Research On Out-of-step Splitting Criterion And Strategy Of Power Systems Considering Oscillation Center Migration

With the expansion of the interconnected power systems, mutual influence of regional grids has been made more complex over time. Some fault in local grid could be spread throughout the whole system through the tie lines, which may result in the blackout. Recent years, blackouts happened frequently at home and abroad, which showed that the traditional third line of defense was hard to adapt to the large scale power system's development demand. Out-of-step splitting control is important guarantee to prevent the blackout as the important control measure of the third line of defense. But the traditional out-of-step splitting control is hard to adapt to the complex grid operation modes influenced by its calculation way, fixed installation site, local control and lack of global information. So how to split the grid quickly and efficiently has important significance to the grid security when out-of-step oscillation occurs.In view of this, combined with the major projects of the national grid in2012, the reason and the factors of oscillation center migration was studied through the phenomenon of oscillation center migration. The influence on out-of-step splitting control was researched. The proper splitting criterion and strategy was considered, and out-of-step oscillation caused by cascading failures was discussed.(1) According to the principle of equal frequency difference, such as voltage, current and other electrical quantities were analyzed based on equivalent two-machine system model. Variation of these electrical quantities was considered and it provided the basis for research on splitting criterion.(2) Based on the phenomenon of oscillation center migration happened in some actual grid accident, linkages with the instability mode and oscillation center was analyzed through the simulation in CEPRI-36node system. Impact of fault clearing time for step-oscillation, the reason and factors caused oscillation center migration, dynamic migration of oscillation center was discussed. The result shows that instability mode and oscillation center is inextricably linked. The simulation result shows that the traditional generator coherent recognition method is invalid, so the improved generator coherent recognition method was proposed. It found the change of instability mode is the reason for oscillation center migration and the influence on splitting control was put forward. The simulation in actual grid proved the conclusions and it was taken as the theoretical basis of research work.(3) The theory of the splitting criterion based on ucos? and phase angle was researched. According to the phenomenon of oscillation center migration, the splitting criterion based on bus frequency difference between two terminals was proposed in the light of the link between the frequency of oscillation center and bus frequency difference between two terminals. The applicability of the splitting criterions were analyzed. The result shows that the traditional splitting criterions are invalid and the splitting criterion prospered is effective.(4) Active splitting strategy based on improved generators coherent recognition method and semi-supervised spectral clustering was proposed. Undirected weighted graph was given using the absolute value of the active power as weights based on graph theory. Coherent generator was identified using the improved method. The graph was divided based on spectral clustering algorithm with minimal impact on the trend of the objective function. The connectivity of subsystems was ensured by semi-supervised spectral clustering algorithm which considered cohomology constraint. The adaptive splitting strategy based on WAMS was proposed. Desynchronization was judged using composite criterions by receiving real-time information of bus frequency and phase angle. The grid was divided when the location of oscillation center could divide the grid into different parts. The simulation results show that the two strategies proposed are effective.(5) Using the fault chains principle, reasonable multi-disturbances were established based on the typical fault mode of flow transferring and overload of transmission lines caused by line faults. The phenomenon of out-of-step oscillation was analyzed. The result showed that the oscillation center located inside the grid which made the equipment out of action. The proposed splitting strategy can solve the problem.