Power System Stability Limit Estimation Based On Quasi-Steady State Model And Power Flow Model

The current power system is increasing large and the amount of electricity load is rising increasingly,which has caused the grid to operate in a state close to its limit.In this case,the operating department of the operating system to determine the limit is particularly important.In recent years,a simplified power flow model has been used to calculate the operating limits of the power system.However,the power flow model may not accurately describe the operation of the system in some cases,thus requiring a more accurate model-quasi-steady state(QSS)model-to describe the power system.The QSS model simplifies the long-term model of the power system,namely the complete model.However,compared with the power flow model,the QSS model describes the dynamic components in the power system more accurately.This paper examines the load margins and bifurcation types of the power flow model and those of the quasi-steady state(QSS)model.This examination shows that load margins of the power flow model are always greater than those of the QSS model,and that the shapes of the P-V curve under these two models are almost the same when the system loading condition is light;with the loading condition increasing,the difference between the P-V curve of the power flow model and that of the QSS model gradually increases;finally,the load margin of the system under these two models varies by about 5%.This raises operational concerns regarding the accuracy of load margins calculated based on the power flow model.Physical explanations are provided for the difference in load margins between the two models.On the other hand,both the power flow model and the QSS model encounter the same type of local bifurcations,which can be either the saddle-node bifurcation(SNB)or the structure-induced bifurcation(SIB).Finally,analytical study and numerical studies of the time-domain comprehensive model were conducted and used as a bench-mark to verify the accuracy of the calculated load margin based on the QSS model.