Modeling And Control Strategy For Excitation System Based On Current Source Converter

The blackouts happened in Europe and America indicates the importance of power system stability. The power grid interconnections among the large area in our country and the increase of transmission capacity contribute to the complexity of power system, and it is generally appeared the problems that power system damping is decreased and the phenomenon of regional low frequency oscillation are frequently emerged, which lead to the higher requirements are needed for the large synchronous power grid. Because excitation systems, as one of the most effective and economical ways contribute to effective control of voltage, are one of the key components for synchronous generators, increasing attention has been focused on the effects of excitation control on the damping of power system oscillations, and the role of excitation systems in power system has been growing more importantly. Even though the implementation of PSS for the fast thyristor excitation systems could inhibit the phenomenon of low frequency oscillation, the effects are limited. Followed the evolution of power semiconductor devices and power electronics, three-phase voltage source converter and three-phase current source converter are proposed to be used in excitation system of synchronous generators, and it is called excitation system based on full-controlled devices. The biggest advantage of this new type excitation system over the conventional is that it could enhance the damping of system and improves transient dynamics of power systems due to injecting reactive power to the generator's terminal when there are disturbances in the system. In this paper, excitation system based on current source converter are mainly discussed as follows:First of all, the basic principles and mathematical model under d-q synchronous axis for main circuit of excitation system based on current source converter are described, which provides the basis for the proposed control strategy. And then, the principle of three-logic SPWM technique is analyzed. The three-logic SPWM is built in Matlab/Simulink and the simulation results demonstrate its correctness.Moreover, according to the aforementioned mathematical model, a novel suitable triple loop decoupled control strategy for CSC based excitation system is proposed to achieve fast and accurate control of energy conversion between the generator terminal and field winding in this paper. And then, the design approach for the parameters of LC filter in the main circuit of CSC based excitation system are briefly discussed. Meanwhile, the model of CSC based excitation system are built in Matlab/Simulink in order to verify the validity of proposed control strategy and design approach of LC filter in the main circuit.In addition, the modified Heffron-Phllips mathematical model of synchronous generator based on CSC excitation system is derived, which helps to explicate the principle of achieving more damping to improve the system stability by injecting reactive power to the generator terminal when there is a disturbance. And then, in order to achieve the goal of maximizing the damping torque, by adopting conventional PSS phase compensation approach, the reactive power injector is designed. And simulation results show that the proposed control strategy can quickly achieve characteristics of the excitation system. What's more, the results show that reactive power injector could provide more damping to improve stability of system, and it has a good robust performance. Most of all, CSC based excitation system could obtain a better performance than thyristor excitation systems.At last, according to several perspectives, which are the processes of start-up and de-excitation, the complexity of control strategies, costs, AC-side harmonics and filters, and the adoption, respectively, the comparative evaluation between VSC based excitation system and CSC based excitation system is discussed. From the comparison, the following conclusions could be made: For excitation system that not using cascaded circuits, CSC based excitation system could be more economical and more reliable. Also, CSC based excitation system could achieve lower THD harmonics under the same switching frequency, which is a big advantage over VSC based excitation system. As for excitation system based on cascaded current source type converter, since the topology doesn't need H-bridge, it has low cost characteristic, but the suitable control strategy is more complex. While as for H-bridge paralleled voltage-source type converter, due to its big amounts of industrial applications, low losses, and more mature control strategies, compared to the cascaded current-source type converter, H-bridge paralleled voltage-source type converter could be implemented more easily for big capacity generators right now,but the disadvantage is its high costs since the bigger needs for more amounts of switching devices.