The Setting Way Of Under Voltage Load Shedding In Power System

The scale of power systems continues to expand and the structure of the network is more complex. Because of environmental and economic factors'confinement, the operation of power system is closer to the limit. Many accidents of the large-scale blackout have occurred caused by voltage collapse since the 1970s due to the appearance of those cases. Power scholars take more attention to voltage stability and throw-in more and more researches. The researches include the mechanism and impact factors of voltage stability, the analysis method of voltage stability, the control measures of prevention and correction of voltage stability. It is of great significance to study on the voltage stability and the measures that prevents voltage collapse.The ultimate goal of the studying on voltage stability is developing and exploiting effective control measures to prevent voltage collapse. It is universal considered that preventing voltage collapse must be paid same attention as the prevention of synchronous operation buckling. The UVLS(under voltage load shedding) is an effective prevention and correction control measures for voltage stability. It has been not studied and applied widely in power system. So the study on UVLS needs to be further studied.The following work has been done in the dissertation:â‘ The difference between the control of UVLS and UFLS(under frequency load shedding) is pointed out based on analyzing the difference between the characteristic of voltage and frequency: the control of UVLS is for the local area; the control of UFLS is for the overall system.â‘¡Voltage stability principle is analyzed using reactive power-voltage characteristic of power system's components and the necessary conditions for large-disturbance voltage stability is obtained. The basic factor leading voltage to collapses is that the reactive power shortage can not be erased fast.â‘¢The effect of delay time to UVLS is analyzed. The delay time should be match with the adjusting speed of the controllable reactive source. Several necessary conditions of effective control of UVLS for voltage stability are pointed out: 1) The balance point of reactive power appears after load shedding; 2) Load shedding is in time; 3) The adjusting speed of controllable reactive source is fast enough.â‘£The simulated system that is used to study the principle fo voltage stability is created. It contains synchronous motor, SVC(Static Var Compensator), constant impedance loads and asynchronous motors. A typical fault is set and analog computation is proceed. The specific program of UVLS is designed and the value of voltage setting and the load shedding quantity are determined.