Research Of The Oscillation Mode Of The Interconnected Power Grid

As the development of the connection between power systems and the inducing of the power economy, the scale of the system is becoming larger and larger. Then the phenomenon of low frequency oscillation (LFO) happens regularly, being a huge threat for the safety and stability of the power grid. Because the operation mode of the power system changes frequently, some problems can not be solved by the outline calculation. And because of the large scale of the connected system, and the data opacity between different power system, it is a big difficulty for the online supervise and analysis of the low frequency oscillation. Then the paper is aimed in supporting some theory and guidance for the online analysis of the LFO. So the main content of the paper is below.First of all, the model of the generator used in LFO analysis is researched. It is proved that the classic two-step model of the generator can incarnate the electric machine oscillation mode instead of the six-step detailed model theoretically, so the dimension of the state matrix is reduced consumedly. Then the characteristic of the matrix latent root of the multi-machine system and the relationship between the oscillation mode and the parameter of the generator are analyzed.Secondly, the oscillation mode of the connection line of the four-machine two- area system is researched. By analyzing the power oscillation curve of the interconnected line using the Prony analysis, the main mode of the oscillation curve is calculated. Then it is compared with the interarea oscillation modes calculated by the small signal stability calculation, finding that the main mode is the same with the interarea oscillation mode. Then the influence of the machine inertia time coefficient and the damping coefficient to the oscillation mode is studied. The result demonstrates the former affect the oscillation frequency and damping ratio, but the later just influents the damping ratio. Based on the result mentioned above, the simple equivalence of the outside grid is studied, using the load, generator and the generator paralleled with the load equivalent to research the different of the LFO mode between the original system and the equivalent system. The simulation results demonstrate that the generator and the generator paralleled with the load equivalence can preserve the interarea LFO better than the load equivalence. Finally, the small signal stability calculation of the real power system of the center china and the north china is carried out. Then the power grids except the Hubei power grid are equivalent as generators to form a equivalent system. The power flow of Hubei grid, the voltage of the boundary nodes and the power flow on the interconnection line are the same as before. Then the small signal stability calculation of the equivalent system is processed too. Compared with the two results, the later can preserve the interior small damping oscillation mode of the Hubei grid and the interarea mode of the different power grid.