Static Voltage Stability Margin Analysis Considering Uncertainty Of Load Variation

In power system operation, there are many random disturbances or uncertain factors, such as the variation of nodal injections, the change in network configuration and the measuring or forecasting errors. By using the voltage stability analysis with uncertain factors of load variation considered in computation, more information about the critical point and the minimal power margin will be offered. Based on the bifurcation theory of differential calculus and the direct approach for the critical point computation, the relationship among loads and the changes of load constitutional ratio with time is considered for the closest bifurcation point computation, which provides evidence to avoid voltage instability effectively.In the introduction for voltage instability and the bifurcation theory, the mechanism and feature of voltage instability are reviewed and analyzed. Conception of the bifurcation theory and its application in voltage stability analysis are introduced. The purpose of voltage instability analysis is to discriminate the main factors that affect the voltage stability, to answer the problem that how the voltage instability happens, as well as the relationship between voltage stability and rotor angle stability. These will be really significance for understanding the essence of voltage instability and selecting effectively measure to defend voltage instability.In the closest bifurcation method for static voltage stability analysis, the power flow equation is formulated based on the bifurcation theory. This approach can figure out the closest collapse point, so as to give the most dangerous load increasing mode and the minimal power margin. They are the indices of judging the degree of voltage stability. In this thesis, the sufficient condition of static bifurcation and the necessary condition of saddle node bifurcation are analyzed, and the equations of the closest bifurcation method are determined. The adoption of parameter transformation avoids the singularity of the Jacobian matrix.In the computation of the closest bifurcation using the probabilistic method, the operational state in a period of time is used as the initial condition, and the load correlation is also considered in the load increasing procedure. Thus more system operation conditions or more random factors are considered into a numerical calculation.The closest bifurcation method based on the bifurcation theory for the static voltage stability analysis is introduced into the probabilistic environment for the probabilistic static voltage stability analysis. The linearization model is adopted in the probabilistic power flow, the mean and covariance are solved alternately. The correction of voltage covariances to nodal power means is considered. Information of load variation is included by using the load within a period of time. The uncertainty and correlation of load variation are obtained from typical daily operational curves and described by the statistic characteristics. Load increasing mode is not in the conventional deterministic, but with load variation included.