Researches On Theory And Some Key Problems Of Power System Voltage Stability

In recent years, much attention has been attracted to the problems of the electrical power system voltage stability. Because of continuous increase of power demands and large scale system interaction as well as considerations of both economic benefits and the environment protection, modern power systems are operated more and more close to their maximum operation conditions. Under such conditions, voltage instability of the systems likely occurs. This dissertation addresses issues in the voltage stability of the power systems.The present investigation on voltage stability is summarized. Definition and classification of voltage stability are introduced and summarization and argument have been given on the static and dynamic analysis method. Static and dynamic models of the load as well as its influence on voltage stability are introduced. Several typical scene of voltage instability are analyzed and the control measures are given.The continuation algorithm of the nonlinear equations is discussed, which can be used to study the variation of the equilibrium point of dynamic systems. The original parameter continuation algorithm, the arclength continuation algorithm and the pseudo-arclength continuation algorithm are introduced, which are generally used in numerically analysis of the dynamic systems. Application of the homotopy continuation algorithm and the locally parameterized continuation method on the voltage stability are studied.Based on the continuation algorithm, the dynamic voltage stability using small signal analysis method is investigated. The equilibrium manifold is tracked by a locally parameterized continuation method with a prediction-correction process. The dynamic stability of each equilibrium point is analyzed using the small disturbance analysis method considering the dynamics of the system components. System state matrix is calculated numerically by applying small perturbation to each state in turn. Simulation results show remarkable differences in the stability critical point between the small signal perturbation dynamic analysis and the static analysis. The relationship between voltage stability and angle stability is investigated. Stability type is identified by modal participation factors of the state variables. State variables are voltage magnitudes and angles in static stability discrimination and they correspond to dynamic component state variables in dynamic stability discrimination. Simulation results demonstrate that stability type can effectively identified by the proposed method.Voltage stability margin (VSM) of power system can be improved by generator reactive power generation rescheduling. The optimal management of the var generation is formulated as an optimization problem and pseudo-gradient evolutionary programming (PGEP) method is used to obtain the optimal solution. Modal analysis techniques are used to determine the searching direction. Compared with the standard evolutionary programming (SEP), better solution can be obtained, and the convergence speed of the algorithm is improved as well. Simulation results show that the reactive power reserves of the system is increased and the active/reactive power losses are decreased after the optimal reactive power rescheduling. Consequently, the VSM of the system can be improved without adding new var compensation equipment.An algorithm about voltage stability assessment is proposed based on wide area measurement system. Only local synchronized phasors are used in the proposed method, which includes the synchronized phasors of the target bus and the coherent buses. The overall system is converted into a Thevenin's equivalent two bus system by multiple measured synchronized phasors. The relationship of load impedance magnitude and the Thevenin's equivalent impedance magnitude is analyzed when voltage stability problem occurrs. A voltage stability index based on this relationship is given. The simulation results are used to show the effectiveness of the proposed method.