A novel criterion on judging long-term voltage stability

Based on the quasi-steady-state (QSS) time domain simulation and the quadratic approximation of the stability region boundary, this paper proposes a criterion on judging long- term voltage stability, and analyzes the long-term voltage stability of a one-machine and one-load system including a multiplicative generic load recovery model and an on-load tap changer (OLTC). Simulations show that the proposed criterion has the advantages of high accuracy, high computing speed, high engineering practicability and easy to implement, and can be applied to obtain the margin of the long-term voltage stability, estimate the critical line/load recovery time of long-term voltage stability and obtain the weakest load bus in complex systems.