  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 inertialess. 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. Moreover,
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 a jump 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 performance is analyzed and verified. The
findings provide new insights into transient stability of power systems hosting
heterogeneous devices.
