Study On The Voltage Stability On-Line Monitor Based On Local Index

In recent years, several foreign large-scale blackouts induced by voltage instability happened, which caused great economical losses and serious sed-effects on people's lives. Voltage instability presented the features of sudden and concealment. Once it happens, it is difficult for power system to survive, whereas the existing voltage stability assessment methods still cannot meet the requirements of real-time analysis. Nevertheless, the assessment method based on voltage stability index is beneficial to on-line real-time application, which just used the relevant measurements and had lower calculation complexity for the network solution. Therefore, it has important theorical and practical significance to study the voltage stability index and the assessment method based on voltage stability index, which provide a new thoerical means for on-line monitoring, preventive and corrective control to avoid voltage collapse in power systems. In this thesis, power system voltage stability local index and its application were focused on, and the issues related to the Thevenin equivalent parameters identification in power systems, the construction of local index of good performance and the application of local index in multi-machine systems were mainly studied, which further developed the voltage stability assessment method based on local index, and contributed a lot to the progress of the realistic application of measure-based local index in the voltage stability real-time assessment of power system.By adding uncertainty to the dynamic model and measurement model, the state-space description of Thevenin equivalent systems with uncertain models was defined. Based on the defined model, altogether with the robust estimation in the state-space and the adaptive modification of prediction covariance matrix, a new algorithm called adaptive robust kalman filter was proposed to track Thevenin equivalent parameters. The uncertain models considers the effects of time-varying factors as operation models, disturbances and nonlinear loads on Thevenin equivalent parameters in power systems, and the new algorithm has good robustness and powerful capabilities of anti-jamming. Otherwise, to solve the problem of divergence of Thevenin equivalent parameters identification caused by the round-off errors in the computer, the measurement updating andtime updating using the UDU~T factorization were used, which guarantee that the filtering has favorable numerical stability.Based on a single source power system, a node load margin index for on-line monitoring voltage stability was proposed to overcome the limitations of existing voltage stability indices, in which a sign function was used to reflect the operation segment. Not only the load margin at a bus, but also the position of operating point--the upper or lower half segment of P-V curve-can be reflected accurately by the proposed index, which also has the characteristic of prelinearity and is insensitive to small load variations in the vicinity of the bifurcation point. Besides, it was found that the voltage instability region sensitive to the reactive power support identified with the reactive load margin index is the same as the one with modal analysis method.To extend the voltage stability index as the above-mentioned load margin index to the multi-machine system, two methods were proposed to compute the equivalent single source system respectively. The first one directly use measured phasors at one operation state to compute the equivalent single source system, in which the equivalent load only included the effects of relevant loads in the surrounding. Compared with L index, this expression reduced the sed-effects of transferred load with improving the index accuracy of voltage stability index, and could also be used to monitor tie buses. Based on the first one, the second only used the information from the surrounding power grids of the monitored bus to compute an equivalent single source system, and the effects of the other parts as the external grids were identified with the information at several operation states by the proposed adaptive robust Kalman filter. Its initial guesses were set to be the computed equivalent system parameters, and the invalid estimated values were filtered out by the estimated scopes of parameters and the voltage magnitude changes at the monitored bus. This approach improves the index accuracy when persistent excitation condition is not met, and can be used to monitor tie buses, and thus further develops the method of extending the voltage stability index to the multi-machine system by Thevenin theorem.In power systems, the load at bus often varied with the change of the bus voltage. Considering the fact that load characteristics affected the voltage stability, by introducing the characteristic equation of saddle bifurcation in a single source power system, an index taking the static load characteristics into account was presented. This approach discarded the method of establishing voltage-stability index by the real-root existing conditions of equations or the determinant of Jacobin matrix and the proposed index accurately reflected the influences of static load characteristics on voltage stability. While computing the local index, NLEQ-RES (global Newton method with residual based convergence criterion and adaptive trust region strategy) was applied to improve good convergence rate and global convergence property for the nonlinear equations.