Research On Transient Voltage Stability Region Of Power System Based On Energy Function Method

Under the background of the power transmission from west to east,with the increase of electrical load,the pressure of power network is becoming much larger.Besides,the component of load at the receiving end is very complex and causes adverse impact on the transient voltage stability of power system.When many thyristor-controlled type High Voltage Direct Current(HVDC)projects are put into use,the transient voltage stability of the system will be worsen since the inverter's characteristic of consuming massive reactive power.Therefore,the research of transient voltage stability is still one of the most important subjects in the industry.Currently,the study of the transient voltage stability is mainly based on the time-domain method.The method obtains the change curve of variables as the time increases.However,massive computation time will be consumed,and the stability margin cannot be given by this method.Therefore,another method,energy function method is concerned in this paper.Energy function method is developed based on the Lyapunov method.Though the precision and the adaptability to the model of this method is needed to be improved,as the important supplement of the time-domain method,energy function method can be used to analyze the power system stability by judging whether the initial state of the post-fault system lies in the stability region,which is one of the important method to characterize the partial boundary of stability region and can be used in the online stability analysis.The inclusion of the new components and the applicability to the transient voltage stability is the current hot topic for energy function method.This paper,firstly according to the energy conservation law,integrates the current balance equation along the system trajectory and derives the energy function including three-order generator model and simplified one-order motor model using Kirchhoff's current law.The effectiveness of the proposed energy function is validated on the three-machine nine-bus system.This method is expanded in the AC/DC system,in which the DC model is considered as the dynamic load and Lagrange's mean value theorem is used,and numerical AC/DC energy function can be obtained.Besides,based on the analysis object of transient voltage stability,a heuristic method is applied to obtain the controlling unstable equilibrium point.In the proposed method,the controlling bus is determined and the initial slip value is obtained through the torque balance equation of motor,the initial guess of other variables is the values of post-fault equilibrium point.The initial guess is iterated by the continuation method and the controlling unstable point can be obtained.Combining the proposed energy function and the obtained controlling unstable equilibrium point,the critical clearing time to maintain the transient voltage stability can be obtained.The proposed method is validated based on the open source simulation software PSAT.Besides,since the research requirement of the DC model in the PSAT cannot be satisfied,based on the mainstream DC control modes,the original codes are improved to add the constant extinction angle control model at the inverter end,which can improve the precision of simulation and make the dynamic behaviors of DC model more real.