  In this paper, a theoretical foundation for the Quasi Steady-State (QSS)
model in power system long-term stability analysis is developed. Sufficient
conditions under which the QSS model gives accurate approximations of the
long-term stability model in terms of trajectory and !-limit set are derived.
These sufficient conditions provide some physical insights regarding the reason
for the failure of the QSS model. Additionally, several numerical examples are
presented to illustrate the analytical results derived.
