  This paper presents a trajectory tracking control strategy that modulates the
active power injected by geographically distributed inverter-based resources to
support transient stability. Each resource is independently controlled, and its
response drives the local bus voltage angle toward a trajectory that tracks the
angle of the center of inertia. The center-of-inertia angle is estimated in
real time from wide-area measurements. The main objectives are to stabilize
transient disturbances and increase the amount of power that can be safely
transferred over key transmission paths without loss of synchronism. Here we
envision the actuators as utility-scale energy storage systems; however,
equivalent examples could be developed for partially-curtailed photovoltaic
generation and/or Type 4 wind turbine generators. The strategy stems from a
time-varying linearization of the equations of motion for a synchronous
machine. The control action produces synchronizing torque in a special
reference frame that accounts for the motion of the center of inertia. This
drives the system states toward the desired trajectory and promotes rotor angle
stability. For testing we employ a reduced-order dynamic model of the North
American Western Interconnection. The results show that this approach improves
system reliability and can increase capacity utilization on stability-limited
transmission corridors.
