contribute to core losses. Thickness deviations in strip production, high levels of impurities in the core, orientation, ageing, surface oxidation, overloading, and hot spot temperature are among the reasons for losses in the core. Some of the losses occur during strip cutting and core assembly. This dissertation focuses on the reduction of losses in the cold rolling, annealing and manufacturing stages. The cold rolling process has a direct influence on the accuracy of the strip thickness and magnetic ageing of sheets. Some disturbances such as eccentricity, working rolls gap deviation, shape and edge deflections have to be removed in order to achieve accurate thickness. Thickness measurement makes up an important portion of loss evaluation in electrical equipment. Impurities and dirty strip surfaces in the cold rolling step can increase the carbon content of strips that pass through the annealing furnaces after cold rolling. The slab should be cleaned before reeling and rewinding. As the strip passes through the annealing furnaces, the temperature should be homogenous over the entire strip. According to simulations of furnace and strip temperature computed in the COMSOL environment, homogenous temperatures may be achieved using high electrical power reflectors which are equipped with molybdenum disilicide (MoSi2) electrical heating elements to replace the gas fired burners that are currently used. Modelling of the cold rolling process is conducted in order to find the correlation between control system parameters. A multivariable mathematical model for the rolling process is derived here, which reveals the interactions of the influencing variables. This approach provides numerically efficient algorithms, which are necessary for running in a real-time environment. A control model is applied in the MATLAB environment in order to determine the strip thickness at online-offline state using a robust algorithm. The critical problem in the thickness control loop is analysed, and an adaptive control algorithm is proposed. A number of control methods are investigated to improve the final strip properties. Cold rolled strip thickness deviations, eccentricities and shape defects are compensated for. The simulation results are verified with measurement data and the most significant sources of disturbances are detected. Finally, to solve the hottest spot problem in large scale electric power transformer, a new apparatus, oil spraying, is proposed and analysed.