  Aggregated load models, such as PQ and ZIP, are used to represent the
approximated load demand at specific buses in grid simulation and optimization
problems. In this paper we examine the impact of model choice on the optimal
power flow solution and demonstrate that it is possible for different load
models to represent the same amount of real and reactive power at the optimal
solution yet correspond to completely different grid operating points. We
introduce the metric derived from the maximum power transfer theorem to
identify the behavior of an aggregated model in the OPF formulation to indicate
its possible limitations. A dataset from the Carnegie Mellon campus is used to
characterize three types of load models using a time-series machine learning
algorithm, from which the optimal power flow results demonstrate that the
choice of load model type has a significant impact on the solution set points.
For example, our results show that the PQ load accurately characterizes the CMU
data behavior correctly for only 16.7% of the cases.
