Theoretical Analysis Of Voltage Stability Phenomenon And Physical Properties Of Load Instability And Study On Voltage Stability Control Strategy

Voltage instability has been considered as one of the major factors which threaten the secure and stable operation of modern large-scale power system. The theoretic analysis of power system voltage stability phenomena is the foundation of unifying the cognition of voltage stability phenomena, and the research on voltage stability control strategy is the premise to improve power system voltage stability. So, the study on these two aspects is very important theoretically and practically to guarantee the secure and stable operation of power system. Based on Thevenin equivalent parameter method and nonlinear dynamical system stability theory, this dissertation investigates systematically the following contents: the analysis of the root cause of load voltage change, the character of dynamic load instability, the relationship between dynamic load instability and voltage stability, voltage stability control strategy and so on. The main achievements are listed as follows.The equivalent parameter method to analyze the voltage magnitude change is presented. In the method, based on Thevenin equivalent parameter method and the functional relationship between load equivalent impedance and load node voltage, the voltage magnitude change of load node is expressed by the algebraic sum of the system characteristic component and load characteristic component. The system characteristic component represents the influence of system Thevenin equivalent parameters change on voltage magnitude, and the load characteristic component represents the influence of load equivalent impedance change on voltage magnitude. So, the equivalent parameter method proposes a valid way to give a quantitative analysis to the cause of load node voltage magnitude change. What's more, the influence of load power change on voltage magnitude is analyzed, and the result reveals that considering the change of load power merely can not explain the voltage stability phenomena accurately.Using nonlinear dynamical system stability theory, the paper discusses the unified model of dynamic load, the definition of load stability and the natural property of voltage stability. At the same time, the paper definitely gives a clear concept of dynamic load instability boundedness and analyzes its character in depth. Using the character of dynamic load instability boundedness, the paper demonstrates the relationship between load stability and voltage stability theoretically, and proposes a scientific theory evidence for the analysis of influence of load instability on voltage stability. Combining the systematic Thevenin equivalent circuit with ideal shunt capacitor model, the paper proposes the functional relationship of the maximum electromagnetic torque of induction motor with capacitor susceptance and the motor slip. Comparing the maximum electromagnetic torque and mechanic torque of induction motor, the paper presents that the collapse process of induction motor load can be separated into controllable stage and uncontrollable stage under the control of shunt capacitor. At the same time, according to the stability condition of electromagnetic torque exceeding mechanic torque, the paper proposes the controllable criterion of the motor load stability. Therefore, the concept of the stage character of dynamic load instability proposed by the paper provides a novel theoretic evidence for the research of voltage stability control strategy.Considering the adjusting capacity of electromagnetic torque of induction motor with capacitor compensation, the paper proposes the stage stability control of SVC dynamic load. When the load loses stability after fault, firstly, the conventional proportion control mode of voltage warp is used to adjust the compensating capacitor susceptance of SVC, and during the control process, the load node voltage is computed dynamically and the controllability of the load node voltage is judged. Secondly, if the load node voltage is judged to be uncontrollable, the proportion control mode of error between electromagnetic torque and mechanic torque is used to restore the system controllable stability stage quickly. Lastly, if the system under torque control mode is in voltage controllable state again, the control mode is converted to the conventional control mode in order to restore the load node voltage to the ideal set value.