Studies On Dynamic Load Characteristic Modeling And Its Influence To Voltage Stability

Electric load dynamic characteristic has great relation to the dynamic behavior of power system, and it is the key factor of causing the voltage instability problem. This dissertation makes the load dynamic characteristic modeling and its influence to voltage stability as keystone; and its research contents include load time-variation character problem, classification and synthesis for dynamic load characteristics, analyzing model and method for small disturbance voltage stability, analyzing model and quick judgment method for transient voltage stability.The classification and synthesis for dynamic load characteristics is used to settle the time-variation character problem in measurement-based load modeling. Based on the random process correlation theory, a new dynamic load characteristics classification method is presented. It uses the field measured response space as the character vector space of dynamic load characteristics. First, the measured response of each dynamic load characteristics sample is standardized in time coordination, then the correlation coefficients of all the samples in measured response space is computed, at last, the system clustering method is used to make the samples classification. The measured response can reflect the load composition character of each sample very good. The project instance shows that the classification result of the method is veracious, and the realization of the method is convenient.In order to build the common load model of a dynamic load characteristics classification, a new dynamic load characteristics synthesis method that directly synthesizes the field measured load characteristics is presented. This method uses the center of gravity of all the samples in a dynamic load characteristics classification as the clustering center of this classification, and by doing parameter identification of the clustering center equivalent sample, the synthetic load model of this classification is gained. The load modeling practice showed that the synthetic model gained from this method can represent different dynamic load characteristics samples in a classification very good, and the method has the advantages of clear physical meaning and small computing. Moreover, the distinguish analysis is used to determine the classification ascription of the new dynamic load characteristics samples, and the new clustering center of each classification including new dynamic load characteristics is gained, then the amendatory synthetic load model parameters of the corresponding classification is also gained. In this way, the amendatory synthetic load model parameters can reflect the new changes of actual load characteristic, and the nicety of synthetic load model parameters'representing the load characteristic is improved.The Thevenin equivalent circuit is used to represent the system side as seen from a load bus of power system at any time, and the third order induction motor paralleled with const impedance dynamic model is used to represent the load, the differential and algebraic equations (DAE) model of analyzing the voltage stability of the system is built up. And the analytic expression of the state matrix of the linear small disturbance equation of the equivalent system is deduced. The relation of the dynamic load model parameters and the position of the small disturbance voltage stability limit point in the PV curve are systematically analyzed. The conclusions that the essence of the small disturbance voltage stability of the equivalent system is the steady stability of the induction motor part in load, and the small disturbance voltage stability of equilibrium points of the system is the same whether first order model or third order model is used to represent the induction motor part in load are gained. So in the condition of the first order model is used to represent the induction motor part in load, a new on-line small disturbance voltage stability state index Ls is presented. The analytic expression of Ls is deduced. The instances analysis shows that the computation of Ls is small, the linear characteristic of Ls is good, and the variation magnitude of Ls is big, so it is suitable for on-line reflecting the voltage stability degree of load buses at the current operating point of the system.A method which can quickly determine the transient voltage stability of power system considering the third order induction motor paralleled with const impedance dynamic load model is presented. The general representation of the mathematics model of power system with this dynamic load model is deduced. Aimed at the problem of choosing the initial value when the Newton method is used to compute the controlling unstable equilibrium point(CUEP) of the post fault system, a practical and rigorous solving scheme is presented: by identifying the controlling load bus of the given fault, and using the Thevenin equivalent circuit to represent the rest of the system at the state of the stable equilibrium point(SEP) of the post fault system, and then using the torque characteristics of induction motor, a point near the CUEP is gained to be the initial value. The second order normal form is used to approximate the stable manifold of CUEP, and the approximating local boundary of the region of attraction of SEP is gained. The instances analysis shows that the nicety of the method is high when it is used to determine the transient voltage stability of power system.