  With increasing prevalence of severe natural hazards and the ignition of
wildfires by power lines, many power system planners are considering converting
overhead lines to underground cables to mitigate risks related to these events.
Systems with a large proportion of underground cables can bring challenges due
to the capacitance and losses of the cables, factors which may limit the
potential for hardening in critical areas. Low frequency AC (LFAC) transmission
solves these problems, as lowering the frequency decreases both the effects of
capacitance and the losses. This paper presents a tractable frequency-dependent
model for underground cables and incorporates it into the open source optimal
power flow tool VariableFrequencyOPF.jl. The model and implementation are used
to demonstrate the benefits of LFAC in a case study involving two
multi-terminal cable upgrades with LFAC, including a comparison with HVDC. The
results demonstrate the value of LFAC systems for both power flow control and
reduction of losses.
