Analiysis On The Power System Line Oscillations

For economic and security reasons, an increasing number of power grid began to interconnect. The common experince of the power grid development in all over the world is the interconnection between the large power grid. The operation experience after interconnection had shown that the greatest threat to the safe operation is the damage to the stability, the most prominent problem is the small signal stability. It is showed that in many cases, the limits of the power transmission in the interconnected power gird are mostly depending on oscillation but not other power flow factors. The stability of the system and the safety of the operating units are heavily affected by low-frequency oscillation, and if the stability of the system is damaged, it may cause the power cut in one or more areas, and for large power grid interconnection, it may lead to even more serious consequences. For the interconnected power system, particularly the long strip structure of the weak Internet communication network, the issue of low-frequency oscillation appears to be particularly prominent. The low-frequency oscillation of AC-DC interconnected power system is one of the important issues that threated to the security and stability of power grid operation.In this paper, focusing on low frequency line oscillations that based on visual linearized model of multi-machine power system, main achievements are as follows:By using simple intuitive calculation method, the visual linearized K₁～K₆ model of multi-machine system described by third-order model of the power generator is derived, this model is based on the expansion of a single-machine infinite bus in the traditional linearized K₁～K₆ model, it is better suited to the requirements of modern power system analysis, the model is simpler and the calculation is more convenience. At the same time, the visual linearized C₁～C₁₅ model of multi-machine system describe by the fifth-order generator model is introduced, the foundation of the analysis for power system by using different models is provided.Based on the visual linearized model of multi-machine system, the incremental expression of node-voltage of the generators, non-generator node voltage and line power is derived, the line power oscillation is analysised by using the mathematical method, which can not be provided by the visual linearized model of multi-machine system. The possibility of a comprehensive analysis of small signal stability in power system is provided by deriving of these expressions.Leading the dominant degree to the analysis of line power oscillation, comparing to the traditional methods of the eigenvalue analysis, by introducing the concept of dominant degree, it can performance better characteristics of the system, and provide a new way for the stability analysis of the system.Based on the visual linearized model of multi-machine system, eigenvalue analysis of small signal stability and line power oscillation are programming by MATLAB. Small signal stability of the 6-machine power system is analysised through the written programme. The results show that the curves of line power oscillation consistent with the results of the theoretical analysis, and analysis of small signal stability with the introduction of the dominant degree also has a degree of correctness.