  The advantages of operating selected transmission lines at frequencies other
than the standard 50 or 60 Hz are numerous, encompassing increased power
transfer capacity and better utilization of existing infrastructure. While high
voltage DC (HVDC) is by far the most well-established example, there has been
an emerging interest low frequency AC (LFAC) transmission in applications
ranging from offshore wind to railway systems and mining. In this paper, we
investigate the use of LFAC as a transmission upgrade and propose models and
analysis methods to determine the optimal choice of frequency. The paper first
presents an optimal power flow model with frequency as a variable, assuming
modular multilevel converters for frequency conversion. Using this model, we
analyze LFAC as an embedded upgrade in a transmission system using existing
lines. We quantify the system-wide advantages from improved power flow control
and frequency reduction and find that an LFAC upgrade achieves similar and
sometimes better results compared with HVDC upgrades. Finally, we analyze the
factors which determine the optimal frequency for these upgraded transmission
lines, and we demonstrate the benefits of changing the frequency in response to
different system topologies and operating conditions.
