  Recent research has demonstrated that the rotor angle stability can be
assessed by identifying the sign of the system maximal Lyapunov exponent (MLE).
A positive (negative) MLE implies unstable (stable) rotor angle dynamics.
However, because the MLE may fluctuate between positive and negative values for
a long time after a severe disturbance, it is difficult to determine the system
stability when observing a positive or negative MLE without knowing its further
fluctuation trend. In this paper, a new approach for online rotor angle
stability assessment is proposed to address this problem. The MLE is estimated
by a recursive least square (RLS) based method based on real-time rotor angle
measurements, and two critical parameters, the Theiler window and the MLE
estimation initial time step, are carefully chosen to make sure the calculated
MLE curves present distinct features for different stability conditions. By
using the proposed stability assessment criteria, the developed approach can
provide timely and reliable assessment of the rotor angle stability. Extensive
tests on the New-England 39-bus system and the Northeast Power Coordinating
Council 140-bus system verify the effectiveness of the proposed approach.
