  Advantages of operating portions of a power system at frequencies different
from the standard 50 or 60 Hz have been demonstrated in the low frequency AC
(LFAC) and high voltage DC (HVDC) literature. Branches constrained by stability
or thermal limits can benefit from increased capacity and flexibility. Since
advances in power electronics enable the choice of an operating frequency,
tools are needed to make this choice. In order to quantify the advantages as
functions of frequency, this paper provides models for steady state
calculations with frequency as a variable and validates the modeling
assumptions. It then introduces an analytical quantification of the power flow
capacity of a transmission branch as a function of frequency, demonstrating
different active constraints across the range of frequency. The modeling and
power flow calculations are demonstrated for a practical transmission line
using manufacturer data. The models presented here provide a foundation for
system level studies with variable frequency using optimal power flow.
