Research On Control Technology Of Power System Low Frequency Oscillation

With the global climate change and rapidly growth of population, the demand of electric energy is continuously increasing globally, which made the load of power system heavier continuously. Meanwhile, power system has stepped into a new stage of large power grid, large generator unit and interregional interconnection, and long distance interconnection electric transmission lines exist extensively. These factors lead to the generation of low frequency oscillation easily. If power system low frequency oscillation can not be suppressed effectively, it will cause cascading outages, large area power cutting and big losses. Due to continuous development of power system and its nonlinear, time-varying, strong constraint and close coupling characteristics, many problems of power system low frequency oscillation control technology are not solved totally and worthy of further research. From the actual needs of power system, the control technology of power system low frequency oscillation is studied.In this dissertation, the mechanism of power system low frequency oscillation is studied. In-depth studies of the production mechanism of low frequency oscillation are helpful for adopting effective control method for suppression of low frequency oscillation. The research status of the mechanism of low frequency oscillation is analyzed in this dissertation. Negative damping mechanism, resonance or harmonic mechanism, and nonlinear mechanism are discussed separately, and the particular features of each mechanism are analyzed. Then various influencing factors of generator damping coefficient are analyzed based on the analytical expression of generator damping coefficient. Finally, the damping coefficient mechanism of power system low frequency oscillation is put forward, and the specific conclusions of the influence of damping coefficient on low frequency oscillation are obtained based on the motion equations of generator rotor.The practical model for power system dynamic research is solved in this dissertation. During the research, the basic power system equations are processed by small deviation linearization and mathematical solution, then the third-order state equation mathematical model for the single-machine infinite-bus power system is obtained, the state variables of the model are local quantities which are easy to measure, and the concrete steps of model solution algorithm are given. At last, the mathematical model of Kaifeng power grid is obtained referring to practical data, which lay a foundation for the design of decentralized controllers based on mathematical model.In this dissertation, the design method of the optimal excitation controller based on the damping ratio of a system is presented. In view of disadvantages of conventional multivariable feedback optimal control, the optimal feedback gain matrix based on the damping ratio of a system can be solved according to the invariance of undamped mechanical oscillation frequency, and then the design method of optimal excitation controller based on the damping ratio of a system is obtained. Finally, the simulation study is carried out on Kaifeng power grid and Yudong power grid. Simulation results show that the controller designed has better control effect relative to the conventional optimal excitation controller and power system stabilizer.Combing the new generation intelligent control technology, the power system intelligent control based on Mamdani fuzzy inference is studied in this dissertation. The proportional integral derivative (PID) excitation controller based on Mamdani fuzzy inference (MFPID) is designed according to PID excitation control principle and the composition principle of fuzzy PID excitation controllers. The detailed design process of fuzzy logic unit based on Mamdani model and algorithm implementation of MFPID controller is presented. Combing advantages of conventional power system stabilizer (PSS), the segmentation switch control strategy is proposed by combing PSS and MFPID further. Finally, the effectiveness of the controller designed is illustrated by simulation.