  This paper investigates the transient stability of power systems co-dominated
by different types of grid-forming (GFM) devices. Synchronous generators (SGs
and VSGs) and droop-controlled inverters are typical GFM devices in modern
power systems. SGs/VSGs are able to provide inertia while droop-controlled
inverters are generally inertia-less. The transient stability of power systems
dominated by homogeneous GFM devices has been extensively studied. Regarding
the hybrid system jointly dominated by heterogeneous GFM devices, the transient
stability is rarely reported. This paper aims to fill this gap. It is found
that the synchronization behavior of the hybrid system can be described by a
second-order motion equation, resembling the swing equation of SGs. More
importantly, two significant differences from conventional power systems are
discovered. The first is that the droop control dramatically enhances the
damping effect, greatly affecting the transient stability region. The second is
that the frequency state variable exhibits an abrupt change at the moment of
fault disturbances, thus impacting the post-fault initial-state location and
stability assessment. The underlying mechanism behind the two new
characteristics is clarified and the impact on the transient stability is
analyzed and verified in this paper. The findings provide new insights into the
stability of power systems hosting heterogeneous devices.
