Power System Static Voltage Stability Study

During recent years the study on voltage stability has already received widespread attention of many researches. Concentrating on the static voltage stability, this paper summarizes the common calculation methods of static voltages stability and pays more attention on the static voltage stability criterion and the static voltage stability index. Many other matters related to static voltage stability are also discussed. Firstly, the paper reviews (?)P/(?)V criterion on a PV curvesimply, discusses the effect of load characteristics on static voltage stability. The paper deduces the condition of static voltage stability with regard to load characteristics on basis of generalized Jacobian matrix, then derives practical criterion of static voltage stability with regard to load characteristic, The paper points out that the system voltage stability operating at up-region of the nose curve depends mainly on the transmission characteristic of the system and the system voltage stability operating at down-region of the nose curve depends greatly on the load characteristic. The relationship between power system voltage instability, voltage collapse and load instability is proposed. That voltage instability and voltage collapse are two different concepts is pointed out. Voltage collapse is not the single final result of voltage stability. The power system voltage stability is an own capacity of the system keeping voltage at bus. Whether the system is voltage stable depends on the transmission characteristics of system's own and is not concerned with the load characteristics. Whether the voltage collapse takes place after the voltage instability relies on the load characteristics. The system voltage instability is different from the load instability, whether the load is unstable relies on that the voltage magnitude at the operating point is acceptable or not. Steady-statevoltage stability indices of power system are summarized and compared in this paper, based on Thevenin theory, the whole system can be equivalent to a simple 2-bus system and on this basis the voltage stability analysis is carried out and a method by which the stability of nodal voltage can be quickly estimated with the defined nodal voltageindex VSI and critical load factor k* is obtained and is expanded tothe situation taking account of load characteristic and reactive power limits. Simulation results shows that the method is simp^ fast and effective. Lastly, influences of many factors on voltage stability are discussed from system characteristic, the obtained result is useful for how to form the index, and it provides the principle for taking the measure to improve voltage stability.