Equivalent model and parameter identification are both analysed in voltage stability analysis based on Thevenin equivalent. This paper demonstrates that Thevenin equivalent impedance is equal to load impedance at stability limiting point. Stability index based on this character provides higher practicability, while the key point is equivalent parameter tracking. Existing parameter tracking methods are compared in this paper and essence of numerical stability in recursive least squares method. A modified method considering estimation error and variable forgetting factor is proposed. Numerical simulations were conducted on 2-bus system and 10-bus system to demonstrate the effectiveness of this criterion.
