  In practice, uncertainties in parameters and model structures always cause a
gap between a model and the corresponding physical entity. Hence, to evaluate
the performance of a model, the bounds of this gap must be assessed. In this
paper, we propose a trajectory-sensitivity--based approach to quantify the
bounds of the gap. The trajectory sensitivity is expressed as a linear
time-varying system. We thus first derive several bounds for a general linear
time-varying system in different scenarios. The derived bounds are then applied
to obtain bounds of the model gap for generator plant models with different
types of structural information, e.g., models of different orders. Case studies
are carried out to show the efficacy of the bounds through synchronous
generator models on different accuracy levels.
