Voltage Stability Analysis Of AC/DC Power System Based On Quadratic Model And Interior Point Method

For its technological and economic advantages, Direct Current transmissiontechnology is now playing an important role in the long distance and large capacity powertransmission and the interconnection of district power grids, and will be critical for thedevelopment of national electricity industry. While the combination of DC and ACtechnology in electricity transmission has already become a certain tendency, it also bringsa series of new problems, among which the voltage stability of the AC buses in convertorstation is a crucial one. DC transmission system needs to absorb huge amount of reactivepower while it provides the active power for the receiving AC side, which will result in alack of reactive power in the AC system and worsen the voltage stability. Precise evaluationabout the voltage stability of AC/DC system is called for in order to assure that the systemcan operate safely and steadily. Therefore, it is necessary to investigate the model andalgorithm used to conduct the evaluation of the voltage stability in the AC/DC system.Available methodologies used to analyze voltage stability in AC/DC system areconcluded in this thesis first, and their advantages and disadvantages are listed, too. Due tothe variety of the control and operation modes of DC system, the analysis of voltagestability is really complex, and there is no systematical method presently. Many of themethods are merely the results of popularization of their counterparts in AC system.For the purpose of improving the speed of calculation of the voltage stability inAC/DC system, a quadratic nonlinear programming model which considers various dcsystem control modes is proposed in this thesis. Hessian matrices in the model are constant,so it does not need to be updated during the iteration process of the interior point methodand will reduce the amount of the calculation greatly. Primal-dual interior point method isused to find a solution for the model proposed, its detailed equations and steps are given.Analytical examples are used to verify the effectiveness of this model and goodconvergence characteristic has been observed.Finally, based on the proposed model, this thesis uses interior point method to find andcompare the load margin when reaching the voltage stability critical points under differentDC system control modes and reactive power compensation modes.