Power System Small Signal Stability Region And Low-Frequency Oscillations

Constructing regional power grids improve significantly the ability of exchanging energy between different regions. But the weak structure of the electric grids interconnection makes the power systems security problem more serious. Currently, more attentions are paid on low-frequency oscillations which had happened several times in system. To give more insights of low-frequency oscillations, we systematically investigate the characteristics of Small Signal Stability Region (SSSR). A novel view of explaining the power oscillations through tie-lines between regions is proposed.First, we show the relationships between critical oscillation modes and the points on boundary of Small Signal Stability Region. It is found that the jumping of the critical oscillation mode and Degenerate Hopf bifurcation are two reasons that could lead to sudden changes of the shape of Small Signal Stability Region's boundary.Then, we analyze the effect of changing equipments'parameters as well as load models on Small Signal Stability Region. The experiential law about parallel of Small Signal Stability Region's boundaries is discovered that can be explored to get Small Signal Stability Region's boundary using inserting value method. By comparing the Small Signal Stability Regions when using different static load models and induction motor load model, it is concluded that induction motor load may deteriorate the small signal stability of system.Thirdly, a method based on fitting function is developed to get Small Signal Stability Region's boundary. After comparing hyperplane fitting method and quadratic fitting method, we show that the boundaries computed by hyperplane fitting method can meet the engineering precision and this method also can save computing time. As the shape of Small Signal Stability Region's boundary may change suddenly, we propose a classifying fitting strategy combining with hyperplane method which improves the fitting precision efficiently.To identify the hyperplane fitting method and the characteristics of Small Signal Stability Region that we have found before, the Small Signal Stability Region is computed in the interconnection of state power grid of China. The small signal stability of system is examined when the power system works at different situations. According to the location of operating point in Small Signal Stability Region, the scheme of generator dispatch could be optimized to avoid low-frequency oscillations in system.At last, a view of the power oscillations through tie-lines is proposed. Since this type of low-frequency oscillation may be resulted from an outside disturbance, we study on the forced oscillation theory at a simple power system model. Research shows that a local persisting period disturbance could lead to an oscillation all over the system. The rules summarized from analysis of forced oscillation and persisting period disturbance show that cleaning the source of disturbance is the available measurement to eliminate forced oscillation. We simulate period disturbance in power system and point out that the nature modes with weak damping are main causes that lead to the low-frequency oscillations.