  Having sufficient grid-forming sources is one of the necessary conditions to
guarantee the stability in a power system hosting a very large share of
inverter-based generation. The grid-forming function has been historically
fulfilled by synchronous machines. However, with the appropriate control, it
can also be provided by voltage source converters (VSC). This work presents a
comparison between two technologies with grid-forming capability: the VSC with
a grid-forming control coupled with an adequate energy storage system, and the
synchronous condensers (SC). Both devices are compared regarding their inertial
response, as well as their contribution to the system strength and
short-circuit current for an equivalent capacity expressed in terms of apparent
power and inertial reserve. Their behaviour following grid disturbances is
assessed through time-domain simulations based on detailed electromagnetic
transient (EMT) models. The results show that both devices achieve similar
performance in the time-scale of seconds. For shorter time-windows, however,
they present a different behavior: the SC ensures a better stiffness in the
first tens of ms following the disturbance, while the VSC offers a faster
resynchronization.
