Voltage Stability Analysis Of AC/DC Power Systems Based On Bifurcation Theory

As a result of its technical and economical advantages, HVDC technology has been playing an important role in the development toward great power grid, high voltage and long distance transmission of power systems. Although AC/DC combined transmission system has been the inevitable trend, it has also brought new challenge. Especially, the voltage stability problem of weak AC system connected to HVDC system becomes an increasingly serious one. It is necessary to put out a kind of theoretical model to analyze voltage stability of AC/DC power systems, which can serve for the design and security operation of power systems.Power system is a large, complicated nonlinear dynamic system. It has been proved that bifurcation theory is one of effective tools for studying the structure stability of nonlinear dynamic system. In recent years, bifurcation theory has been widely applied in many engineering fields, and a great deal of results have been obtained, but it is only in the initial stage for the application of voltage stability in power systems. Based on the bifurcation theory and the eigenvalue analysis approach for small disturbance stability, a static and dynamic bifurcation analysis method for voltage stability of AC/DC power systems is introduced in this thesis after the study of on the algorithm of power flow and bifurcation points. Main research works and contributions are summarized as follows.Firstly, according to two kinds of HVDC models, the corresponding differential algebra equation (DAE) models of AC/DC power systems are modeled for voltage stability analysis, following the mathematic models of generators, loads and HVDC given in this paper.Secondly, a new power flow algorithm of AC/DC -"partial DC variables elimination method"is proposed in this thesis on the basis of assesment about the existing algorithms. The new algorithm not only keeps the advantage of classical unified approach-good convergence, but also overcomes the disadvantage of "DC variables elimination method" -complicated programme design for dealing with control model convertion of HVDC system.Thirdly, the identification method of Hopf bifurcation point is studied in this thesis. After review on normal methods to calculate SNB point and Hopf bifurcation point, a new matrix reduction method is presented to directly calculate the Hopf bifurcation point with the resolution-iterative-technique, which is verified to be more efficient than the classic direct method in the samples. Meanwhile, when the continuation method is adapted to locate Hopf bifurcation, equilibrium points are detected inverse the direct of equilibrium manifold beginning from the SNB point, which uses the bifurcation characteristic of power systems. Sample power systems show its effecience. In addition, the procedure of bifurcation point identification in AC/DC power systems is stated in detail.After power flow and bifurcation points of AC/DC power systems are obtained, the static and dynamic bifurcation analysis method for voltage stability is proposed by the use of byproducts from biburcation point calculation, combining the eigenvalue approach for small disturbance stability. Three classic examples are given in order to verity this method and to illustrate the procedure of voltage stability analysis in this way.Finally, HVDC's influence on the converter bus voltage stability is analyzed in the method presented in this paper. Its results show that it is necessary to select proper control parameters and models of DC system for the security of AC/DC power systems.