  The electric power system is witnessing a shift in the technology of
generation. Conventional thermal generation based on synchronous machines is
gradually being replaced by power electronics interfaced renewable generation.
This new mode of generation, however, lacks the natural inertia and governor
damping which are quintessential features of synchronous machines. The loss of
these features results in increasing frequency excursions and, ultimately,
system instability. Among the numerous studies on mitigating these undesirable
effects, the main approach involves virtual inertia emulation to mimic the
behavior of synchronous machines. In this work, explicit models of
grid-following and grid-forming virtual inertia (VI) devices are developed for
inertia emulation in low-inertia systems. An optimization problem is formulated
to optimize the parameters and location of these devices in a power system to
increase its resilience. Finally, a case study based on a high-fidelity model
of the South-East Australian system is used to illustrate the effectiveness of
such devices.
